Hyperfile Cleaner

Segments cleaner for Hyperfile.

What it does

Hyperfile is a copy-on-write log-structured file format on S3. Each flush produces a new segment object. Over time, older segments lose references as the metadata tree gets rewritten. This crate enumerates and (optionally) deletes segment objects that are no longer referenced by the latest published checkpoint.

Two operations on the [Cleaner] trait:

• collect(low, high) — dry-run inspection. Walks the metadata tree, returns (referenced, unreferenced) lists of segment objects.
• prune(low, high) — collect + delete the unreferenced list via S3 DeleteObjects.

low and high are checkpoint numbers (Cno), inclusive at both ends; high == 0 is a sentinel for "use the latest checkpoint".

What is reported and deleted is a SegmentId, which names a stored object. Under BlockPtrFormat::PartedSegment — the default since hyperfile 0.7.0 — one checkpoint may be several objects, 7.0, 7.1, … — and they are reclaimed separately, because a checkpoint abandoned partway leaves its partials behind under the same number the retry then reuses. Under every other format an object is a checkpoint and prints as the plain number.

Compaction

prune reclaims an object nothing points into. An object that is 5% live is not that object: rewriting those few blocks elsewhere is what turns it into one prune can take. The same rewrite is what makes a read cheaper, since a request breaks where the object changes.

Two more operations, on S3Cleaner itself:

• report(low, high) — read-only. Per-object liveness and what could be given back: dead() is prune's work, compactable(ratio) estimates what a rewrite would cost and free.
• compact(low, high, &opts) — rewrites the live blocks of objects at or below max_live_ratio, as one checkpoint. Deletes nothing; prune does that afterwards.

use hyperfile_cleaner::{Cleaner, compact::CompactOptions, s3::S3Cleaner};

# async fn run() -> std::io::Result<()> {
let cleaner = S3Cleaner::new("s3://my-bucket/path/to/file").await?;

let report = cleaner.report(1, 0).await?;
let (objects, rewrite, freed) = report.compactable(0.5);
println!("{objects} objects: rewrite {rewrite} bytes to free {freed}");

let out = cleaner.compact(1, 0, &CompactOptions::default()).await?;
assert!(!out.content_moved());
cleaner.prune(1, 0).await?;
# Ok(())
# }

The rewrite is hyperfile's, not this crate's: fs_relocate marks the blocks to be rewritten with their current bytes, reading them with the read path's own coalescing, and one flush writes them sorted by block index and packed. Holes and explicit zero blocks are left alone, so a sparse file stays sparse, and neither mtime nor ctime moves.

A compaction is not a change of content. It has to publish a new checkpoint, so last_cno advances by one. hyperfile marks that checkpoint FLAG_COMPACTED, and fs_content_cno() stays where it was: that is the number to compare when asking whether a file changed, not last_cno. The mark holds for one checkpoint, so two passes in a row would move content_cno on the second; compact refuses to start on a checkpoint that is already marked unless allow_consecutive is set. CompactOutcome::content_moved() says whether consumers will see the pass as a change — they will when an earlier flush failed and gave its number up, which the mark does not span.

Compaction requires exclusive write access. It is a writer, and it writes bytes it read earlier, so it runs with FlushConflictPolicy::FailFast and a concurrent publish makes the pass fail rather than overwrite the other writer's newer data. It also refuses to run when the container's prefix holds keys that are neither the inode nor segment objects, because a WAL is the usual reason and a compactor that does not read the log would publish over entries recovery has not replayed.

Usage

use hyperfile_cleaner::{Cleaner, s3::S3Cleaner};

# async fn run() -> std::io::Result<()> {
let cleaner = S3Cleaner::new("s3://my-bucket/path/to/file").await?;

// Dry-run: see what would be deleted.
let (refed, unrefed) = cleaner.collect(1, 0).await?;
println!("referenced segments  : {:?}", refed);
println!("unreferenced segments: {:?}", unrefed);

// Actually delete.
cleaner.prune(1, 0).await?;
# Ok(())
# }

For tools that already have a configured aws_sdk_s3::Client, use S3Cleaner::with_client(&client, root_uri) instead of new.

Limitations

• The cleaner does not coordinate with in-flight writers. It snapshots the inode at the start of the call. New segments published after that point are outside the configured range and remain untouched, as are the partials of a checkpoint a writer has open — those carry the number above the published one.
• Compaction is offline: it needs exclusive write access for the pass, and a container holding a WAL is refused unless the caller overrides it. Compacting a container a writer is actively publishing to needs a background task inside hyperfile, not a separate process.
• A pass holds everything it relocates dirty in memory until its one flush, bounded by max_pass_bytes (512 MiB by default). A container with more to rewrite takes several passes, and each pass after the first in a row moves content_cno once.
• An inline file or a device node has no segments; collect, prune, report and compact refuse them.
• An object holding a live metadata node is not a compaction candidate. A node is relocated when the flush rewrites it, which needs something under it dirtied, and which keys a node covers is not visible from here.

Status

:warning: Under development. Not recommended for production.

License

Apache-2.0.

Contributing

PRs welcome.