Re: [pbs-devel] [PATCH v2 proxmox-backup 4/4] fix #5331: garbage collection: avoid multiple chunk atime updates

[Christian Ebner <c.ebner@proxmox.com>]

On 3/17/25 15:55, Fabian Grünbichler wrote:
> On March 10, 2025 12:16 pm, Christian Ebner wrote:
>> Reduce the number of atime updates on the same chunk by logically
>> iterating over image index files, following the incremental backup
>> logic. By inserting paths for encountered images during
>> `list_images` using the GroupedImageList structure, the iteration
>> happens now for the same image filenames in the same image namespace
>> and group in a order based on the snapshot timestamp. For each image,
>> keep track of the encountered chunk digests, and remember these as
>> seen for the next snapshot. Chunks which have been encountered in the
>> previous image index, but are not present anymore are removed from
>> the list after each image, in order to reduce memory footprint.
>>
>> Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5331
>> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
> 
> technically this could be split into two parts - one for the iterating
> change, one for the performance optimization? with the latter part maybe
> being combined with patch#3 and the order switched around? ;)

Okay, will reorganize the patches accordingly.

> 
>> ---
>> changes since version 1:
>> - Use pre-existing datastore iterator helpers, following the logic other
>>    datastore operations take.
>> - Chunks are now remembered for all archives per snapshot, not just a
>>    single archive per snapshot as previously, this mimics more closely
>>    the backup behaviour.
>>
>>   pbs-datastore/src/datastore.rs | 117 +++++++++++++++++++++++----------
>>   1 file changed, 84 insertions(+), 33 deletions(-)
>>
>> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
>> index fdbb33a98..a80343d9b 100644
>> --- a/pbs-datastore/src/datastore.rs
>> +++ b/pbs-datastore/src/datastore.rs
>> @@ -25,7 +25,7 @@ use pbs_api_types::{
>>       MaintenanceMode, MaintenanceType, Operation, UPID,
>>   };
>>   
>> -use crate::backup_info::{BackupDir, BackupGroup};
>> +use crate::backup_info::{BackupDir, BackupGroup, BackupInfo};
>>   use crate::chunk_store::ChunkStore;
>>   use crate::dynamic_index::{DynamicIndexReader, DynamicIndexWriter};
>>   use crate::fixed_index::{FixedIndexReader, FixedIndexWriter};
>> @@ -970,10 +970,10 @@ impl DataStore {
>>           ListGroups::new(Arc::clone(self), ns)?.collect()
>>       }
>>   
>> -    fn list_images(&self) -> Result<Vec<PathBuf>, Error> {
>> +    fn list_images(&self) -> Result<HashSet<PathBuf>, Error> {
>>           let base = self.base_path();
>>   
>> -        let mut list = vec![];
>> +        let mut list = HashSet::new();
>>   
>>           use walkdir::WalkDir;
>>   
>> @@ -1021,7 +1021,7 @@ impl DataStore {
>>                   if archive_type == ArchiveType::FixedIndex
>>                       || archive_type == ArchiveType::DynamicIndex
>>                   {
>> -                    list.push(path);
>> +                    list.insert(path);
>>                   }
>>               }
>>           }
>> @@ -1071,6 +1071,7 @@ impl DataStore {
>>           &self,
>>           index: Box<dyn IndexFile>,
>>           file_name: &Path, // only used for error reporting
>> +        touched_chunks: &mut TouchedChunks,
>>           status: &mut GarbageCollectionStatus,
>>           worker: &dyn WorkerTaskContext,
>>       ) -> Result<(), Error> {
>> @@ -1081,6 +1082,12 @@ impl DataStore {
>>               worker.check_abort()?;
>>               worker.fail_on_shutdown()?;
>>               let digest = index.index_digest(pos).unwrap();
>> +
>> +            // Avoid multiple expensive atime updates by utimensat
>> +            if touched_chunks.insert(*digest) {
>> +                continue;
>> +            }
>> +
>>               if !self.inner.chunk_store.cond_touch_chunk(digest, false)? {
>>                   let hex = hex::encode(digest);
>>                   warn!(
>> @@ -1107,43 +1114,87 @@ impl DataStore {
>>           status: &mut GarbageCollectionStatus,
>>           worker: &dyn WorkerTaskContext,
>>       ) -> Result<(), Error> {
>> -        let image_list = self.list_images()?;
>> -        let image_count = image_list.len();
>> -
>> +        // Iterate twice over the datastore to fetch images, even if this comes with an additional
>> +        // runtime cost:
>> +        // - First iteration to find all index files, no matter if they are in a location expected
>> +        //   by the datastore's hierarchy
>> +        // - Iterate using the datastore's helpers, so the namespaces, groups and snapshots are
>> +        //   looked up given the expected hierarchy and iterator logic
>> +        //
>> +        // 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.
>> +
>> +        let mut unprocessed_image_list = self.list_images()?;
>> +        let image_count = unprocessed_image_list.len();
>> +
>> +        // Optimize for avoiding updates of chunks atime in same group for consecutive
>> +        // snapshots multiple times.
>> +        let mut touched_chunks = TouchedChunks::new();
>> +        let mut processed_images = 0;
>>           let mut last_percentage: usize = 0;
>>   
>> -        let mut strange_paths_count: u64 = 0;
>> -
>> -        for (i, img) in image_list.into_iter().enumerate() {
>> -            worker.check_abort()?;
>> -            worker.fail_on_shutdown()?;
>> -
>> -            if let Some(backup_dir_path) = img.parent() {
>> -                let backup_dir_path = backup_dir_path.strip_prefix(self.base_path())?;
>> -                if let Some(backup_dir_str) = backup_dir_path.to_str() {
>> -                    if pbs_api_types::parse_ns_and_snapshot(backup_dir_str).is_err() {
>> -                        strange_paths_count += 1;
>> +        let arc_self = Arc::new(self.clone());
>> +        for namespace in arc_self
>> +            .recursive_iter_backup_ns(BackupNamespace::root())
>> +            .context("creating namespace iterator failed")?
>> +        {
>> +            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()?;
>> +
>> +                        let mut path = snapshot.backup_dir.full_path();
>> +                        path.push(file);
>> +
>> +                        let index = match self.open_index_reader(&path)? {
>> +                            Some(index) => index,
>> +                            None => continue,
>> +                        };
>> +                        self.index_mark_used_chunks(
>> +                            index,
>> +                            &path,
>> +                            &mut touched_chunks,
>> +                            status,
>> +                            worker,
>> +                        )?;
>> +
>> +                        unprocessed_image_list.remove(&path);
>> +
>> +                        let percentage = (processed_images + 1) * 100 / image_count;
>> +                        if percentage > last_percentage {
>> +                            info!(
>> +                                "marked {percentage}% ({} of {image_count} index files)",
>> +                                processed_images + 1,
>> +                            );
>> +                            last_percentage = percentage;
>> +                        }
>> +                        processed_images += 1;
>>                       }
>> +                    touched_chunks.reset();
>>                   }
>>               }
>> -
>> -            if let Some(index) = self.open_index_reader(&img)? {
>> -                self.index_mark_used_chunks(index, &img, status, worker)?;
>> -            }
>> -
>> -            let percentage = (i + 1) * 100 / image_count;
>> -            if percentage > last_percentage {
>> -                info!(
>> -                    "marked {percentage}% ({} of {image_count} index files)",
>> -                    i + 1,
>> -                );
>> -                last_percentage = percentage;
>> -            }
>>           }
>>   
>> +        let strange_paths_count = unprocessed_image_list.len();
>>           if strange_paths_count > 0 {
>> -            info!(
>> -                "found (and marked) {strange_paths_count} index files outside of expected directory scheme"
>> +            warn!("found {strange_paths_count} index files outside of expected directory scheme");
>> +        }
>> +        for path in unprocessed_image_list {
>> +            let index = match self.open_index_reader(&path)? {
>> +                Some(index) => index,
>> +                None => continue,
>> +            };
>> +            self.index_mark_used_chunks(index, &path, &mut touched_chunks, status, worker)?;
>> +            warn!(
>> +                "Marked chunks for unexpected index file at '{}'",
>> +                path.to_string_lossy()
>>               );
>>           }
>>   
>> -- 
>> 2.39.5
>>
>>
>>
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>>
>>
>>
> 
> 
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