[PATCH 0/3] Update LZ4 compressor module

[Sven Schmidt <4sschmid@informatik.uni-hamburg.de>]

This patchset is for updating the LZ4 compression module to a version based
on LZ4 v1.7.2 allowing to use the fast compression algorithm aka LZ4 fast
which provides an "acceleration" parameter as a tradeoff between
compression ratio and compression speed.

We will use LZ4 fast in order to support compression in lustre. LZ4 fast empowers
us to do client-side as well as server-side compression/decompression while
being able to provide appropriate parameters to enable users to tune lustre's
behaviour to obtain the best performance/compression/etc. on their behalf
(adapative compression).

Also, it will be useful for other users of LZ4 compression,
as with LZ4 fast it is possible to enable applications to use fast and/or high
compression depending of the usecase. E.g. a developer can use very
high compression (low acceleration) for sending data over a network with
limited rate of transmission or he trades the compression ratio for higher
compression speed.

LZ4 homepage: http://www.lz4.org/
LZ4 source repository: https://github.com/lz4/lz4
Source version: 1.7.2
Compression in lustre: http://wiki.lustre.org/Enhanced_Adaptive_Compression_in_Lustre

Benchmark (taken from [1], Core i5-4300U @1.9GHz):
----------------|--------------|----------------|----------
Compressor      | Compression  | Decompression  | Ratio
----------------|--------------|----------------|----------
memcpy          |  4200 MB/s   |  4200 MB/s     | 1.000
LZ4 fast 50     |  1080 MB/s   |  2650 MB/s     | 1.375
LZ4 fast 17     |   680 MB/s   |  2220 MB/s     | 1.607
LZ4 fast 5      |   475 MB/s   |  1920 MB/s     | 1.886
LZ4 default     |   385 MB/s   |  1850 MB/s     | 2.101

[1] http://fastcompression.blogspot.de/2015/04/sampling-or-faster-lz4.html

[PATCH 1/3] crypto: Change lz4 modules to work with new lz4
[PATCH 2/3] fs/pstore: fs/squashfs: Change lz4 compressor functions
[PATCH 3/3] lib: Update LZ4 compressor module based on LZ4 v1.7.2.