Adjusting the TCP window size for a SnapMirror relationship

Recently, I learnt on how to adjust the TCP window size for a snapmirror relationship, thought it would be worth sharing it here.

Most of us complain that we don't fully utilize the bandwidth given for the snapmirror. We can utilize the bandwidth precisely provided we give the TCP window size correctly in snapmirror.conf file.

Let me explain with an example: 

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. ( this can be got from the network team or the TTL value using ping from source to destination filer) ( 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 its 80msec x 400Mbps

the Mbps and MBps are totally different its better to use the online calculator for this rather than making mistakes: available at the link here: http://www.numion.com/calculators/units.html

paste 400 in the calculator link and convert them 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

so 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 * * * *

For setting the TCP window size on all the volumes, the window size has to be added to the options snapmirror.