Yuqi Yan created CASSANDRA-21533:
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Summary: Configurable compression algorithm for each LCS level
Key: CASSANDRA-21533
URL: https://issues.apache.org/jira/browse/CASSANDRA-21533
Project: Apache Cassandra
Issue Type: Improvement
Components: Local/Compaction
Reporter: Yuqi Yan
Assignee: Yuqi Yan
Allow an LCS table to use a different sstable compressor per level. The most
useful case: keep the high-churn L0 tier on a fast codec (LZ4) while deeper
levels retain a high-ratio codec (Zstd). L0 is repeatedly rewritten by
STCS-in-L0 and L0→L1 compactions, so paying full Zstd cost there wastes
compaction CPU for data that is short-lived at that level; the ratio only
matters once data settles into the deeper, stable levels.
*Motivation*
This generalizes the idea already accepted in CASSANDRA-15379
({{{}flush_compression{}}}), which flushes with fast LZ4 and lets compaction
re-compress to the table codec. Today there is no way to keep the churny
compaction tier on a fast codec — the table's single compressor applies to
every level.
*Proposed change*
* New LCS compaction sub-option {{{}per_level_compression{}}}: a
comma-separated {{level:CompressorClass}} list (e.g.
{{{}'0:LZ4Compressor'{}}}). Levels not listed inherit the table
{{{}compression{}}}.
* Only the compressor _class_ is overridden; {{chunk_length_in_kb}} and
{{min_compress_ratio}} are inherited from the table compression so chunk sizing
stays consistent.
* Applied in the compaction write path, keyed on the target sstable level.
I'll share a patch for 4.1 targeting LCS and will create a trunk PR targeting
both UCS / LCS. Not very sure how UCS handles the "level" concept if configured
with size tiered like "T4" but I think the idea of using faster compression in
shallow level for "L10" like setup still make sense
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