Hi, I would like to run this through ML to gather feedback as we are contemplating about making this happen.
Currently, snapshots are just hardlinks located in a snapshot directory to live data directory. That is super handy as it occupies virtually zero disk space etc (as long as underlying SSTables are not compacted away, then their size would "materialize"). On the other hand, because it is a hardlink, it is not possible to make hard links across block devices (infamous "Invalid cross-device link" error). That means that snapshots can ever be located on the very same disk Cassandra has its datadirs on. Imagine there is a company ABC which has 10 TiB disk (or NFS share) mounted to a Cassandra node and they would like to use that as a cheap / cold storage of snapshots. They do not care about the speed of such storage nor they care about how much space it occupies etc. when it comes to snapshots. On the other hand, they do not want to have snapshots occupying a disk space where Cassandra has its data because they consider it to be a waste of space. They would like to utilize fast disk and disk space for production data to the max and snapshots might eat a lot of that space unnecessarily. There might be a configuration property like "snapshot_root_dir: /mnt/nfs/cassandra" and if a snapshot is taken, it would just copy SSTables there, but we need to be a little bit smart here (By default, it would all work as it does now - hard links to snapshot directories located under Cassandra's data_file_directories.) Because it is a copy, it occupies disk space. But if we took 100 snapshots on the same SSTables, we would not want to copy the same files 100 times. There is a very handy way to prevent this - unique SSTable identifiers (under already existing uuid_sstable_identifiers_enabled property) so we could have a flat destination hierarchy where all SSTables would be located in the same directory and we would just check if such SSTable is already there or not before copying it. Snapshot manifests (currently under manifest.json) would then contain all SSTables a logical snapshot consists of. This would be possible only for _user snapshots_. All snapshots taken by Cassandra itself (diagnostic snapshots, snapshots upon repairs, snapshots against all system tables, ephemeral snapshots) would continue to be hard links and it would not be possible to locate them outside of live data dirs. The advantages / characteristics of this approach for user snapshots: 1. Cassandra will be able to create snapshots located on different devices. 2. From an implementation perspective it would be totally transparent, there will be no specific code about "where" we copy. We would just copy, from Java perspective, as we copy anywhere else. 3. All the tooling would work as it does now - nodetool listsnapshots / clearsnapshot / snapshot. Same outputs, same behavior. 4. No need to use external tools copying SSTables to desired destination, custom scripts, manual synchronisation ... 5. Snapshots located outside of Cassandra live data dirs would behave the same when it comes to snapshot TTL. (TTL on snapshot means that after so and so period of time, they are automatically removed). This logic would be the same. Hence, there is not any need to re-invent a wheel when it comes to removing expired snapshots from the operator's perspective. 6. Such a solution would deduplicate SSTables so it would be as space-efficient as possible (but not as efficient as hardlinks, because of obvious reasons mentioned above). It seems to me that there is recently a "push" to add more logic to Cassandra where it was previously delegated for external toolings, for example CEP around automatic repairs are basically doing what external tooling does, we just move it under Cassandra. We would love to get rid of a lot of tooling and customly written logic around copying snapshot SSTables. From the implementation perspective it would be just plain Java, without any external dependencies etc. There seems to be a lot to gain for relatively straightforward additions to the snapshotting code. We did a serious housekeeping in CASSANDRA-18111 where we consolidated and centralized everything related to snapshot management so we feel comfortable to build logic like this on top of that. In fact, CASSANDRA-18111 was a prerequisite for this because we did not want to base this work on pre-18111 state of things when it comes to snapshots (it was all over the code base, fragmented and duplicated logic etc). WDYT? Regards
