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From man lz4:

--fast[=#]
       Switch  to ultra-fast compression levels. The higher the value, the faster the compres‐
       sion speed, at the cost of some compression ratio. If =# is not present, it defaults to
       1.  This setting overrides compression level if one was set previously. Similarly, if a
       compression level is set after --fast, it overrides it.

So what is the max ultra-fast compression level? Is it the same (12) as for compression level?

pmor
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2 Answers2

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It’s currently 65537, as determined by the LZ4_ACCELERATION_MAX constant.

See also the library documentation for “fast” compression.

Stephen Kitt
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Looking at the command line source code, the compression level is set to the negative of the --fast=# number; this is the only use of this number. As you said the max compression level is 12 so, while I can't be 100% certain that there is symmetry here, it could be that the max effective fast number is also 12.

Anthony Kelly
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