P4500 Snapshot Question

[Anne D Brown]

Hello,

Apologies for what may be a simple post, but I'm new to LeftHand storage SANs and have not as yet had any training.

When my SAN takes a Snapshot is it the entire size of the volume rather than just what data is being used? Is there a way to configure the Snapshot to only save files that have changed since my current Snapshot data size is 5 times the volume size.
Finally when I delete all of the Snapshots my volume used space increases - why?
The reason for these questions is that I’m constantly running out of space on my SAN – often it’s at 99% capacity.

Thanks for any help!

[Patrick Terlisten]

Hello Anne,

if you delete a snapshot, the data is written to the source volume. In this case you source volume can grow.

Regards,
Patrick

[teledata]

When a new snapshot is taken, no data is "copied" (other than rollups for snapshots being deleted). The newest volume contains only data that changes 'after' the snapshot took place.

If you have a volume with heavy change rates (like a database with a ever-changing transaction log) you can cause each delta (snapshot) to consume a significant volume of space.

With these volumes you may want to employ a different snapshot strategy depending on what you are trying to protect from.

For example, you could perform frequent hourly snapshots, but only keep the last 3-4 snapshots. This would keep the size down, yet still allow you to recover from a corruption problem if identified quickly.

In addition to that you could perform a daily snapshot, but only keep them for 2 days (realize the delta size from yesterday, to an hourly 24 hours later could be significant if you have a high change rate)

When you delete a snapshot the CMC incorrectly reports the data being consumed twice: Once on the snapshot being deleted, and once on the snapshot it is being rolled into. I've noticed that when I query using SNMP commands via Nagios (http://www.tdonline.com/training/lefthand/)
It properly reports the size, and doesn't show the extra usage, The CMC can often actually show a negative amount of space available, so clearly the CMC has some bugs in the reporting method.