jojochuang commented on code in PR #8514: URL: https://github.com/apache/ozone/pull/8514#discussion_r2114844327
########## hadoop-hdds/docs/content/feature/SnapshotCompaction.md: ########## @@ -0,0 +1,87 @@ +# Improving Snapshot Scale: + +[HDDS-13003](https://issues.apache.org/jira/browse/HDDS-13003) + +# Problem Statement + +In Apache Ozone, snapshots currently take a checkpoint of the Active Object Store (AOS) RocksDB each time a snapshot is created and track the compaction of SST files over time. This model works efficiently when snapshots are short-lived, as they merely serve as hard links to the AOS RocksDB. However, over time, if an older snapshot persists while significant churn occurs in the AOS RocksDB (due to compactions and writes), the snapshot RocksDB may diverge significantly from both the AOS RocksDB and other snapshot RocksDB instances. This divergence increases storage requirements linearly with the number of snapshots. + +# Solution Proposal: + +The primary inefficiency in the current snapshotting mechanism stems from constant RocksDB compactions in AOS, which can cause a key, file, or directory entry to appear in multiple SST files. Ideally, each unique key, file, or directory entry should reside in only one SST file, eliminating redundant storage and mitigating the multiplier effect caused by snapshots. If implemented correctly, the total RocksDB size would be proportional to the total number of unique keys in the system rather than the number of snapshots. + +## Snapshot Compaction: + +Currently, automatic RocksDB compactions are disabled for snapshot RocksDB to preserve snapshot diff performance, preventing any form of compaction. However, snapshots can be compacted if the next snapshot in the chain is a checkpoint of the previous snapshot plus a diff stored in a separate SST file. The proposed approach involves rewriting snapshots iteratively from the beginning of the snapshot chain and restructuring them in a separate directory. P.S This has got nothing to do with compacting snapshot’s rocksdb, we are not going to enable rocksdb auto compaction on snapshot rocksdb. + +1. ### Introducing a last compaction time: + + A new boolean flag (`needsCompaction`) and timestamp (`lastCompactionTime`) will be added to snapshot metadata. If absent, `needsCompaction` will default to `true`. + A new list of Map\<String, List\<Longs\>\> (`sstFiles`) also needs to be added to snapshot info; this would be storing the original list of sst files in the uncompacted copy of the snapshot corresponding to keyTable/fileTable/DirectoryTable. + Since this is not going to be consistent across all OMs this would have to be written to a local yaml file inside the snapshot directory and this can be maintained in the SnapshotChainManager in memory on startup. So all updates should not go via ratis. Review Comment: If we end up using a YAML file, I'd recommend adding a checksum for the file to prevent from corruption. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: issues-unsubscr...@ozone.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@ozone.apache.org For additional commands, e-mail: issues-h...@ozone.apache.org
