Files missing after the snapshot is taken (only applicable when the TTL for the TimeToLiveHFileCleaner is small, like the default 5 mins)
- SnapshotManifest#addRegion visits store_file_A, but is yet to write it to the manifest.
- store_file_A is marked as compacted away and HFileArchiver moves the file to archive.
- HFileCleaner comes in and sees the store_file_A in archive. It adds the file to the list of files that might need to be cleaned up.
- HFileCleaner's SnapshotHFileCleaner plugin is kicked in.
- SnapshotFileCache#getUnreferencedFiles also says that store_file_A is unreferenced and should be cleaned up (It has not yet been written to the manifest).
- SnapshotHFileCleaner is still going through rest of the files in archive.
- store_file_A reference is created and written to snapshot manifest.
- Snapshot verification runs and sees the store_file_A is present in archive, and thus the verification passes.
- Now, the SnapshotHFileCleaner finishes and TimeToLiveHFileCleaner is triggered. If TTL has passed since the store_file_A was moved to archive (SnapshotHFileCleaner could take easily several minutes to go through rest of the files), the TimeToLiveHFileCleaner also marks the file as deletable.
- Since all cleaner plugins marked file as deletable, the store_file_A is deleted.