Using Compression with SnapMirror transfers in ONTAP 7.3.2

mtellinghuisen | Nov 23, 2009 | Comments 0

One of the new features in Data ONTAP 7.3.2 is that you can now enable compression for your SnapMirror/SnapVault transfers. This is great news for customers with limited bandwidth on their WAN links. We have had customers in the past in this situation with SnapMirror transfers that would never finish and they had to look into WAN accelerators, nice to know that now there may be another option that in included free with ONTAP 7.3.2. The compression is done on the controllers by using a standard gzip compression.

Obviously you need to be aware that enabling compression will add additional load onto your system but keep in mind you can use FlexShare to set lower priority to system level (e.g. SnapMirror) operations. Another thing to keep in mind is that FlexShare is assigned per volume, so it doesn’t have to assign ALL of your SnapMirror transfers a low priority.

Enabling compression is as easy as modifying the /etc/snapmirror.conf file, you can enable compression on existing SnapMirror relationships. The changes you need to make are as follows:

At the top of snapmirror.conf you need to establish a connection name and assign the source and destination filers to it. For my example this will be:

sm1=multi(940-1,940-2)

<connection name>=multi(<sourcefiler>,<destinationfiler>)

Now you need to modify the existing line for the SnapMirror schedule, my example looks like this:

sm1:test_vol 940-2:test_vol_recv compression=enable – 10 * * *

Example:

For my example I was setting this up on a FAS940, I ran ‘sysstat –s 1’ before running the snapmirror and it averaged about 1% CPU usage

Source NetApp:

Destination NetApp:

After kicking off the initial snapmirror transfer I re-ran the same command and it returned an average CPU usage of 72% on the destination and 100% on the source.

Source NetApp:

Destination NetApp:

So again if you have a busy system, you will need to decide if the decreased WAN traffic outweighs the added load on your system. I kicked off a ’snapmirror initialize’ and then monitored the transfer with ’snapmirror status -l’ and saw that I was getting a steady 8.0:1 compression ratio on my transfer.