Hi Sage,

Am 25.10.2017 um 21:54 schrieb Sage Weil:
> On Wed, 25 Oct 2017, Stefan Priebe - Profihost AG wrote:
>> Hello,
>>
>> in the lumious release notes is stated that zstd is not supported by
>> bluestor due to performance reason. I'm wondering why btrfs instead
>> states that zstd is as fast as lz4 but compresses as good as zlib.
>>
>> Why is zlib than supported by bluestor? And why does btrfs / facebook
>> behave different?
>>
>> "BlueStore supports inline compression using zlib, snappy, or LZ4. (Ceph
>> also supports zstd for RGW compression but zstd is not recommended for
>> BlueStore for performance reasons.)"
>
> zstd will work but in our testing the performance wasn't great for
> bluestore in particular.  The problem was that for each compression run
> there is a relatively high start-up cost initializing the zstd
> context/state (IIRC a memset of a huge memory buffer) that dominated the
> execution time... primarily because bluestore is generally compressing
> pretty small chunks of data at a time, not big buffers or streams.
>
> Take a look at unittest_compression timings on compressing 16KB buffers
> (smaller than bluestore needs usually, but illustrated of the problem):
>
> [ RUN      ] Compressor/CompressorTest.compress_16384/0
> [plugin zlib (zlib/isal)]
> [       OK ] Compressor/CompressorTest.compress_16384/0 (294 ms)
> [ RUN      ] Compressor/CompressorTest.compress_16384/1
> [plugin zlib (zlib/noisal)]
> [       OK ] Compressor/CompressorTest.compress_16384/1 (1755 ms)
> [ RUN      ] Compressor/CompressorTest.compress_16384/2
> [plugin snappy (snappy)]
> [       OK ] Compressor/CompressorTest.compress_16384/2 (169 ms)
> [ RUN      ] Compressor/CompressorTest.compress_16384/3
> [plugin zstd (zstd)]
> [       OK ] Compressor/CompressorTest.compress_16384/3 (4528 ms)
>
> It's an order of magnitude slower than zlib or snappy, which probably
> isn't acceptable--even if it is a bit smaller.
>
> We just updated to a newer zstd the other day but I haven't been paying
> attention to the zstd code changes.  When I was working on this the plugin
> was initially also misusing the zstd API, but it was also pointed out
> that the size of the memset is dependent on the compression level.
> Maybe a different (default) choice there woudl help.
>
> https://github.com/facebook/zstd/issues/408#issuecomment-252163241

thanks for the fast reply. Btrfs uses a default compression level of 3
but i think this is the default anyway.

Does the zstd plugin of ceph already uses the mentioned
ZSTD_resetCStream instead of creating and initializing a new one every time?

So if performance matters ceph would recommand snappy?

Greets,
Stefan
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