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pageserver: drop layer map lock in Timeline::get (#7217)

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## Problem
We currently hold the layer map read lock while doing IO on the read
path. This is not required for correctness.

## Summary of changes
Drop the layer map lock after figuring out which layer we wish to read
from.
Why is this correct:
* `Layer` models the lifecycle of an on disk layer. In the event the
layer is removed from local disk, it will be on demand downloaded
* `InMemoryLayer` holds the `EphemeralFile` which wraps the on disk
file. As long as the `InMemoryLayer` is in scope, it's safe to read from it.

Related https://github.com/neondatabase/neon/issues/6833
This commit is contained in:
Vlad Lazar
2024-03-26 14:35:36 +00:00
committed by GitHub
parent ad072de420
commit de03742ca3
1 changed files with 12 additions and 8 deletions
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20
pageserver/src/tenant/timeline.rs
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@@ -2587,6 +2587,10 @@ impl Timeline {
// Get all the data needed to reconstruct the page version from this layer.
// But if we have an older cached page image, no need to go past that.
let lsn_floor = max(cached_lsn + 1, start_lsn);
let open_layer = open_layer.clone();
drop(guard);
result = match open_layer
.get_value_reconstruct_data(
key,
@@ -2604,10 +2608,7 @@ impl Timeline {
traversal_path.push((
result,
cont_lsn,
Box::new({
let open_layer = Arc::clone(open_layer);
move || open_layer.traversal_id()
}),
Box::new(move || open_layer.traversal_id()),
));
continue 'outer;
}
@@ -2617,6 +2618,10 @@ impl Timeline {
if cont_lsn > start_lsn {
//info!("CHECKING for {} at {} on frozen layer {}", key, cont_lsn, frozen_layer.filename().display());
let lsn_floor = max(cached_lsn + 1, start_lsn);
let frozen_layer = frozen_layer.clone();
drop(guard);
result = match frozen_layer
.get_value_reconstruct_data(
key,
@@ -2634,10 +2639,7 @@ impl Timeline {
traversal_path.push((
result,
cont_lsn,
Box::new({
let frozen_layer = Arc::clone(frozen_layer);
move || frozen_layer.traversal_id()
}),
Box::new(move || frozen_layer.traversal_id()),
));
continue 'outer;
}
@@ -2645,6 +2647,8 @@ impl Timeline {
if let Some(SearchResult { lsn_floor, layer }) = layers.search(key, cont_lsn) {
let layer = guard.get_from_desc(&layer);
drop(guard);
// Get all the data needed to reconstruct the page version from this layer.
// But if we have an older cached page image, no need to go past that.
let lsn_floor = max(cached_lsn + 1, lsn_floor);