Re: [pbs-devel] [RFC proxmox-backup 4/4] garbage collection: read pruned snapshot index files from trash

["Fabian Grünbichler" <f.gruenbichler@proxmox.com>]

On April 16, 2025 4:18 pm, Christian Ebner wrote:
> Snapshots pruned during phase 1 are now also assured to be included
> in the marking phase by reading the index files from trash and
> touching these chunks as well.
> 
> Clear any trash before starting with phase 1, so that only snapshots
> pruned during GC are consided.
> 
> Further, drop the retry logic used before to assure eventually newly
> written index files are included in the marking phase, if the
> previously last one was pruned. This is not necessary anymore as the
> previously last one will be read from trash.
> 
> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
> ---
>  pbs-datastore/src/datastore.rs | 141 +++++++++++++++------------------
>  1 file changed, 65 insertions(+), 76 deletions(-)
> 
> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
> index 97b78f000..688e65247 100644
> --- a/pbs-datastore/src/datastore.rs
> +++ b/pbs-datastore/src/datastore.rs
> @@ -1137,7 +1137,13 @@ impl DataStore {
>          //
>          // By this it is assured that all index files are used, even if they would not have been
>          // seen by the regular logic and the user is informed by the garbage collection run about
> -        // the detected index files not following the iterators logic.
> +        // the detected index files not following the iterators logic. Further, include trashed
> +        // snapshots which have been pruned during garbage collections marking phase.
> +
> +        let trash = PathBuf::from(".trash/");
> +        let mut full_trash_path = self.base_path();
> +        full_trash_path.push(&trash);
> +        let _ = std::fs::remove_dir_all(full_trash_path);

I think this would need some locking, else we start recursively deleting
here while at the same time a prune operation is moving something into
the trash..

>  
>          let mut unprocessed_index_list = self.list_index_files(None)?;
>          let mut index_count = unprocessed_index_list.len();
> @@ -1154,88 +1160,63 @@ 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 = match group.list_backups() {
> +                    Ok(snapshots) => snapshots,
> +                    Err(err) => {
> +                        if group.exists() {
> +                            return Err(err).context("listing snapshots failed")?;
> +                        }
> +                        // vanished, will be covered by trashed list below to avoid
> +                        // not touching known chunks.
> +                        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 snapshots = match group.list_backups() {
> -                        Ok(snapshots) => snapshots,
> -                        Err(err) => {
> -                            if group.exists() {
> -                                return Err(err).context("listing snapshots failed")?;
> -                            }
> -                            break 'retry;
> +                BackupInfo::sort_list(&mut snapshots, true);
> +                for snapshot in snapshots.into_iter() {
> +                    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,
>                          }
> -                    };
> -
> -                    // 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 mut path = snapshot.backup_dir.full_path();
> +                        path.push(file);
>  
> -                            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;
> +                        let index = match self.open_index_reader(&path)? {
> +                            Some(index) => index,
> +                            None => {
> +                                unprocessed_index_list.remove(&path);
> +                                continue;
>                              }
> -                            processed_index_files += 1;
> +                        };
> +
> +                        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;
> +                        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;
> +                    }
>                  }
>              }
>          }
> @@ -1257,6 +1238,14 @@ impl DataStore {
>              warn!("Found {strange_paths_count} index files outside of expected directory scheme");
>          }
>  
> +        let trashed_index_list = self.list_index_files(Some(trash))?;
> +        for path in trashed_index_list {
> +            if let Some(index) = self.open_index_reader(&path)? {
> +                info!("Mark chunks for pruned index file found in {path:?}");
> +                self.index_mark_used_chunks(index, &path, &mut chunk_lru_cache, status, worker)?;
> +            };
> +        }
> +

I think we'd want to support undoing moving a snapshot to trash, if we
have a trash can (that's the other half of wanting this feature after
all). if we do so, we need to be careful to not re-introduce a race here
(e.g., by keeping a copy in the trash can when undoing, or by using a
trash can mechanism that doesn't require separate iteration over regular
and trashed snapshots).

>          Ok(())
>      }
>  
> -- 
> 2.39.5
> 
> 
> 
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> 
> 
> 


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