On-Wire Compression + Decompression

The 'compression translator' compresses and decompresses data in-flight between client and bricks.

Working

When a writev call occurs, the client compresses the data before sending it to brick. On the brick, compressed data is decompressed. Similarly, when a readv call occurs, the brick compresses the data before sending it to client. On the client, the compressed data is decompressed. Thus, the amount of data sent over the wire is minimized. Compression/Decompression is done using Zlib library.

During normal operation, this is the format of data sent over wire:

<compressed-data> + trailer(8 bytes)

The trailer contains the CRC32 checksum and length of original uncompressed data. This is used for validation.

Usage

Turning on compression xlator:

gluster volume set <vol_name> network.compression on

Configurable parameters (optional)

Compression level

gluster volume set <vol_name> network.compression.compression-level 8

 0  : no compression
 1  : best speed
 9  : best compression
-1  : default compression

Minimum file size

gluster volume set <vol_name> network.compression.min-size 50

Data is compressed only when its size exceeds the above value in bytes.

Other paramaters

Other less frequently used parameters include network.compression.mem-level and network.compression.window-size. More details can about these options can be found by running gluster volume set help command.

Known Issues and Limitations

• Compression translator cannot work with striped volumes.
• Mount point hangs when writing a file with write-behind xlator turned on. To overcome this, turn off performance.write-behind entirely OR setperformance.strict-write-ordering to on.
• For glusterfs versions <= 3.5, compression traslator can ONLY work with pure distribute volumes. This limitation is caused by AFR not being able to propagate xdata. This issue has been fixed in glusterfs versions > 3.5

TODO

Although zlib offers high compression ratio, it is very slow. We can make the translator pluggable to add support for other compression methods such as lz4 compression