Summary:
Add the `--leader_path` option to specify the directory path of the leader for a follower RocksDB instance. This PR also adds a `count` command to the repl shell. While not specific to followers, it is useful for testing purposes.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12682
Reviewed By: jowlyzhang
Differential Revision: D57642296
Pulled By: anand1976
fbshipit-source-id: 53767d496ecadc363ff92cd958b8e15a7bf3b151
Summary:
This PR implements deletion of obsolete files in a follower RocksDB instance. The follower tails the leader's MANIFEST and creates links to newly added SST files. These links need to be deleted once those files become obsolete in order to reclaim space. There are three cases to be considered -
1. New files added and links created, but the Version could not be installed due to some missing files. Those links need to be preserved so a subsequent catch up attempt can succeed. We insert the next file number in the `VersionSet` to `pending_outputs_` to prevent their deletion.
2. Files deleted from the previous successfully installed `Version`. These are deleted as usual in `PurgeObsoleteFiles`.
3. New files added by a `VersionEdit` and deleted by a subsequent `VersionEdit`, both processed in the same catchup attempt. Links will be created for the new files when verifying a candidate `Version`. Those need to be deleted explicitly as they're never added to `VersionStorageInfo`, and thus not deleted by `PurgeObsoleteFiles`.
Test plan -
New unit tests in `db_follower_test`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12657
Reviewed By: jowlyzhang
Differential Revision: D57462697
Pulled By: anand1976
fbshipit-source-id: 898f15570638dd4930f839ffd31c560f9cb73916
Summary:
A basic implementation of RocksDB follower mode, which opens a remote database (referred to as leader) on a distributed file system by tailing its MANIFEST. It leverages the secondary instance mode, but is different in some key ways -
1. It has its own directory with links to the leader's database
2. Periodically refreshes itself
3. (Future) Snapshot support
4. (Future) Garbage collection of obsolete links
5. (Long term) Memtable replication
There are two main classes implementing this functionality - `DBImplFollower` and `OnDemandFileSystem`. The former is derived from `DBImplSecondary`. Similar to `DBImplSecondary`, it implements recovery and catch up through MANIFEST tailing using the `ReactiveVersionSet`, but does not consider logs. In a future PR, we will implement memtable replication, which will eliminate the need to catch up using logs. In addition, the recovery and catch-up tries to avoid directory listing as repeated metadata operations are expensive.
The second main piece is the `OnDemandFileSystem`, which plugs in as an `Env` for the follower instance and creates the illusion of the follower directory as a clone of the leader directory. It creates links to SSTs on first reference. When the follower tails the MANIFEST and attempts to create a new `Version`, it calls `VerifyFileMetadata` to verify the size of the file, and optionally the unique ID of the file. During this process, links are created which prevent the underlying files from getting deallocated even if the leader deletes the files.
TODOs: Deletion of obsolete links, snapshots, robust checking against misconfigurations, better observability etc.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12540
Reviewed By: jowlyzhang
Differential Revision: D56315718
Pulled By: anand1976
fbshipit-source-id: d19e1aca43a6af4000cb8622a718031b69ebd97b
