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create_delta_layer: improve misleading TODO comment (#4488)

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Context:
https://github.com/neondatabase/neon/pull/4441#discussion_r1228086608
This commit is contained in:
Christian Schwarz
2023-06-16 13:23:55 +02:00
committed by GitHub
parent 472cc17b7a
commit 14d495ae14
1 changed files with 10 additions and 7 deletions
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17
pageserver/src/tenant/timeline.rs
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@@ -3015,16 +3015,19 @@ impl Timeline {
// Sync it to disk.
//
// We must also fsync the timeline dir to ensure the directory entries for
// new layer files are durable
// new layer files are durable.
//
// NB: timeline dir must be synced _after_ the file contents are durable.
// So, two separate fsyncs are required, they mustn't be batched.
//
// TODO: If we're running inside 'flush_frozen_layers' and there are multiple
// files to flush, it might be better to first write them all, and then fsync
// them all in parallel.
// First sync the delta layer. We still use par_fsync here to keep everything consistent. Feel free to replace
// this with a single fsync in future refactors.
// files to flush, the fsync overhead can be reduces as follows:
// 1. write them all to temporary file names
// 2. fsync them
// 3. rename to the final name
// 4. fsync the parent directory.
// Note that (1),(2),(3) today happen inside write_to_disk().
par_fsync::par_fsync(&[new_delta_path.clone()]).context("fsync of delta layer")?;
// Then sync the parent directory.
par_fsync::par_fsync(&[self_clone
.conf
.timeline_path(&self_clone.timeline_id, &self_clone.tenant_id)])