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Revert "iter12: fallocate without KEEP_SIZE + truncate_at_sync; chunk back to 32 MiB"

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This reverts commit e2c7e20329.
This commit is contained in:
MattJackson
2026-05-17 09:48:35 -07:00
parent e2c7e20329
commit 3394a5b3fe
2 changed files with 22 additions and 50 deletions
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src/io/writeback_file/mod.rs
+8 -36
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@@ -73,23 +73,18 @@ use std::path::Path;
use super::writeback::WritebackPipeline;
/// Granularity at which the Linux writeback pipeline issues
/// `sync_file_range` pairs. 32 MiB is the empirically best value
/// (iter8: 28.7; iter9 64 MiB: 27.5; iter11 128 MiB: 16.6; iter6
/// 8 MiB: 15.8). Locking in.
const WRITEBACK_CHUNK_BYTES: u64 = 32 * 1024 * 1024;
/// `sync_file_range` pairs.
///
/// iter11 (2026-05-17): 32 → 128 MiB. 0.21.14 tried this under Phase
/// 2.5 and reverted; with Phase 2.5 disabled (iter8 baseline) the
/// tradeoff is different. iter8 (32 MiB) = 28.7, iter9 (64 MiB) = 27.5.
/// Trying 128 to see if the iter9 dip was noise or a real trend.
const WRITEBACK_CHUNK_BYTES: u64 = 128 * 1024 * 1024;
pub(crate) struct WritebackFile {
file: File,
pipeline: WritebackPipeline,
pos: u64,
/// Highest position ever reached by `write`/`write_all`. Used by
/// `sync_all` to truncate the file down to the actual content
/// extent if `preallocate` over-reserved.
high_water: u64,
/// True if the file was preallocated AND extended to a hint size
/// at construction. `sync_all` will `ftruncate` to `high_water`
/// when this is set, to discard any over-reservation.
truncate_at_sync: bool,
}
impl WritebackFile {
@@ -104,8 +99,6 @@ impl WritebackFile {
file,
pipeline,
pos,
high_water: pos,
truncate_at_sync: false,
})
}
@@ -138,14 +131,7 @@ impl WritebackFile {
pub(crate) fn create_with_size_hint(path: &Path, size_bytes: u64) -> io::Result<Self> {
let file = File::create(path)?;
platform::preallocate(&file, size_bytes);
let mut wbf = Self::new(file)?;
// `preallocate` (on Linux/macOS where it's implemented) extends
// the file's reported size to `size_bytes`. We mark this so
// `sync_all` will truncate down to actual content extent at
// mux end. If the hint was an underestimate, writes simply
// extend past it as normal.
wbf.truncate_at_sync = true;
Ok(wbf)
Self::new(file)
}
/// Open an existing file at `path` for writing (no truncation) and
@@ -168,14 +154,6 @@ impl WritebackFile {
/// effort, but bounded.
pub(crate) fn sync_all(&mut self) -> io::Result<()> {
self.pipeline.finalize();
if self.truncate_at_sync {
// Truncate down to the actual content extent. If
// `preallocate` extended past the muxer's real output
// size, the tail is otherwise zero-filled garbage.
// `set_len` is ftruncate; safe to call even when
// high_water == current file size (no-op).
self.file.set_len(self.high_water)?;
}
platform::durable_sync(&self.file)
}
}
@@ -184,9 +162,6 @@ impl Write for WritebackFile {
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
let n = self.file.write(buf)?;
self.pos += n as u64;
if self.pos > self.high_water {
self.high_water = self.pos;
}
self.pipeline.note_progress(self.pos);
Ok(n)
}
@@ -194,9 +169,6 @@ impl Write for WritebackFile {
fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
self.file.write_all(buf)?;
self.pos += buf.len() as u64;
if self.pos > self.high_water {
self.high_water = self.pos;
}
self.pipeline.note_progress(self.pos);
Ok(())
}