turbo_persistence/

db.rs

use std::{
    borrow::Cow,
    collections::HashSet,
    fs::{self, File, OpenOptions, ReadDir},
    io::{BufWriter, Write},
    mem::take,
    ops::RangeInclusive,
    path::{Path, PathBuf},
    sync::{
        OnceLock,
        atomic::{AtomicBool, AtomicU32, Ordering},
    },
};

use anyhow::{Context, Result, bail};
use byteorder::{BE, ReadBytesExt, WriteBytesExt};
use dashmap::DashSet;
use jiff::Timestamp;
use memmap2::Mmap;
use nohash_hasher::BuildNoHashHasher;
use parking_lot::{Mutex, RwLock};
use smallvec::SmallVec;
use tracing::span::EnteredSpan;

pub use crate::compaction::selector::CompactConfig;
use crate::{
    DbConfig, FamilyKind, QueryKey,
    arc_bytes::ArcBytes,
    compaction::selector::{Compactable, get_merge_segments},
    compression::{checksum_block, decompress_into_arc},
    constants::{
        DATA_THRESHOLD_PER_COMPACTED_FILE, KEY_BLOCK_AVG_SIZE, KEY_BLOCK_CACHE_SIZE,
        MAX_ENTRIES_PER_COMPACTED_FILE, VALUE_BLOCK_AVG_SIZE, VALUE_BLOCK_CACHE_SIZE,
    },
    key::{StoreKey, hash_key},
    lookup_entry::{IterValue, LookupEntry, LookupValue},
    merge_iter::MergeIter,
    meta_file::{MetaEntryFlags, MetaFile, MetaLookupResult, StaticSortedFileRange},
    meta_file_builder::MetaFileBuilder,
    mmap_helper::advise_mmap_for_persistence,
    parallel_scheduler::ParallelScheduler,
    rc_bytes::RcBytes,
    sst_filter::SstFilter,
    static_sorted_file::{BlockCache, SstLookupResult, StaticSortedFileIter},
    static_sorted_file_builder::{StaticSortedFileBuilderMeta, StreamingSstWriter},
    write_batch::{FinishResult, WriteBatch},
};

#[cfg(feature = "stats")]
#[derive(Debug)]
pub struct CacheStatistics {
    pub hit_rate: f32,
    pub fill: f32,
    pub items: usize,
    pub size: u64,
    pub hits: u64,
    pub misses: u64,
}

#[cfg(feature = "stats")]
impl CacheStatistics {
    fn new<Key, Val, We, B, L>(cache: &quick_cache::sync::Cache<Key, Val, We, B, L>) -> Self
    where
        Key: Eq + std::hash::Hash,
        Val: Clone,
        We: quick_cache::Weighter<Key, Val> + Clone,
        B: std::hash::BuildHasher + Clone,
        L: quick_cache::Lifecycle<Key, Val> + Clone,
    {
        let size = cache.weight();
        let hits = cache.hits();
        let misses = cache.misses();
        Self {
            hit_rate: hits as f32 / (hits + misses) as f32,
            fill: size as f32 / cache.capacity() as f32,
            items: cache.len(),
            size,
            hits,
            misses,
        }
    }
}

#[cfg(feature = "stats")]
#[derive(Debug)]
pub struct Statistics {
    pub meta_files: usize,
    pub sst_files: usize,
    pub key_block_cache: CacheStatistics,
    pub value_block_cache: CacheStatistics,
    pub hits: u64,
    pub misses: u64,
    pub miss_family: u64,
    pub miss_range: u64,
    pub miss_amqf: u64,
    pub miss_key: u64,
}

#[cfg(feature = "stats")]
#[derive(Default)]
struct TrackedStats {
    hits_deleted: std::sync::atomic::AtomicU64,
    hits_small: std::sync::atomic::AtomicU64,
    hits_blob: std::sync::atomic::AtomicU64,
    miss_family: std::sync::atomic::AtomicU64,
    miss_range: std::sync::atomic::AtomicU64,
    miss_amqf: std::sync::atomic::AtomicU64,
    miss_key: std::sync::atomic::AtomicU64,
    miss_global: std::sync::atomic::AtomicU64,
}

/// State of the active write slot.
enum ActiveWriteState {
    /// A write operation or compaction is in progress.
    /// The string is a human-readable name used in error messages.
    Active(&'static str),
    /// A previous write or compaction failed and recovery also failed.
    /// No further writes are possible.
    Error,
}

/// A single superseded file whose deletion failed and is being retried.
///
/// On Linux/macOS, deleting a memory-mapped file is safe and this list is
/// normally empty. On Windows, open memory maps prevent deletion; failed files
/// are collected here and retried on the next commit or shutdown.
enum DeferredDeletion {
    Sst(u32),
    Meta(u32),
    Blob(u32),
}

/// RAII guard for an active write operation.
///
/// When dropped without [`WriteOperationGuard::success`] being called first, the guard rolls back
/// the operation by deleting any files whose sequence number exceeds `seq_before` (the sequence
/// number at the time the operation started). If rollback itself fails the write slot is set to
/// [`ActiveWriteState::Error`], permanently disabling further writes.
pub(crate) struct WriteOperationGuard<'a> {
    /// Reference to the active-write-operation slot, so we can clear or error it on drop.
    active: &'a Mutex<Option<ActiveWriteState>>,
    /// Database directory path, needed for orphan-file deletion during rollback.
    path: &'a Path,
    /// Sequence number at the time the operation started (= the last committed seq on disk).
    /// Files with seq > this were created by the current operation and must be deleted on
    /// rollback.
    seq_before: u32,
    /// Set to `true` by [`WriteOperationGuard::success`] to skip rollback on drop.
    succeeded: bool,
}

impl WriteOperationGuard<'_> {
    /// Mark the operation as successfully completed.
    ///
    /// After this call the guard's `Drop` impl will release the write slot without rolling back.
    pub(crate) fn success(&mut self) {
        self.succeeded = true;
    }
}

/// Deletes all files in `path` whose numeric stem is greater than `seq_before`.
///
/// Called on rollback to clean up any SST, meta, blob, or del files written during a
/// failed write operation or compaction.
fn delete_orphan_files(path: &Path, seq_before: u32) -> Result<()> {
    // Restore CURRENT to seq_before first. The failure may have happened mid-write
    // to CURRENT, leaving it partially written. Writing seq_before makes the
    // on-disk state consistent before we start deleting orphan files.
    let mut current_file = OpenOptions::new()
        .write(true)
        .truncate(false)
        .read(false)
        .open(path.join("CURRENT"))?;
    current_file.write_u32::<BE>(seq_before)?;
    current_file.sync_all()?;

    for entry in fs::read_dir(path)? {
        let entry = entry?;
        let path = entry.path();
        if let Some(ext) = path.extension().and_then(|s| s.to_str())
            && let Some(seq) = path
                .file_stem()
                .and_then(|s| s.to_str())
                .and_then(|s| s.parse::<u32>().ok())
            && seq > seq_before
        {
            match ext {
                "sst" | "meta" | "blob" | "del" => fs::remove_file(&path)?,
                _ => {}
            }
        }
    }
    Ok(())
}

impl Drop for WriteOperationGuard<'_> {
    fn drop(&mut self) {
        if self.succeeded {
            // Happy path: just release the slot.
            *self.active.lock() = None;
            return;
        }

        // Unhappy path: the operation failed (or was dropped without commit).
        // Delete every file that was created during this operation (seq > seq_before).
        match delete_orphan_files(self.path, self.seq_before) {
            Ok(()) => *self.active.lock() = None,
            Err(_) => *self.active.lock() = Some(ActiveWriteState::Error),
        }
    }
}

/// TurboPersistence is a persistent key-value store. It is limited to a single writer at a time
/// using a single write batch. It allows for concurrent reads.
pub struct TurboPersistence<S: ParallelScheduler, const FAMILIES: usize> {
    parallel_scheduler: S,
    /// The path to the directory where the database is stored
    path: PathBuf,
    /// If true, the database is opened in read-only mode. In this mode, no writes are allowed and
    /// no modification on the database is performed.
    read_only: bool,
    /// The inner state of the database. Writing will update that.
    inner: RwLock<Inner<FAMILIES>>,
    /// A flag to indicate if the database is empty (no meta files). This is an atomic mirror of
    /// `inner.meta_files.is_empty()` to avoid taking a lock on the hot path.
    is_empty: AtomicBool,
    /// Tracks whether a write operation is in progress or has permanently failed.
    /// `None` = idle, `Some(Active)` = in progress, `Some(Error)` = permanently disabled.
    active_write_operation: Mutex<Option<ActiveWriteState>>,
    /// Files from superseded commits whose deletion failed (e.g. on Windows due to open memory
    /// maps) and will be retried on the next commit or at shutdown.
    /// Protected by `active_write_operation` (only mutated inside a write operation).
    deferred_deletions: Mutex<Vec<DeferredDeletion>>,
    /// A cache for decompressed key blocks. Allocated lazily on first read via
    /// [`Self::key_block_cache`] so write-only or empty sessions never pay the cache's fixed
    /// hash-table overhead.
    key_block_cache: OnceLock<BlockCache>,
    /// A cache for decompressed value blocks. Allocated lazily on first read via
    /// [`Self::value_block_cache`]; see [`Self::key_block_cache`].
    value_block_cache: OnceLock<BlockCache>,
    /// Per-family configuration for file limits.
    config: DbConfig<FAMILIES>,
    /// Statistics for the database.
    #[cfg(feature = "stats")]
    stats: TrackedStats,
}

/// The inner state of the database.
struct Inner<const FAMILIES: usize> {
    /// The list of meta files in the database. This is used to derive the SST files.
    meta_files: Vec<MetaFile>,
    /// The current sequence number for the database.
    current_sequence_number: u32,
    /// The in progress set of hashes of keys that have been accessed.
    /// It will be flushed onto disk (into a meta file) on next commit.
    /// It's a dashset to allow modification while only tracking a read lock on Inner.
    accessed_key_hashes: [DashSet<u64, BuildNoHashHasher<u64>>; FAMILIES],
}

pub struct CommitOptions {
    new_meta_files: Vec<(u32, File)>,
    new_sst_files: Vec<(u32, File)>,
    new_blob_files: Vec<(u32, File)>,
    sst_seq_numbers_to_delete: Vec<u32>,
    blob_seq_numbers_to_delete: Vec<u32>,
    sequence_number: u32,
    keys_written: u64,
}

struct OpenOpts<S: ParallelScheduler, const FAMILIES: usize> {
    path: PathBuf,
    read_only: bool,
    parallel_scheduler: S,
    config: DbConfig<FAMILIES>,
}

impl<S: ParallelScheduler + Default, const FAMILIES: usize> TurboPersistence<S, FAMILIES> {
    /// Open a TurboPersistence database at the given path.
    /// This will read the directory and might performance cleanup when the database was not closed
    /// properly. Cleanup only requires to read a few bytes from a few files and to delete
    /// files, so it's fast.
    pub fn open(path: PathBuf) -> Result<Self> {
        Self::open_with_parallel_scheduler(path, Default::default())
    }

    /// Open a TurboPersistence database at the given path with custom per-family configuration.
    pub fn open_with_config(path: PathBuf, config: DbConfig<FAMILIES>) -> Result<Self> {
        Self::open_with_config_and_parallel_scheduler(path, config, Default::default())
    }

    /// Open a TurboPersistence database at the given path in read only mode.
    /// This will read the directory. No Cleanup is performed.
    pub fn open_read_only_with_config(path: PathBuf, config: DbConfig<FAMILIES>) -> Result<Self> {
        Self::open_read_only_with_parallel_scheduler(path, config, Default::default())
    }

    /// Construct an empty, read-only `TurboPersistence` that owns no on-disk state and never
    /// touches the filesystem. Reads return None; writes bail via the existing `read_only` guard.
    /// Used to provide a "noop" backing storage with the same concrete type as the real one.
    pub fn empty_in_memory_with_config(config: DbConfig<FAMILIES>) -> Self {
        // `path` is `PathBuf::new()` but never read because `meta_files` is empty and
        // `read_only` is true (so no write/compaction path is reachable).
        Self::new(OpenOpts {
            path: PathBuf::new(),
            read_only: true,
            parallel_scheduler: Default::default(),
            config,
        })
    }
}

impl<S: ParallelScheduler, const FAMILIES: usize> TurboPersistence<S, FAMILIES> {
    fn new(
        OpenOpts {
            path,
            read_only,
            parallel_scheduler,
            config,
        }: OpenOpts<S, FAMILIES>,
    ) -> Self {
        Self {
            parallel_scheduler,
            path,
            read_only,
            inner: RwLock::new(Inner {
                meta_files: Vec::new(),
                current_sequence_number: 0,
                accessed_key_hashes: [(); FAMILIES]
                    .map(|_| DashSet::with_hasher(BuildNoHashHasher::default())),
            }),
            is_empty: AtomicBool::new(true),
            active_write_operation: Mutex::new(None),
            deferred_deletions: Mutex::new(Vec::new()),
            key_block_cache: OnceLock::new(),
            value_block_cache: OnceLock::new(),
            config,
            #[cfg(feature = "stats")]
            stats: TrackedStats::default(),
        }
    }

    /// Open a TurboPersistence database at the given path.
    /// This will read the directory and might performance cleanup when the database was not closed
    /// properly. Cleanup only requires to read a few bytes from a few files and to delete
    /// files, so it's fast.
    pub fn open_with_parallel_scheduler(path: PathBuf, parallel_scheduler: S) -> Result<Self> {
        Self::open_with_config_and_parallel_scheduler(path, DbConfig::default(), parallel_scheduler)
    }

    /// Open a TurboPersistence database at the given path with custom per-family configuration.
    pub fn open_with_config_and_parallel_scheduler(
        path: PathBuf,
        config: DbConfig<FAMILIES>,
        parallel_scheduler: S,
    ) -> Result<Self> {
        let mut db = Self::new(OpenOpts {
            path,
            read_only: false,
            parallel_scheduler,
            config,
        });
        db.open_directory(false)?;
        Ok(db)
    }

    /// Open a TurboPersistence database at the given path in read only mode.
    /// This will read the directory. No Cleanup is performed.
    fn open_read_only_with_parallel_scheduler(
        path: PathBuf,
        config: DbConfig<FAMILIES>,
        parallel_scheduler: S,
    ) -> Result<Self> {
        let mut db = Self::new(OpenOpts {
            path,
            read_only: true,
            parallel_scheduler,
            config,
        });
        db.open_directory(false)?;
        Ok(db)
    }

    /// Performs the initial check on the database directory.
    fn open_directory(&mut self, read_only: bool) -> Result<()> {
        match fs::read_dir(&self.path) {
            Ok(entries) => {
                if !self
                    .load_directory(entries, read_only)
                    .context("Loading persistence directory failed")?
                {
                    if read_only {
                        bail!("Failed to open database");
                    }
                    self.init_directory()
                        .context("Initializing persistence directory failed")?;
                }
                Ok(())
            }
            Err(e) => {
                if !read_only && e.kind() == std::io::ErrorKind::NotFound {
                    self.create_and_init_directory()
                        .context("Creating and initializing persistence directory failed")?;
                    Ok(())
                } else {
                    Err(e).context("Failed to open database")
                }
            }
        }
    }

    /// Creates the directory and initializes it.
    fn create_and_init_directory(&mut self) -> Result<()> {
        fs::create_dir_all(&self.path)?;
        self.init_directory()
    }

    /// Initializes the directory by creating the CURRENT file.
    fn init_directory(&mut self) -> Result<()> {
        let mut current = File::create(self.path.join("CURRENT"))?;
        current.write_u32::<BE>(0)?;
        current.flush()?;
        Ok(())
    }

    /// Loads an existing database directory and performs cleanup if necessary.
    fn load_directory(&mut self, entries: ReadDir, read_only: bool) -> Result<bool> {
        let mut meta_files = Vec::new();
        let mut current_file = match File::open(self.path.join("CURRENT")) {
            Ok(file) => file,
            Err(e) => {
                if !read_only && e.kind() == std::io::ErrorKind::NotFound {
                    return Ok(false);
                } else {
                    return Err(e).context("Failed to open CURRENT file");
                }
            }
        };
        let current = current_file.read_u32::<BE>()?;
        drop(current_file);

        let mut deleted_files = HashSet::new();
        for entry in entries {
            let entry = entry?;
            let path = entry.path();
            if let Some(ext) = path.extension().and_then(|s| s.to_str()) {
                let seq: u32 = path
                    .file_stem()
                    .context("File has no file stem")?
                    .to_str()
                    .context("File stem is not valid utf-8")?
                    .parse()?;
                if deleted_files.contains(&seq) {
                    continue;
                }
                if seq > current {
                    if !read_only {
                        fs::remove_file(&path)?;
                    }
                } else {
                    match ext {
                        "meta" => {
                            meta_files.push(seq);
                        }
                        "del" => {
                            let mut content = &*fs::read(&path)?;
                            let mut no_existing_files = true;
                            while !content.is_empty() {
                                let seq = content.read_u32::<BE>()?;
                                deleted_files.insert(seq);
                                if !read_only {
                                    // Remove the files that are marked for deletion
                                    let sst_file = self.path.join(format!("{seq:08}.sst"));
                                    let meta_file = self.path.join(format!("{seq:08}.meta"));
                                    let blob_file = self.path.join(format!("{seq:08}.blob"));
                                    for path in [sst_file, meta_file, blob_file] {
                                        if fs::exists(&path)? {
                                            fs::remove_file(path)?;
                                            no_existing_files = false;
                                        }
                                    }
                                }
                            }
                            if !read_only && no_existing_files {
                                fs::remove_file(&path)?;
                            }
                        }
                        "blob" | "sst" => {
                            // ignore blobs and sst, they are read when needed
                        }
                        _ => {
                            if !path
                                .file_name()
                                .is_some_and(|s| s.as_encoded_bytes().starts_with(b"."))
                            {
                                bail!("Unexpected file in persistence directory: {:?}", path);
                            }
                        }
                    }
                }
            } else {
                match path.file_stem().and_then(|s| s.to_str()) {
                    Some("CURRENT") => {
                        // Already read
                    }
                    Some("LOG") => {
                        // Ignored, write-only
                    }
                    _ => {
                        if !path
                            .file_name()
                            .is_some_and(|s| s.as_encoded_bytes().starts_with(b"."))
                        {
                            bail!("Unexpected file in persistence directory: {:?}", path);
                        }
                    }
                }
            }
        }

        meta_files.retain(|seq| !deleted_files.contains(seq));
        meta_files.sort_unstable();
        let mut meta_files = self
            .parallel_scheduler
            .parallel_map_collect::<_, _, Result<Vec<MetaFile>>>(&meta_files, |&seq| {
                let meta_file = MetaFile::open(&self.path, seq)?;
                Ok(meta_file)
            })?;

        let mut sst_filter = SstFilter::new();
        for meta_file in meta_files.iter_mut().rev() {
            sst_filter.apply_filter(meta_file);
        }

        let inner = self.inner.get_mut();
        self.is_empty
            .store(meta_files.is_empty(), Ordering::Relaxed);
        inner.meta_files = meta_files;
        inner.current_sequence_number = current;
        Ok(true)
    }

    /// Reads and decompresses a blob file. This is not backed by any cache.
    #[tracing::instrument(level = "info", name = "reading database blob", skip_all)]
    fn read_blob(&self, seq: u32) -> Result<ArcBytes> {
        let path = self.path.join(format!("{seq:08}.blob"));
        let file = File::open(&path)
            .with_context(|| format!("Failed to open blob file {}", path.display()))?;
        let mmap = unsafe { Mmap::map(&file) }.with_context(|| {
            format!(
                "Failed to mmap blob file {} ({} bytes)",
                path.display(),
                file.metadata().map(|m| m.len()).unwrap_or(0)
            )
        })?;
        #[cfg(unix)]
        mmap.advise(memmap2::Advice::Sequential)?;
        #[cfg(unix)]
        mmap.advise(memmap2::Advice::WillNeed)?;
        advise_mmap_for_persistence(&mmap)?;
        let mut reader = &mmap[..];
        let uncompressed_length = reader
            .read_u32::<BE>()
            .context("Failed to read uncompressed length from blob file")?;
        let expected_checksum = reader.read_u32::<BE>()?;

        // Verify checksum on the compressed on-disk data before decompression.
        let actual_checksum = checksum_block(reader);
        if actual_checksum != expected_checksum {
            bail!(
                "Cache corruption detected: checksum mismatch in blob file {:08}.blob (expected \
                 {:08x}, got {:08x})",
                seq,
                expected_checksum,
                actual_checksum
            );
        }

        let buffer = decompress_into_arc(uncompressed_length, reader)?;
        Ok(ArcBytes::from(buffer))
    }

    /// Returns true if the database is empty.
    pub fn is_empty(&self) -> bool {
        self.is_empty.load(Ordering::Relaxed)
    }

    /// Returns `true` if a previous write or compaction left the database in an unrecoverable error
    /// state, permanently disabling further writes.
    pub fn has_unrecoverable_write_error(&self) -> bool {
        matches!(
            *self.active_write_operation.lock(),
            Some(ActiveWriteState::Error)
        )
    }

    /// Acquires the write-operation slot, returning an RAII guard that rolls back and releases it
    /// on drop. Only one write operation (write batch or compaction) is allowed at a time.
    /// `name` is a short human-readable label used in error messages (e.g. `"write batch"`).
    fn acquire_write_operation(&self, name: &'static str) -> Result<WriteOperationGuard<'_>> {
        if self.read_only {
            bail!("Cannot perform write operations on a read-only database");
        }
        let mut slot = self.active_write_operation.lock();
        match &*slot {
            Some(ActiveWriteState::Active(active_name)) => {
                bail!(
                    "Another {active_name} is already active (only a single write operation is \
                     allowed at a time)"
                );
            }
            Some(ActiveWriteState::Error) => {
                bail!(
                    "A previous write operation failed with an unrecoverable error; no further \
                     writes are possible"
                );
            }
            None => {}
        }
        *slot = Some(ActiveWriteState::Active(name));
        drop(slot); // release before acquiring inner read lock
        let seq_before = self.inner.read().current_sequence_number;
        Ok(WriteOperationGuard {
            active: &self.active_write_operation,
            path: &self.path,
            seq_before,
            succeeded: false,
        })
    }

    /// Starts a new WriteBatch for the database. Only a single write operation is allowed at a
    /// time. The WriteBatch need to be committed with [`TurboPersistence::commit_write_batch`].
    /// Note that the WriteBatch might start writing data to disk while it's filled up with data.
    /// This data will only become visible after the WriteBatch is committed.
    pub fn write_batch<K: StoreKey + Send + Sync>(&self) -> Result<WriteBatch<'_, K, S, FAMILIES>> {
        let guard = self.acquire_write_operation("write batch")?;
        // seq_before is already the current sequence number, no second read needed.
        let current = guard.seq_before;
        Ok(WriteBatch::new(
            guard,
            self.path.clone(),
            current,
            self.parallel_scheduler.clone(),
            self.config.family_configs,
        ))
    }

    fn key_block_cache(&self) -> &BlockCache {
        self.key_block_cache.get_or_init(|| {
            BlockCache::with(
                KEY_BLOCK_CACHE_SIZE as usize / KEY_BLOCK_AVG_SIZE,
                KEY_BLOCK_CACHE_SIZE,
                Default::default(),
                Default::default(),
                Default::default(),
            )
        })
    }

    fn value_block_cache(&self) -> &BlockCache {
        self.value_block_cache.get_or_init(|| {
            BlockCache::with(
                VALUE_BLOCK_CACHE_SIZE as usize / VALUE_BLOCK_AVG_SIZE,
                VALUE_BLOCK_CACHE_SIZE,
                Default::default(),
                Default::default(),
                Default::default(),
            )
        })
    }

    /// Clears all caches of the database.
    pub fn clear_cache(&self) {
        self.clear_block_caches();
        for meta in self.inner.write().meta_files.iter_mut() {
            meta.clear_cache();
        }
    }

    /// Clears block caches of the database. Caches that have not been allocated yet are left
    /// uninitialized, so clearing never forces allocation.
    pub fn clear_block_caches(&self) {
        if let Some(cache) = self.key_block_cache.get() {
            cache.clear();
        }
        if let Some(cache) = self.value_block_cache.get() {
            cache.clear();
        }
    }

    /// Prefetches all SST files which are usually lazy loaded. This can be used to reduce latency
    /// for the first queries after opening the database.
    pub fn prepare_all_sst_caches(&self) {
        for meta in self.inner.write().meta_files.iter_mut() {
            meta.prepare_sst_cache();
        }
    }

    fn open_log(&self) -> Result<BufWriter<File>> {
        if self.read_only {
            unreachable!("Only write operations can open the log file");
        }
        let log_path = self.path.join("LOG");
        let log_file = OpenOptions::new()
            .create(true)
            .append(true)
            .open(log_path)?;
        Ok(BufWriter::new(log_file))
    }

    /// Commits a WriteBatch to the database. This will finish writing the data to disk and make it
    /// visible to readers.
    pub fn commit_write_batch<K: StoreKey + Send + Sync>(
        &self,
        mut write_batch: WriteBatch<'_, K, S, FAMILIES>,
    ) -> Result<()> {
        if self.read_only {
            unreachable!("It's not possible to create a write batch for a read-only database");
        }
        let FinishResult {
            sequence_number,
            new_meta_files,
            new_sst_files,
            new_blob_files,
            keys_written,
        } = write_batch.finish(|family| {
            let inner = self.inner.read();
            let set = &inner.accessed_key_hashes[family as usize];
            // len is only a snapshot at that time and it can change while we create the filter.
            // So we give it 5% more space to make resizes less likely.
            let initial_capacity = set.len() * 20 / 19;
            // TODO: Using u64::BITS as fingerprint size is wasteful for a
            // probabilistic membership filter. A smaller fingerprint (e.g. via
            // Filter::new with a target fp_rate) would significantly reduce size,
            // but would make merging slower since mismatched fingerprint sizes
            // fall back to one-by-one insertion instead of sorted merge.
            let mut amqf =
                qfilter::Filter::with_fingerprint_size(initial_capacity as u64, u64::BITS as u8)
                    .unwrap();
            // This drains items from the set. But due to concurrency it might not be empty
            // afterwards, but that's fine. It will be part of the next commit.
            set.retain(|hash| {
                // Performance-wise it would usually be better to insert sorted fingerprints, but we
                // assume that hashes are equally distributed, which makes it unnecessary.
                // Good for cache locality is that we insert in the order of the dashset's buckets.
                amqf.insert_fingerprint(false, *hash)
                    .expect("Failed to insert fingerprint");
                false
            });
            amqf
        })?;
        self.commit(CommitOptions {
            new_meta_files,
            new_sst_files,
            new_blob_files,
            sst_seq_numbers_to_delete: vec![],
            blob_seq_numbers_to_delete: vec![],
            sequence_number,
            keys_written,
        })?;
        // Mark the guard inside the write batch as succeeded so it skips the rollback on drop.
        write_batch.mark_succeeded();
        Ok(())
    }

    /// fsyncs the new files and updates the CURRENT file. Updates the database state to include the
    /// new files.
    fn commit(
        &self,
        CommitOptions {
            mut new_meta_files,
            new_sst_files,
            new_blob_files,
            mut sst_seq_numbers_to_delete,
            mut blob_seq_numbers_to_delete,
            sequence_number: mut seq,
            keys_written,
        }: CommitOptions,
    ) -> Result<(), anyhow::Error> {
        let time = Timestamp::now();

        new_meta_files.sort_unstable_by_key(|(seq, _)| *seq);

        let sync_span = tracing::trace_span!("sync new files").entered();

        enum SyncItem {
            Meta(u32, File),
            Sst(File),
            Blob(u32, File),
        }
        enum SyncResult {
            Meta(MetaFile),
            Sst,
            Blob(u32, File),
        }

        let mut sync_items: Vec<SyncItem> =
            Vec::with_capacity(new_meta_files.len() + new_sst_files.len() + new_blob_files.len());
        for (seq, file) in new_meta_files {
            sync_items.push(SyncItem::Meta(seq, file));
        }
        for (_, file) in new_sst_files {
            sync_items.push(SyncItem::Sst(file));
        }
        for (seq, file) in new_blob_files {
            sync_items.push(SyncItem::Blob(seq, file));
        }

        let results: Vec<SyncResult> = self
            .parallel_scheduler
            .parallel_map_collect_owned::<_, _, Result<Vec<_>>>(sync_items, |item| match item {
                SyncItem::Meta(seq, file) => {
                    file.sync_data()?;
                    let meta_file = MetaFile::open(&self.path, seq)?;
                    Ok(SyncResult::Meta(meta_file))
                }
                SyncItem::Sst(file) => {
                    file.sync_data()?;
                    Ok(SyncResult::Sst)
                }
                SyncItem::Blob(seq, file) => {
                    file.sync_data()?;
                    Ok(SyncResult::Blob(seq, file))
                }
            })?;

        let mut new_meta_files: Vec<MetaFile> = Vec::new();
        let mut new_blob_files: Vec<(u32, File)> = Vec::new();
        for result in results {
            match result {
                SyncResult::Meta(mf) => new_meta_files.push(mf),
                SyncResult::Sst => {}
                SyncResult::Blob(seq, file) => new_blob_files.push((seq, file)),
            }
        }

        let mut sst_filter = SstFilter::new();
        for meta_file in new_meta_files.iter_mut().rev() {
            sst_filter.apply_filter(meta_file);
        }

        // Sync the directory to ensure the new directory entries (file name → inode mappings)
        // are durable before we update CURRENT. Without this, a crash could leave CURRENT pointing
        // to files whose directory entries were lost even though their data was flushed.
        File::open(&self.path)?.sync_data()?;
        drop(sync_span);

        let new_meta_info = new_meta_files
            .iter()
            .map(|meta| {
                let ssts = meta
                    .entries()
                    .iter()
                    .map(|entry| {
                        let seq = entry.sequence_number();
                        let range = entry.range();
                        let size = entry.size();
                        let flags = entry.flags();
                        (seq, range.min_hash, range.max_hash, size, flags)
                    })
                    .collect::<Vec<_>>();
                (
                    meta.sequence_number(),
                    meta.family(),
                    ssts,
                    meta.obsolete_sst_files().to_vec(),
                )
            })
            .collect::<Vec<_>>();

        // ── Phase A: compute what will change without modifying inner. ──
        //
        // We need `meta_seq_numbers_to_delete` and `has_delete_file` to write
        // the .del file BEFORE writing CURRENT. We must not modify `inner` at
        // all — if a disk error occurs before CURRENT is durable, the
        // WriteOperationGuard rollback can only clean up orphan files, not undo
        // in-memory mutations. The MetaFile in-memory optimization
        // (retain_entries) is deferred to Phase C.
        let has_delete_file;
        let mut meta_seq_numbers_to_delete = Vec::new();
        let entries_to_remove;

        {
            let inner = self.inner.read();

            // (A1) Run the SST filter on existing meta files. This only
            // updates the SstFilter state — the MetaFile in-memory layout is
            // not modified yet (that happens in Phase C via retain_entries).
            // Collects the set of SST entry sequence numbers to remove from
            // each meta file, keyed by position in `inner.meta_files`.
            entries_to_remove = inner
                .meta_files
                .iter()
                .rev()
                .map(|meta_file| sst_filter.apply_filter_collect(meta_file))
                .collect::<Vec<_>>();

            // (A2) Determine which meta files are fully obsolete by running
            // `apply_and_get_remove` in newest-first order. Process new metas
            // first (they are newer than existing ones) to advance the filter
            // state, then existing ones. New metas are never candidates for
            // removal (just created), so only their filter-state side-effects
            // matter.
            for meta_file in new_meta_files.iter().rev() {
                let should_remove = sst_filter.apply_and_get_remove(meta_file);
                debug_assert!(
                    !should_remove,
                    "newly created meta file should never be a candidate for removal"
                );
            }
            for i in (0..inner.meta_files.len()).rev() {
                if sst_filter.apply_and_get_remove(&inner.meta_files[i]) {
                    meta_seq_numbers_to_delete.push(inner.meta_files[i].sequence_number());
                }
            }

            // (A3) Compute the final sequence number that will be written to
            // CURRENT. A .del file is created only when there are files to
            // delete, which consumes one extra sequence number.
            has_delete_file = !sst_seq_numbers_to_delete.is_empty()
                || !blob_seq_numbers_to_delete.is_empty()
                || !meta_seq_numbers_to_delete.is_empty();
        }
        if has_delete_file {
            seq += 1;
        }

        self.parallel_scheduler.block_in_place(|| {
            if has_delete_file {
                sst_seq_numbers_to_delete.sort_unstable();
                meta_seq_numbers_to_delete.sort_unstable();
                blob_seq_numbers_to_delete.sort_unstable();
                // Write *.del file, marking the selected files as to delete
                let mut buf = Vec::with_capacity(
                    (sst_seq_numbers_to_delete.len()
                        + meta_seq_numbers_to_delete.len()
                        + blob_seq_numbers_to_delete.len())
                        * size_of::<u32>(),
                );
                for seq in sst_seq_numbers_to_delete.iter() {
                    buf.write_u32::<BE>(*seq)?;
                }
                for seq in meta_seq_numbers_to_delete.iter() {
                    buf.write_u32::<BE>(*seq)?;
                }
                for seq in blob_seq_numbers_to_delete.iter() {
                    buf.write_u32::<BE>(*seq)?;
                }
                let mut file = File::create(self.path.join(format!("{seq:08}.del")))?;
                file.write_all(&buf)?;
                file.sync_data()?;
            }

            let mut current_file = OpenOptions::new()
                .write(true)
                .truncate(false)
                .read(false)
                .open(self.path.join("CURRENT"))?;
            current_file.write_u32::<BE>(seq)?;
            current_file.sync_data()?;

            // ── Point of no return ──────────────────────────────────────────
            //
            // CURRENT has been durably updated. The commit is now visible to
            // future readers (including after a crash/restart via
            // `load_directory`). Everything below is best-effort cleanup:
            //
            // • Writing the LOG is purely informational.
            //
            // • Superseded files are NOT deleted here — Phase C handles that
            //   after `inner` is updated. On Linux/macOS they are deleted
            //   immediately; on Windows (where open memory maps prevent
            //   deletion) they are retried on the next commit or shutdown.
            //
            // Errors here must NOT propagate, because the WriteOperationGuard
            // would then run its rollback and delete the *newly committed*
            // files, corrupting the database.

            if let Err(e) = (|| {
                let mut log = self.open_log()?;
                writeln!(log, "Time {time}")?;
                let span = time.until(Timestamp::now())?;
                writeln!(log, "Commit {seq:08} {keys_written} keys in {span:#}")?;
                writeln!(log, "FAM | META SEQ | SST SEQ         | RANGE")?;
                for (meta_seq, family, ssts, obsolete) in new_meta_info {
                    for (seq, min, max, size, flags) in ssts {
                        writeln!(
                            log,
                            "{family:3} | {meta_seq:08} | {seq:08} SST    | {} ({} MiB, {})",
                            range_to_str(min, max),
                            size / 1024 / 1024,
                            flags
                        )?;
                    }
                    for obsolete in obsolete.chunks(15) {
                        write!(log, "{family:3} | {meta_seq:08} |")?;
                        for seq in obsolete {
                            write!(log, " {seq:08}")?;
                        }
                        writeln!(log, " OBSOLETE SST")?;
                    }
                }

                fn write_seq_numbers<W: std::io::Write, T>(
                    log: &mut W,
                    items: &[T],
                    label: &str,
                    extract_seq: fn(&T) -> u32,
                ) -> std::io::Result<()> {
                    for chunk in items.chunks(15) {
                        write!(log, "    |          |")?;
                        for item in chunk {
                            write!(log, " {:08}", extract_seq(item))?;
                        }
                        writeln!(log, " {}", label)?;
                    }
                    Ok(())
                }

                new_blob_files.sort_unstable_by_key(|(seq, _)| *seq);
                write_seq_numbers(&mut log, &new_blob_files, "NEW BLOB", |&(seq, _)| seq)?;
                write_seq_numbers(
                    &mut log,
                    &blob_seq_numbers_to_delete,
                    "BLOB DELETED",
                    |&seq| seq,
                )?;
                write_seq_numbers(
                    &mut log,
                    &sst_seq_numbers_to_delete,
                    "SST DELETED",
                    |&seq| seq,
                )?;
                write_seq_numbers(
                    &mut log,
                    &meta_seq_numbers_to_delete,
                    "META DELETED",
                    |&seq| seq,
                )?;
                anyhow::Ok(())
            })() {
                eprintln!("turbo-persistence: failed to write LOG after commit {seq:08}: {e:#}");
            }

            anyhow::Ok(())
        })?;

        // ── Phase C: structurally update inner (CURRENT is already durable). ──
        //
        // Between Phase A's read-lock drop and this point no other writer can
        // run (WriteOperationGuard ensures exclusivity) and readers never mutate
        // inner, so the snapshot from Phase A is still valid.
        {
            let mut inner = self.inner.write();

            // Apply the deferred MetaFile mutations from Phase A1. apply_filter
            // was called read-only earlier; now we actually move superseded
            // entries from active to obsolete inside each MetaFile.
            // entries_to_remove was collected in reverse order, so iterate it
            // in reverse to match the forward order of inner.meta_files.
            for (meta_file, to_remove) in inner
                .meta_files
                .iter_mut()
                .zip(entries_to_remove.into_iter().rev())
            {
                if !to_remove.is_empty() {
                    meta_file.retain_entries(|seq| !to_remove.contains(&seq));
                }
            }

            inner.meta_files.append(&mut new_meta_files);
            if !meta_seq_numbers_to_delete.is_empty() {
                let to_delete: HashSet<u32> = meta_seq_numbers_to_delete.iter().copied().collect();
                inner
                    .meta_files
                    .retain(|meta| !to_delete.contains(&meta.sequence_number()));
            }
            inner.current_sequence_number = seq;
            self.is_empty
                .store(inner.meta_files.is_empty(), Ordering::Relaxed);
        }

        // Try to delete superseded files immediately. On Linux/macOS this always
        // works even if readers have the files memory-mapped. On Windows, open
        // memory maps prevent deletion; any file that fails is kept in
        // `deferred_deletions` and retried on the next commit or at shutdown.
        self.deferred_deletions.lock().extend(
            Self::try_delete_files(&self.path, &sst_seq_numbers_to_delete, "sst")
                .map(DeferredDeletion::Sst)
                .chain(
                    Self::try_delete_files(&self.path, &meta_seq_numbers_to_delete, "meta")
                        .map(DeferredDeletion::Meta),
                )
                .chain(
                    Self::try_delete_files(&self.path, &blob_seq_numbers_to_delete, "blob")
                        .map(DeferredDeletion::Blob),
                ),
        );

        // Retry any deletions that failed in earlier commits.
        self.retry_deferred_deletions();

        // Best-effort verbose log of the new database state after Phase C.
        #[cfg(feature = "verbose_log")]
        {
            let _: Result<(), _> = (|| -> anyhow::Result<()> {
                let mut log = self.open_log()?;
                writeln!(log, "New database state:")?;
                writeln!(log, "FAM | META SEQ | SST SEQ  FLAGS | RANGE")?;
                let inner = self.inner.read();
                let families = inner.meta_files.iter().map(|meta| meta.family()).filter({
                    let mut set = HashSet::new();
                    move |family| set.insert(*family)
                });
                for family in families {
                    for meta in inner.meta_files.iter() {
                        if meta.family() != family {
                            continue;
                        }
                        let meta_seq = meta.sequence_number();
                        for entry in meta.entries().iter() {
                            let seq = entry.sequence_number();
                            let range = entry.range();
                            writeln!(
                                log,
                                "{family:3} | {meta_seq:08} | {seq:08} {:>6} | {}",
                                entry.flags(),
                                range_to_str(range.min_hash, range.max_hash)
                            )?;
                        }
                    }
                }
                Ok(())
            })();
        }

        Ok(())
    }

    /// Runs a full compaction on the database. This will rewrite all SST files, removing all
    /// duplicate keys and separating all key ranges into unique files.
    pub fn full_compact(&self) -> Result<()> {
        self.compact(&CompactConfig {
            min_merge_count: 2,
            optimal_merge_count: usize::MAX,
            max_merge_count: usize::MAX,
            max_merge_bytes: u64::MAX,
            min_merge_duplication_bytes: 0,
            optimal_merge_duplication_bytes: u64::MAX,
            max_merge_segment_count: usize::MAX,
        })?;
        Ok(())
    }

    /// Runs a (partial) compaction. Compaction will only be performed if the coverage of the SST
    /// files is above the given threshold. The coverage is the average number of SST files that
    /// need to be read to find a key. It also limits the maximum number of SST files that are
    /// merged at once, which is the main factor for the runtime of the compaction.
    pub fn compact(&self, compact_config: &CompactConfig) -> Result<bool> {
        let mut guard = self.acquire_write_operation("compaction")?;

        // Free block caches and SST mmaps before compaction. The block caches
        // are not used during compaction (we iterate uncached), and any cached
        // SST mmaps would use MADV_RANDOM which is wrong for sequential scans.
        // Clearing them upfront frees memory for the merge work.
        self.clear_cache();

        let mut sequence_number;
        let mut new_meta_files = Vec::new();
        let mut new_sst_files = Vec::new();
        let mut sst_seq_numbers_to_delete = Vec::new();
        let mut blob_seq_numbers_to_delete = Vec::new();
        let mut keys_written = 0;

        {
            let inner = self.inner.read();
            sequence_number = AtomicU32::new(inner.current_sequence_number);
            self.compact_internal(
                &inner.meta_files,
                &sequence_number,
                &mut new_meta_files,
                &mut new_sst_files,
                &mut sst_seq_numbers_to_delete,
                &mut blob_seq_numbers_to_delete,
                &mut keys_written,
                compact_config,
            )
            .context("Failed to compact database")?;
        }

        let has_changes = !new_meta_files.is_empty();
        if has_changes {
            self.commit(CommitOptions {
                new_meta_files,
                new_sst_files,
                new_blob_files: Vec::new(),
                sst_seq_numbers_to_delete,
                blob_seq_numbers_to_delete,
                sequence_number: *sequence_number.get_mut(),
                keys_written,
            })
            .context("Failed to commit the database compaction")?;
        }

        guard.success();
        Ok(has_changes)
    }

    /// Internal function to perform a compaction.
    fn compact_internal(
        &self,
        meta_files: &[MetaFile],
        sequence_number: &AtomicU32,
        new_meta_files: &mut Vec<(u32, File)>,
        new_sst_files: &mut Vec<(u32, File)>,
        sst_seq_numbers_to_delete: &mut Vec<u32>,
        blob_seq_numbers_to_delete: &mut Vec<u32>,
        keys_written: &mut u64,
        compact_config: &CompactConfig,
    ) -> Result<()> {
        if meta_files.is_empty() {
            return Ok(());
        }

        struct SstWithRange {
            meta_index: usize,
            index_in_meta: u32,
            seq: u32,
            range: StaticSortedFileRange,
            size: u64,
            flags: MetaEntryFlags,
        }

        impl Compactable for SstWithRange {
            fn range(&self) -> RangeInclusive<u64> {
                self.range.min_hash..=self.range.max_hash
            }

            fn size(&self) -> u64 {
                self.size
            }

            fn category(&self) -> u8 {
                // Cold and non-cold files are placed separately so we pass different category
                // values to ensure they are not merged together.
                if self.flags.cold() { 1 } else { 0 }
            }
        }

        let ssts_with_ranges = meta_files
            .iter()
            .enumerate()
            .flat_map(|(meta_index, meta)| {
                meta.entries()
                    .iter()
                    .enumerate()
                    .map(move |(index_in_meta, entry)| SstWithRange {
                        meta_index,
                        index_in_meta: index_in_meta as u32,
                        seq: entry.sequence_number(),
                        range: entry.range(),
                        size: entry.size(),
                        flags: entry.flags(),
                    })
            })
            .collect::<Vec<_>>();

        let mut sst_by_family = [(); FAMILIES].map(|_| Vec::new());

        for sst in ssts_with_ranges {
            sst_by_family[sst.range.family as usize].push(sst);
        }

        let path = &self.path;

        let log_mutex = Mutex::new(());

        struct PartialResultPerFamily {
            new_meta_file: Option<(u32, File)>,
            new_sst_files: Vec<(u32, File)>,
            sst_seq_numbers_to_delete: Vec<u32>,
            blob_seq_numbers_to_delete: Vec<u32>,
            keys_written: u64,
        }

        let mut compact_config = compact_config.clone();
        let merge_jobs = sst_by_family
            .into_iter()
            .enumerate()
            .filter_map(|(family, ssts_with_ranges)| {
                if compact_config.max_merge_segment_count == 0 {
                    return None;
                }
                let (merge_jobs, real_merge_job_size) =
                    get_merge_segments(&ssts_with_ranges, &compact_config);
                compact_config.max_merge_segment_count -= real_merge_job_size;
                Some((family, ssts_with_ranges, merge_jobs))
            })
            .collect::<Vec<_>>();

        let result = self
            .parallel_scheduler
            .parallel_map_collect_owned::<_, _, Result<Vec<_>>>(
                merge_jobs,
                |(family, ssts_with_ranges, merge_jobs)| {
                    let family = family as u32;

                    if merge_jobs.is_empty() {
                        return Ok(PartialResultPerFamily {
                            new_meta_file: None,
                            new_sst_files: Vec::new(),
                            sst_seq_numbers_to_delete: Vec::new(),
                            blob_seq_numbers_to_delete: Vec::new(),
                            keys_written: 0,
                        });
                    }

                    // Deserialize and merge used key hash filters per-family into
                    // a single filter. This avoids O(entries × N) filter probes
                    // during the merge loop. Empty filters (from commits with no
                    // reads) are discarded.
                    let used_key_hashes: Option<qfilter::Filter> = {
                        let filters: Vec<qfilter::FilterRef<'_>> = meta_files
                            .iter()
                            .filter(|m| m.family() == family)
                            .filter_map(|meta_file| {
                                meta_file.deserialize_used_key_hashes_amqf().transpose()
                            })
                            .collect::<Result<Vec<_>>>()?
                            .into_iter()
                            .filter(|amqf| !amqf.is_empty())
                            .collect();
                        if filters.is_empty() {
                            None
                        } else if filters.len() == 1 {
                            // Just directly use the single item
                            Some(filters[0].to_owned())
                        } else {
                            let total_len: u64 = filters.iter().map(|f| f.len()).sum();
                            // Fingerprint size must match the source filters to
                            // enable the efficient sorted merge path in qfilter.
                            let mut merged =
                                qfilter::Filter::with_fingerprint_size(total_len, u64::BITS as u8)
                                    .expect("Failed to create merged AMQF filter");
                            for filter in &filters {
                                merged
                                    .merge(false, filter)
                                    .expect("Failed to merge AMQF filters");
                            }
                            merged.shrink_to_fit();
                            Some(merged)
                        }
                    };

                    // Later we will remove the merged files
                    let sst_seq_numbers_to_delete = merge_jobs
                        .iter()
                        .filter(|l| l.len() > 1)
                        .flat_map(|l| l.iter().copied())
                        .map(|index| ssts_with_ranges[index].seq)
                        .collect::<Vec<_>>();

                    // Merge SST files
                    let span = tracing::trace_span!(
                        "merge files",
                        family = self.config.family_configs[family as usize].name
                    );
                    enum PartialMergeResult<'l> {
                        Merged {
                            new_sst_files: Vec<(u32, File, StaticSortedFileBuilderMeta<'static>)>,
                            blob_seq_numbers_to_delete: Vec<u32>,
                            keys_written: u64,
                            indices: SmallVec<[usize; 1]>,
                        },
                        Move {
                            seq: u32,
                            meta: StaticSortedFileBuilderMeta<'l>,
                        },
                    }
                    let merge_result = self
                        .parallel_scheduler
                        .parallel_map_collect_owned::<_, _, Result<Vec<_>>>(merge_jobs, |indices| {
                            let _span = span.clone().entered();
                            if indices.len() == 1 {
                                // If we only have one file, we can just move it
                                let index = indices[0];
                                let meta_index = ssts_with_ranges[index].meta_index;
                                let index_in_meta = ssts_with_ranges[index].index_in_meta;
                                let meta_file = &meta_files[meta_index];
                                let entry = meta_file.entry(index_in_meta);
                                let amqf = Cow::Borrowed(entry.raw_amqf(meta_file.amqf_data()));
                                let meta = StaticSortedFileBuilderMeta {
                                    min_hash: entry.min_hash(),
                                    max_hash: entry.max_hash(),
                                    amqf,
                                    block_count: entry.block_count(),
                                    size: entry.size(),
                                    flags: entry.flags(),
                                    entries: 0,
                                };
                                return Ok(PartialMergeResult::Move {
                                    seq: entry.sequence_number(),
                                    meta,
                                });
                            }

                            // Open SST files independently for compaction.
                            // Uses MADV_SEQUENTIAL for better OS page management
                            // and avoids caching mmaps on MetaEntry's OnceLock.
                            let iters = indices
                                .iter()
                                .map(|&index| {
                                    let meta_index = ssts_with_ranges[index].meta_index;
                                    let index_in_meta = ssts_with_ranges[index].index_in_meta;
                                    let entry = meta_files[meta_index].entry(index_in_meta);
                                    StaticSortedFileIter::open(path, entry.sst_metadata())
                                })
                                .collect::<Result<Vec<_>>>()?;

                            let iter = MergeIter::new(iters.into_iter())?;

                            let mut blob_seq_numbers_to_delete: Vec<u32> = Vec::new();

                            struct Collector {
                                /// The active writer and its sequence number. `None` if no
                                /// entries have been added since the last flush. We defer
                                /// allocation to avoid creating empty SST files for collectors
                                /// that receive no entries (e.g., the unused_collector when
                                /// all keys are in the
                                /// used set).
                                writer: Option<(u32, StreamingSstWriter<LookupEntry>)>,
                                flags: MetaEntryFlags,
                                new_sst_files:
                                    Vec<(u32, File, StaticSortedFileBuilderMeta<'static>)>,
                                /// Hash of the last key added. Used to ensure we only split
                                /// SST files at key boundaries (not mid-key-group for MultiValue).
                                last_hash: Option<u64>,
                            }
                            impl Collector {
                                fn new(flags: MetaEntryFlags) -> Self {
                                    Self {
                                        writer: None,
                                        flags,
                                        new_sst_files: Vec::new(),
                                        last_hash: None,
                                    }
                                }

                                /// Ensures a writer is open, creating one if needed.
                                fn ensure_writer(
                                    &mut self,
                                    path: &Path,
                                    sequence_number: &AtomicU32,
                                ) -> Result<&mut StreamingSstWriter<LookupEntry>>
                                {
                                    if self.writer.is_none() {
                                        let seq =
                                            sequence_number.fetch_add(1, Ordering::SeqCst) + 1;
                                        let sst_path = path.join(format!("{seq:08}.sst"));
                                        let writer = StreamingSstWriter::new(
                                            &sst_path,
                                            self.flags,
                                            MAX_ENTRIES_PER_COMPACTED_FILE as u64,
                                        )?;
                                        self.writer = Some((seq, writer));
                                    }
                                    Ok(&mut self.writer.as_mut().unwrap().1)
                                }

                                /// Closes the current SST file (flushing remaining blocks and
                                /// writing the index) and records it in the completed files
                                /// list.
                                fn close_sst_file(&mut self, keys_written: &mut u64) -> Result<()> {
                                    if let Some((seq, writer)) = self.writer.take() {
                                        let _span =
                                            tracing::trace_span!("close merged sst file").entered();
                                        let (meta, file) = writer.close()?;
                                        *keys_written += meta.entries;
                                        self.new_sst_files.push((seq, file, meta));
                                    }
                                    Ok(())
                                }

                                /// Adds an entry to the collector. Only splits the SST file at
                                /// key boundaries to avoid breaking key groups for MultiValue
                                /// families.
                                fn add_entry(
                                    &mut self,
                                    entry: LookupEntry,
                                    path: &Path,
                                    sequence_number: &AtomicU32,
                                    keys_written: &mut u64,
                                ) -> Result<()> {
                                    let key_changed = self.last_hash != Some(entry.hash);
                                    // Only check fullness at key boundaries to avoid splitting
                                    // a key group across two SST files.
                                    if key_changed
                                        && let Some((_, ref writer)) = self.writer
                                        && writer.is_full(
                                            MAX_ENTRIES_PER_COMPACTED_FILE,
                                            DATA_THRESHOLD_PER_COMPACTED_FILE,
                                        )
                                    {
                                        self.close_sst_file(keys_written)?;
                                    }
                                    self.last_hash = Some(entry.hash);
                                    let writer = self.ensure_writer(path, sequence_number)?;
                                    writer.add(entry)?;
                                    Ok(())
                                }
                            }
                            #[cfg(debug_assertions)]
                            impl Drop for Collector {
                                fn drop(&mut self) {
                                    if !std::thread::panicking() {
                                        assert!(
                                            self.writer.is_none(),
                                            "Collector dropped with an open writer"
                                        );
                                    }
                                }
                            }
                            let mut used_collector = Collector::new(MetaEntryFlags::WARM);
                            let mut unused_collector = Collector::new(MetaEntryFlags::COLD);
                            let mut current_key: Option<RcBytes> = None;
                            let mut keys_written = 0;

                            // MergeIter yields entries from newer SSTs first (by SST sequence
                            // number). Within each SST, tombstones sort last within key groups.
                            // Use a skip flag to handle:
                            // - SingleValue: skip all older entries after writing the first
                            // - MultiValue: skip all older entries after encountering a tombstone
                            //   (which signals deletion of all prior values for this key)
                            let mut skip_remaining_for_this_key = false;
                            let family_config = &self.config.family_configs[family as usize];

                            for entry in iter {
                                let entry = entry?;
                                if current_key.as_ref() != Some(&entry.key) {
                                    // we changed keys so undo this flag
                                    skip_remaining_for_this_key = false;
                                    current_key = Some(entry.key.clone());
                                }
                                if !skip_remaining_for_this_key {
                                    let is_used = used_key_hashes
                                        .as_ref()
                                        .is_some_and(|amqf| amqf.contains_fingerprint(entry.hash));
                                    let collector = if is_used {
                                        &mut used_collector
                                    } else {
                                        &mut unused_collector
                                    };
                                    match family_config.kind {
                                        FamilyKind::MultiValue => {
                                            // For MultiValue families we only skip remaining if we
                                            // see a tombstone
                                            if matches!(entry.value, IterValue::Deleted) {
                                                skip_remaining_for_this_key = true;
                                            }
                                        }
                                        FamilyKind::SingleValue => {
                                            // Since MergeItr is in newest to oldest order anything
                                            // else that comes out must be skipped
                                            skip_remaining_for_this_key = true;
                                        }
                                    }
                                    collector.add_entry(
                                        entry,
                                        path,
                                        sequence_number,
                                        &mut keys_written,
                                    )?;
                                } else {
                                    // Entry is being dropped (superseded by newer entry or
                                    // pruned by tombstone). If it references a blob file,
                                    // mark that blob for deletion.
                                    if let IterValue::Blob { sequence_number } = &entry.value {
                                        blob_seq_numbers_to_delete.push(*sequence_number);
                                    }
                                }
                            }

                            // Close remaining writers
                            used_collector.close_sst_file(&mut keys_written)?;
                            unused_collector.close_sst_file(&mut keys_written)?;

                            let mut new_sst_files = take(&mut unused_collector.new_sst_files);
                            new_sst_files.append(&mut used_collector.new_sst_files);
                            Ok(PartialMergeResult::Merged {
                                new_sst_files,
                                blob_seq_numbers_to_delete,
                                keys_written,
                                indices,
                            })
                        })
                        .with_context(|| {
                            format!("Failed to merge database files for family {family}")
                        })?;

                    let Some((sst_files_len, blob_delete_len)) = merge_result
                        .iter()
                        .map(|r| {
                            if let PartialMergeResult::Merged {
                                new_sst_files,
                                blob_seq_numbers_to_delete,
                                indices: _,
                                keys_written: _,
                            } = r
                            {
                                (new_sst_files.len(), blob_seq_numbers_to_delete.len())
                            } else {
                                (0, 0)
                            }
                        })
                        .reduce(|(a1, a2), (b1, b2)| (a1 + b1, a2 + b2))
                    else {
                        unreachable!()
                    };

                    let mut new_sst_files = Vec::with_capacity(sst_files_len);
                    let mut blob_seq_numbers_to_delete = Vec::with_capacity(blob_delete_len);

                    let meta_seq = sequence_number.fetch_add(1, Ordering::SeqCst) + 1;
                    let mut meta_file_builder = MetaFileBuilder::new(family);

                    let mut keys_written = 0;
                    self.parallel_scheduler.block_in_place(|| {
                        let guard = log_mutex.lock();
                        let mut log = self.open_log()?;
                        writeln!(log, "{family:3} | {meta_seq:08} | Compaction:",)?;
                        for result in merge_result {
                            match result {
                                PartialMergeResult::Merged {
                                    new_sst_files: merged_new_sst_files,
                                    blob_seq_numbers_to_delete: merged_blob_seq_numbers_to_delete,
                                    keys_written: merged_keys_written,
                                    indices,
                                } => {
                                    writeln!(
                                        log,
                                        "{family:3} | {meta_seq:08} | MERGE \
                                         ({merged_keys_written} keys):"
                                    )?;
                                    for i in indices.iter() {
                                        let seq = ssts_with_ranges[*i].seq;
                                        let (min, max) = ssts_with_ranges[*i].range().into_inner();
                                        writeln!(
                                            log,
                                            "{family:3} | {meta_seq:08} | {seq:08} INPUT  | {}",
                                            range_to_str(min, max)
                                        )?;
                                    }
                                    for (seq, file, meta) in merged_new_sst_files {
                                        let min = meta.min_hash;
                                        let max = meta.max_hash;
                                        writeln!(
                                            log,
                                            "{family:3} | {meta_seq:08} | {seq:08} OUTPUT | {} \
                                             ({})",
                                            range_to_str(min, max),
                                            meta.flags
                                        )?;

                                        meta_file_builder.add(seq, meta);
                                        new_sst_files.push((seq, file));
                                    }
                                    blob_seq_numbers_to_delete
                                        .extend(merged_blob_seq_numbers_to_delete);
                                    keys_written += merged_keys_written;
                                }
                                PartialMergeResult::Move { seq, meta } => {
                                    let min = meta.min_hash;
                                    let max = meta.max_hash;
                                    writeln!(
                                        log,
                                        "{family:3} | {meta_seq:08} | {seq:08} MOVED  | {}",
                                        range_to_str(min, max)
                                    )?;

                                    meta_file_builder.add(seq, meta);
                                }
                            }
                        }
                        drop(log);
                        drop(guard);

                        anyhow::Ok(())
                    })?;

                    for &seq in sst_seq_numbers_to_delete.iter() {
                        meta_file_builder.add_obsolete_sst_file(seq);
                    }

                    let meta_file = {
                        let _span = tracing::trace_span!("write meta file").entered();
                        self.parallel_scheduler
                            .block_in_place(|| meta_file_builder.write(&self.path, meta_seq))?
                    };

                    Ok(PartialResultPerFamily {
                        new_meta_file: Some((meta_seq, meta_file)),
                        new_sst_files,
                        sst_seq_numbers_to_delete,
                        blob_seq_numbers_to_delete,
                        keys_written,
                    })
                },
            )?;

        for PartialResultPerFamily {
            new_meta_file: inner_new_meta_file,
            new_sst_files: mut inner_new_sst_files,
            sst_seq_numbers_to_delete: mut inner_sst_seq_numbers_to_delete,
            blob_seq_numbers_to_delete: mut inner_blob_seq_numbers_to_delete,
            keys_written: inner_keys_written,
        } in result
        {
            new_meta_files.extend(inner_new_meta_file);
            new_sst_files.append(&mut inner_new_sst_files);
            sst_seq_numbers_to_delete.append(&mut inner_sst_seq_numbers_to_delete);
            blob_seq_numbers_to_delete.append(&mut inner_blob_seq_numbers_to_delete);
            *keys_written += inner_keys_written;
        }

        Ok(())
    }

    /// Get a value from the database. Returns None if the key is not found. The returned value
    /// might hold onto a block of the database and it should not be hold long-term.
    pub fn get<K: QueryKey>(&self, family: usize, key: &K) -> Result<Option<ArcBytes>> {
        debug_assert!(family < FAMILIES, "Family index out of bounds");
        if self.config.family_configs[family].kind != FamilyKind::SingleValue {
            // This is an error in our caller so just panic
            panic!(
                "only single valued tables can be queried with `get', call `get_multiple` instead"
            )
        }
        let span = tracing::trace_span!(
            "database read",
            name = self.config.family_configs[family].name,
            result_size = tracing::field::Empty
        )
        .entered();
        let results = self.get_impl::<K, false>(family, key, &span)?;
        debug_assert!(results.len() <= 1, "get() should return at most one result");
        Ok(results.into_iter().next())
    }

    /// Looks up a key and returns all matching values.
    ///
    /// This is useful for keyspaces where keys are not unique and multiple mappings are possible.
    /// Unlike `get`, which returns only the first match, this method returns all
    /// entries with the same key from all SST files.  By default however we assume these
    /// collections are small and thus optimize for there being exactly 0 or 1 results.
    ///
    /// The order of returned values is undefined and duplicates are preserved. Callers must not
    /// rely on any particular ordering (neither insertion order nor byte order).
    pub fn get_multiple<K: QueryKey>(
        &self,
        family: usize,
        key: &K,
    ) -> Result<SmallVec<[ArcBytes; 1]>> {
        debug_assert!(family < FAMILIES, "Family index out of bounds");
        if self.config.family_configs[family].kind != FamilyKind::MultiValue {
            // This is an error in our caller so just panic
            panic!("only multi-valued tables can be queried with `get_multiple`")
        }
        let span = tracing::trace_span!(
            "database read multiple",
            name = self.config.family_configs[family].name,
            result_count = tracing::field::Empty,
            result_size = tracing::field::Empty
        )
        .entered();
        let results = self.get_impl::<K, true>(family, key, &span)?;
        Ok(results)
    }

    /// Shared implementation for `get` and `get_multiple`.
    ///
    /// If `FIND_ALL` is false, stops after finding the first match.
    /// If `FIND_ALL` is true, continues to find all matches across all meta files.
    fn get_impl<K: QueryKey, const FIND_ALL: bool>(
        &self,
        family: usize,
        key: &K,
        span: &EnteredSpan,
    ) -> Result<SmallVec<[ArcBytes; 1]>> {
        let hash = hash_key(key);
        let inner = self.inner.read();
        let mut output: SmallVec<[ArcBytes; 1]> = SmallVec::new();
        // Track whether we found the key in any SST (even if deleted).
        // Used for miss_global stat: only fires if key was never found anywhere.
        #[cfg(feature = "stats")]
        let mut found_in_sst = false;

        let mut size = 0;

        for meta in inner.meta_files.iter().rev() {
            match meta.lookup::<K, FIND_ALL>(
                family as u32,
                hash,
                key,
                self.key_block_cache(),
                self.value_block_cache(),
            )? {
                MetaLookupResult::FamilyMiss => {
                    #[cfg(feature = "stats")]
                    self.stats.miss_family.fetch_add(1, Ordering::Relaxed);
                }
                MetaLookupResult::RangeMiss => {
                    #[cfg(feature = "stats")]
                    self.stats.miss_range.fetch_add(1, Ordering::Relaxed);
                }
                MetaLookupResult::QuickFilterMiss => {
                    #[cfg(feature = "stats")]
                    self.stats.miss_amqf.fetch_add(1, Ordering::Relaxed);
                }
                MetaLookupResult::SstLookup(result) => match result {
                    SstLookupResult::Found(values) => {
                        #[cfg(feature = "stats")]
                        {
                            found_in_sst = true;
                        }
                        inner.accessed_key_hashes[family].insert(hash);
                        // Process values. Tombstones sort last within a key group,
                        // so when we see a tombstone, we can return immediately.
                        for value in values {
                            match value {
                                LookupValue::Deleted => {
                                    #[cfg(feature = "stats")]
                                    self.stats.hits_deleted.fetch_add(1, Ordering::Relaxed);
                                    if !FIND_ALL {
                                        span.record("result_size", "deleted");
                                        return Ok(SmallVec::new());
                                    }
                                    // Tombstone is last in key group. Return accumulated
                                    // values (from this SST and newer layers). Stop
                                    // searching older SSTs.
                                    if output.is_empty() {
                                        span.record("result_size", "deleted");
                                    } else {
                                        span.record("result_size", size);
                                    }
                                    return Ok(output);
                                }
                                LookupValue::Slice { value } => {
                                    #[cfg(feature = "stats")]
                                    self.stats.hits_small.fetch_add(1, Ordering::Relaxed);
                                    if !FIND_ALL {
                                        span.record("result_size", value.len());
                                        return Ok(SmallVec::from_buf([value]));
                                    }
                                    size += value.len();
                                    output.push(value);
                                }
                                LookupValue::Blob { sequence_number } => {
                                    #[cfg(feature = "stats")]
                                    self.stats.hits_blob.fetch_add(1, Ordering::Relaxed);
                                    let blob = self.read_blob(sequence_number)?;
                                    if !FIND_ALL {
                                        span.record("result_size", blob.len());
                                        return Ok(SmallVec::from_buf([blob]));
                                    }
                                    size += blob.len();
                                    output.push(blob);
                                }
                            }
                        }
                    }
                    SstLookupResult::NotFound => {
                        #[cfg(feature = "stats")]
                        self.stats.miss_key.fetch_add(1, Ordering::Relaxed);
                    }
                },
            }
        }

        #[cfg(feature = "stats")]
        if !found_in_sst {
            self.stats.miss_global.fetch_add(1, Ordering::Relaxed);
        }

        if FIND_ALL {
            span.record("result_count", output.len());
        }
        if output.is_empty() {
            span.record("result_size", "not_found");
        } else {
            span.record("result_size", size);
        }
        Ok(output)
    }

    pub fn batch_get<K: QueryKey>(
        &self,
        family: usize,
        keys: &[K],
    ) -> Result<Vec<Option<ArcBytes>>> {
        debug_assert!(family < FAMILIES, "Family index out of bounds");
        if self.config.family_configs[family].kind != FamilyKind::SingleValue {
            // This is an error in our caller so just panic
            panic!("only single valued tables can be queried with `batch_get'")
        }
        let span = tracing::trace_span!(
            "database batch read",
            name = self.config.family_configs[family].name,
            keys = keys.len(),
            not_found = tracing::field::Empty,
            deleted = tracing::field::Empty,
            result_size = tracing::field::Empty
        )
        .entered();
        let mut cells: Vec<(u64, usize, Option<LookupValue>)> = Vec::with_capacity(keys.len());
        let mut empty_cells = keys.len();
        for (index, key) in keys.iter().enumerate() {
            let hash = hash_key(key);
            cells.push((hash, index, None));
        }
        cells.sort_by_key(|(hash, _, _)| *hash);
        let inner = self.inner.read();
        for meta in inner.meta_files.iter().rev() {
            let _result = meta.batch_lookup(
                family as u32,
                keys,
                &mut cells,
                &mut empty_cells,
                self.key_block_cache(),
                self.value_block_cache(),
            )?;

            #[cfg(feature = "stats")]
            {
                let crate::meta_file::MetaBatchLookupResult {
                    family_miss,
                    range_misses,
                    quick_filter_misses,
                    sst_misses,
                    hits: _,
                } = _result;
                if family_miss {
                    self.stats.miss_family.fetch_add(1, Ordering::Relaxed);
                }
                if range_misses > 0 {
                    self.stats
                        .miss_range
                        .fetch_add(range_misses as u64, Ordering::Relaxed);
                }
                if quick_filter_misses > 0 {
                    self.stats
                        .miss_amqf
                        .fetch_add(quick_filter_misses as u64, Ordering::Relaxed);
                }
                if sst_misses > 0 {
                    self.stats
                        .miss_key
                        .fetch_add(sst_misses as u64, Ordering::Relaxed);
                }
            }

            if empty_cells == 0 {
                break;
            }
        }
        let mut deleted = 0;
        let mut not_found = 0;
        let mut result_size = 0;
        let mut results = vec![None; keys.len()];
        for (hash, index, result) in cells {
            if let Some(result) = result {
                inner.accessed_key_hashes[family].insert(hash);
                let result = match result {
                    LookupValue::Deleted => {
                        #[cfg(feature = "stats")]
                        self.stats.hits_deleted.fetch_add(1, Ordering::Relaxed);
                        deleted += 1;
                        None
                    }
                    LookupValue::Slice { value } => {
                        #[cfg(feature = "stats")]
                        self.stats.hits_small.fetch_add(1, Ordering::Relaxed);
                        result_size += value.len();
                        Some(value)
                    }
                    LookupValue::Blob { sequence_number } => {
                        #[cfg(feature = "stats")]
                        self.stats.hits_blob.fetch_add(1, Ordering::Relaxed);
                        let blob = self.read_blob(sequence_number)?;
                        result_size += blob.len();
                        Some(blob)
                    }
                };
                results[index] = result;
            } else {
                #[cfg(feature = "stats")]
                self.stats.miss_global.fetch_add(1, Ordering::Relaxed);
                not_found += 1;
            }
        }
        span.record("not_found", not_found);
        span.record("deleted", deleted);
        span.record("result_size", result_size);
        Ok(results)
    }

    /// Returns database statistics.
    #[cfg(feature = "stats")]
    pub fn statistics(&self) -> Statistics {
        let inner = self.inner.read();
        Statistics {
            meta_files: inner.meta_files.len(),
            sst_files: inner.meta_files.iter().map(|m| m.entries().len()).sum(),
            key_block_cache: CacheStatistics::new(self.key_block_cache()),
            value_block_cache: CacheStatistics::new(self.value_block_cache()),
            hits: self.stats.hits_deleted.load(Ordering::Relaxed)
                + self.stats.hits_small.load(Ordering::Relaxed)
                + self.stats.hits_blob.load(Ordering::Relaxed),
            misses: self.stats.miss_global.load(Ordering::Relaxed),
            miss_family: self.stats.miss_family.load(Ordering::Relaxed),
            miss_range: self.stats.miss_range.load(Ordering::Relaxed),
            miss_amqf: self.stats.miss_amqf.load(Ordering::Relaxed),
            miss_key: self.stats.miss_key.load(Ordering::Relaxed),
        }
    }

    pub fn meta_info(&self) -> Result<Vec<MetaFileInfo>> {
        Ok(self
            .inner
            .read()
            .meta_files
            .iter()
            .rev()
            .map(|meta_file| {
                let entries = meta_file
                    .entries()
                    .iter()
                    .map(|entry| {
                        let amqf = entry.raw_amqf(meta_file.amqf_data());
                        MetaFileEntryInfo {
                            sequence_number: entry.sequence_number(),
                            min_hash: entry.min_hash(),
                            max_hash: entry.max_hash(),
                            sst_size: entry.size(),
                            flags: entry.flags(),
                            amqf_size: entry.amqf_size(),
                            amqf_entries: amqf.len(),
                            block_count: entry.block_count(),
                        }
                    })
                    .collect();
                MetaFileInfo {
                    sequence_number: meta_file.sequence_number(),
                    family: meta_file.family(),
                    obsolete_sst_files: meta_file.obsolete_sst_files().to_vec(),
                    entries,
                }
            })
            .collect())
    }

    /// Shuts down the database. This will print statistics if the `print_stats` feature is enabled.
    /// Retries deletion of all previously-deferred files and clears successfully deleted batches.
    pub fn shutdown(&self) -> Result<()> {
        #[cfg(feature = "print_stats")]
        println!("{:#?}", self.statistics());
        self.retry_deferred_deletions();
        Ok(())
    }

    /// Attempts to delete files with the given extension, returning an iterator of sequence
    /// numbers for files that could not be deleted (e.g. due to open memory maps on Windows).
    fn try_delete_files<'a>(
        dir: &'a Path,
        seqs: &'a [u32],
        ext: &'a str,
    ) -> impl Iterator<Item = u32> + 'a {
        seqs.iter()
            .copied()
            .filter(move |&seq| fs::remove_file(dir.join(format!("{seq:08}.{ext}"))).is_err())
    }

    /// Retries deletion of files that previously failed (typically due to open memory maps on
    /// Windows). Any file that still fails is kept for the next retry.
    /// Best-effort: persistent failures are acceptable because `load_directory` cleans up
    /// any leftover files on the next open via the `.del` file.
    fn retry_deferred_deletions(&self) {
        let mut deferred = self.deferred_deletions.lock();
        deferred.retain(|entry| {
            let (seq, ext) = match *entry {
                DeferredDeletion::Sst(seq) => (seq, "sst"),
                DeferredDeletion::Meta(seq) => (seq, "meta"),
                DeferredDeletion::Blob(seq) => (seq, "blob"),
            };
            // Keep the entry only if deletion still fails.
            fs::remove_file(self.path.join(format!("{seq:08}.{ext}"))).is_err()
        });
    }
}

fn range_to_str(min: u64, max: u64) -> String {
    use std::fmt::Write;
    const DISPLAY_SIZE: usize = 100;
    const TOTAL_SIZE: u64 = u64::MAX;
    let start_pos = (min as u128 * DISPLAY_SIZE as u128 / TOTAL_SIZE as u128) as usize;
    let end_pos = (max as u128 * DISPLAY_SIZE as u128 / TOTAL_SIZE as u128) as usize;
    let mut range_str = String::new();
    for i in 0..DISPLAY_SIZE {
        if i == start_pos && i == end_pos {
            range_str.push('O');
        } else if i == start_pos {
            range_str.push('[');
        } else if i == end_pos {
            range_str.push(']');
        } else if i > start_pos && i < end_pos {
            range_str.push('=');
        } else {
            range_str.push(' ');
        }
    }
    write!(range_str, " | {min:016x}-{max:016x}").unwrap();
    range_str
}

pub struct MetaFileInfo {
    pub sequence_number: u32,
    pub family: u32,
    pub obsolete_sst_files: Vec<u32>,
    pub entries: Vec<MetaFileEntryInfo>,
}

pub struct MetaFileEntryInfo {
    pub sequence_number: u32,
    pub min_hash: u64,
    pub max_hash: u64,
    pub amqf_size: u32,
    pub amqf_entries: usize,
    pub sst_size: u64,
    pub flags: MetaEntryFlags,
    pub block_count: u16,
}