On-Wire Compression + Decompression
The 'compression translator' compresses and decompresses data in-flight between client and bricks.
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.
Turning on compression xlator:
gluster volume set <vol_name> network.compression on
Configurable parameters (optional)
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 less frequently used parameters include
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-behindentirely OR set
- 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
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