Change TCP Windows Size for NetApp SnapMirror

Most of the customers complain that the storage systems don’t fully utilize the bandwidth given for the snapmirror. You can utilize the bandwidth precisely provided given the TCP window size correctly in snapmirror.conf file.


The following are the pre requistes for adjusting the Window size:
Ascertain the round-trip time between the source and the destination for a SnapMirror relationship. ( 80msec)
Determine the bandwidth available for the SnapMirror relationship. ( 400Mbps)

The default TCP window size for a SnapMirror relationship is 1,994,752 bytes.
Adjustment of the TCP window size is applicable only for asynchronous SnapMirror relationships.
For qtree SnapMirror relationships, TCP window sizes higher than the default value are not supported.

Formula : window size = (round-trip time) × (available bandwidth)

Here in our example it is 80msec x 400Mbps.
The Mbps and MBps are totally different, so it is better to use a calculator for this rather than making mistakes.
Convert the bandwidth to bits per second i.e: 400000000
Convert the 80msec into seconds ( this can be achieved by dividing by 1000 which is 0.08 seconds
Now the formula would be : window size = (((0.08sec) × (400000000 bps)) / 8) bytes = 4000000 bytes

Here we divide by 8 to make it to bytes
The TCP window size for the SnapMirror relationship to 4000000 bytes.
It can be added to snapmirror.conf as below

sourcefiler:src_vol destinationfiler:dst_vol wsize=4000000 * * * *



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