VariableCOMPRESSION_TYPE
DescriptionSets default compression levels for segments and minisegments
Defaulthigh
Valid Valuesfast 
 high 

Compress (fast) in digest pipeline or (highly) later. For fast, compress using LZ4 in the digest pipeline. This is what all versions up to 1.5.x did. High: compress using LZ4 in the digest pipeline, then re-compress using Zstd when merging mini-segments into proper segments later. For extreme, compress using Zstd in the digest pipeline, then re-compress using Zstd when merging mini-segments into proper segments later. Extreme is not recommended as the extra compression is not worth the extra CPU time spent.

Type String
Default Value high
ini files
COMPRESSION_TYPE=high
COMPRESSION_LEVEL=9
COMPRESSION_LEVEL_MINI=0
#VALUE_DEDUP_LEVEL=COMPRESSION_LEVEL

Recommended setting depends on the hardware and use case. The rule of thumb is that high provides a two-times compression ratio over fast at the cost of using more CPU time for decompressing while searching. Set high as the default for fresh installs, and keep fast on existing systems to allow rolling back to 1.5.x. See also VALUE_DEDUP_LEVEL.