Integrate lz4 compression with glusterfs

Issue

• The lz4 compression algorithm is currently the fastest algorithm for compression and decompression. When told to use fast compression, the compression ratio is not as high as with other compression methods such as xz. But for many filesystems, the reduced disk usage of a filesystem compressed via lz4 makes both reading and writing faster despite the slight CPU overhead of lz4 compression and decompression.

• lz4 is currently available in ZFS and there is an lz4 tool in Fedora 18 and newer as well as in EPEL.

• It would be useful to have a "gluster volume" command that would lz4 compress the existing files in a volume once lz4 compression is enabled on a volume. Also, a "volume decompress" command would be needed to decompress the files for disabling lz4 on a volume.

• A "volume high-compress" command would be useful for high-level compressing all files in a volume that is used mostly read-only. This would result in files that are even more highly compressed and faster to read.