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garbage collection: fix rare race in chunk marking phase

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During phase 1 of garbage collection referenced chunks are marked as
in use by iterating over all index files and updating the atime on
the chunks referenced by these.

In an edge case for long running garbage collection jobs, where a
newly added snapshot (created after the start of GC) reused known
chunks from a previous snapshot, but the previous snapshot index
referencing them disappeared before the marking phase could reach
that index (e.g. pruned because only 1 snapshot to be kept by
retention setting), known chunks from that previous index file might
not be marked (given that by none of the other index files it was
marked).

Since commit 74361da8 ("garbage collection: generate index file list
via datastore iterators") this is even less likely as now the
iteration reads also index files added during phase 1, and
therefore either the new or the previous index file will account for
these chunks (the previous backup snapshot can only be pruned after
the new one finished, since locked). There remains however a small
race window between the reading of the snapshots in the backup group
and the reading of the actual index files for marking.

Fix this race by:
1. Checking if the last snapshot of a group disappeared and if so
2. generate the list again, looking for new index files previously
   not accounted for
3. To avoid possible endless looping, lock the group if the snapshot
   list changed even after the 10th time (which will lead to
   concurrent operations to this group failing).

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
Acked-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Link: https://lore.proxmox.com/20250416105000.270166-3-c.ebner@proxmox.com
This commit is contained in:
Christian Ebner 2025-04-16 12:50:00 +02:00 committed by Thomas Lamprecht
parent 31dbaf69ab
commit cb9814e331
1 changed files with 75 additions and 39 deletions
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114
pbs-datastore/src/datastore.rs
View File

@ -1145,52 +1145,88 @@ impl DataStore {
let namespace = namespace.context("iterating namespaces failed")?;
for group in arc_self.iter_backup_groups(namespace)? {
let group = group.context("iterating backup groups failed")?;
let mut snapshots = group.list_backups().context("listing snapshots failed")?;
// Sort by snapshot timestamp to iterate over consecutive snapshots for each image.
BackupInfo::sort_list(&mut snapshots, true);
for snapshot in snapshots {
for file in snapshot.files {
worker.check_abort()?;
worker.fail_on_shutdown()?;
match ArchiveType::from_path(&file) {
Ok(ArchiveType::FixedIndex) | Ok(ArchiveType::DynamicIndex) => (),
Ok(ArchiveType::Blob) | Err(_) => continue,
}
// Avoid race between listing/marking of snapshots by GC and pruning the last
// snapshot in the group, following a new snapshot creation. Otherwise known chunks
// might only be referenced by the new snapshot, so it must be read as well.
let mut retry_counter = 0;
'retry: loop {
let _lock = match retry_counter {
0..=9 => None,
10 => Some(
group
.lock()
.context("exhausted retries and failed to lock group")?,
),
_ => bail!("exhausted retries and unexpected counter overrun"),
};
let mut path = snapshot.backup_dir.full_path();
path.push(file);
let index = match self.open_index_reader(&path)? {
Some(index) => index,
None => {
unprocessed_index_list.remove(&path);
continue;
let mut snapshots = match group.list_backups() {
Ok(snapshots) => snapshots,
Err(err) => {
if group.exists() {
return Err(err).context("listing snapshots failed")?;
}
};
self.index_mark_used_chunks(
index,
&path,
&mut chunk_lru_cache,
status,
worker,
)?;
if !unprocessed_index_list.remove(&path) {
info!("Encountered new index file '{path:?}', increment total index file count");
index_count += 1;
break 'retry;
}
};
let percentage = (processed_index_files + 1) * 100 / index_count;
if percentage > last_percentage {
info!(
"marked {percentage}% ({} of {index_count} index files)",
processed_index_files + 1,
);
last_percentage = percentage;
// Always start iteration with the last snapshot of the group to reduce race
// window with concurrent backup+prune previous last snapshot. Allows to retry
// without the need to keep track of already processed index files for the
// current group.
BackupInfo::sort_list(&mut snapshots, true);
for (count, snapshot) in snapshots.into_iter().rev().enumerate() {
for file in snapshot.files {
worker.check_abort()?;
worker.fail_on_shutdown()?;
match ArchiveType::from_path(&file) {
Ok(ArchiveType::FixedIndex) | Ok(ArchiveType::DynamicIndex) => (),
Ok(ArchiveType::Blob) | Err(_) => continue,
}
let mut path = snapshot.backup_dir.full_path();
path.push(file);
let index = match self.open_index_reader(&path)? {
Some(index) => index,
None => {
unprocessed_index_list.remove(&path);
if count == 0 {
retry_counter += 1;
continue 'retry;
}
continue;
}
};
self.index_mark_used_chunks(
index,
&path,
&mut chunk_lru_cache,
status,
worker,
)?;
if !unprocessed_index_list.remove(&path) {
info!("Encountered new index file '{path:?}', increment total index file count");
index_count += 1;
}
let percentage = (processed_index_files + 1) * 100 / index_count;
if percentage > last_percentage {
info!(
"marked {percentage}% ({} of {index_count} index files)",
processed_index_files + 1,
);
last_percentage = percentage;
}
processed_index_files += 1;
}
processed_index_files += 1;
}
break;
}
}
}