doc: Add doc for network compression xlator

Change-Id: Iaed94f1bfe7c97eade95923759980a1d10805663
BUG: 1086752
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/7479
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>

doc/network_compression.md

@@ -0,0 +1,71 @@
+#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
+set`performance.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](https://code.google.com/p/lz4/)