Summary:
... and remove some old code and tech debt in the process.
This is arguably a great milestone and precendent in RocksDB history as for the first time we are explicitly dropping support for the ability to read source-of-truth data in old formats. (We previously dropped support for reading some old bloom filters, but those are performance optimizers not source-of-truth. https://github.com/facebook/rocksdb/issues/10184) However, DBs written with default settings since release 4.6.0, which is very nearly 10 years ago, can still be read. And by using compaction with intermediate versions, there's an upgrade path going back to (AFAIK) early releases of LevelDB (from which RocksDB was forked).
Some detail:
* The magic number for LevelDB SST files (0xdb4775248b80fb57, most recently called kLegacyBlockBasedTableMagicNumber) now only exists in the code to provide a good error message and to test that good error message.
* There is some notable refactoring and renaming around format_version handling. This is a bit of a messy area of code because the footer code being shared between different table formats (block-based, plain, cuckoo) means format_version in the footer is in ways tied to all of them, but in other ways is just tied to block-based table where we have been making updates. Hopefully code comments keep this clear.
* Now that there are old format_versions we can't read (and can't write authoritatively in tests), I've needed to split out kMinSupportedFormatVersion into a constant for reads and for writes, currently the same at format_version=2. Comments describe how to update these in the future.
* The idea of versioning the compression format is basically going away, though we're keeping BuiltinV2 in places just because it's already there. There's lots of room in the BuiltinV2 schema to expand to new built-in compression types, or new ways of handling existing compression algorithms. CompressionManager with CompatibilityName gives users the power to customize compression without the need for versions tied to format_version.
Immediate follow-up:
* Clean up compression loose ends like OLD_Compress, OLD_Uncompress
Suggested follow-up:
* Update plain table builder to migrate to new footer version so that we can drop support for legacy footer. We have to be careful that the (likely untested) forward compatibility path I put in place a while back works (or fix it and wait a while) before dropping support for plain table with legacy footer.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14315
Test Plan:
* Some tests updated / added
* A couple tests are obsolete: removed
* Also updated format compatible test, which now doesn't need to dig as far back into history building RocksDB.
Reviewed By: hx235
Differential Revision: D92577766
Pulled By: pdillinger
fbshipit-source-id: a23be846189d901ce087af4ca9a99cef18445cb7
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