Summary:
kLZ4Compression and kLZ4HCCompression share the same on-disk format and decompressor, but historically kLZ4Compression only honored negative (acceleration) levels while kLZ4HCCompression only honored positive levels. This unifies them so `compression_opts.level` alone selects the variant: level <= 0 uses LZ4 fast (acceleration = -level) and level >= 1 uses LZ4HC (1..12), regardless of which of the two types is configured.
The configured type now only determines the default compression level (LZ4: acceleration 1, equivalent to level -1; LZ4HC: level 9).
For code simplicity, the recorded per-block type comes from the compression type derived from the level, which could differ from the configured type. To preserve the originally configured choice for debugging/tracking, it is recorded as a `_type=<decimal>` pseudo-option in the rocksdb.compression_options SST table property.
Out-of-range non-default levels are clamped to the nearest effective value (LZ4 acceleration capped at 65537, which also avoids signed overflow negating INT_MIN; LZ4HC level capped at 12). The cost-aware (auto-tune) compressor's LZ4 level grid is changed to negative accelerations so it actually exercises fast LZ4 (positive levels now route to LZ4HC).
Related inclusion: The ZSTD library has a discontinuity at level=0, which maps to level 3, which is more aggressive than levels 1 and 2, which are more aggressive than levels -1, -2, etc. For better friendliness to auto-tuning (etc.), we now map level 0 to be the same as level -1, so that increasing compression level numbers have non-decreasing aggressiveness.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14819
Test Plan:
New unit tests in compression_test.cc:
- UnifiedLZ4LZ4HCLevels: for a representative set of non-default levels, both configured types produce identical output and the same recorded type (selected by the level), levels that clamp to the same effective parameter compress identically, and each round-trips; plus per-type default-level behavior.
- ConfiguredCompressionTypeRecordedInProperties: the `_type=` pseudo- option appears in the SST table property for each configured type.
- ZSTDLevelZeroMapsToMinusOne: level 0 behaves like -1, not like 3.
Reviewed By: joshkang97
Differential Revision: D107536580
Pulled By: pdillinger
fbshipit-source-id: a1281956f70a75d0620cb73d0bfb9ad76c52cca3
Peter Dillinger
6d4a8144e0