Snapmirror and rolling snapshots

[ianaforbes]

Hopefully someone can clear some things up for me. Let's say I've created my snapmirror source - destination relationship. I'll be replicating the volumes containing the SQL databases and transaction logs. I know that the destination volumes need to be at least as large as the source. Are there situations where the destination volumes should be larger? The volumes on the destination wouldn't need to grow because of snapmirroring would they?

I'd also like to do rolling transaction log snapshots. Do I need to account for any additional volume size on the destination volume to account for rolling snapshots? In overview I'd like to schedule a full SMSQL backup nightly and rolling log snapshots on the hour. Do I need to account for additional volume space at the destination to make this happen, or size the source/destination volumes the same?

Cheers

[mikepapalia]

Hi Ian,

If your destination volume is a bit larger than the source and restricted (in addition to the other setup requirements, like access command, etc.) you will be fine. For your SQL Log volume that contains the Log LUN, you will have to do the calculations for the amount of space needed for data, fractional reserve, number of logs, log size, SnapInfo space, and the number of logs kept online--including all those retained for up-to-the-minute recovery. Both volumes--source and dest--will need to account for this space. This will require monitoring (usualy requires *close* monitoring at first, if you are not completely sure about database activity). Some utilize AutoGrow on the source to alleviate the need for monitoring, but this can lead to your main concern--that of the source outgrowing the destination without your knowledge--not initially recommended.

I hope this helps.

[ianaforbes]

Hi Mike. Thanks very much for the reply. I've actually performed all the relevent sizing already. My first question is why does the destination volume need to be a bit larger than the source?

My second question - Does performing rolling log snapshots mean increased space on the destination?

[mikepapalia]

Ian,

Never been sure on that. My sense is that it actually just needs to be the same size, but it has been best practice to slightly oversize it to avoice mistakes or growth during configuration ("cutting it too close" so to speak).

If "rolling" snapshots means that they will expire and be displaced as new snapshots arrive, then you really just need to have the space for the number of snaps you expect to retain and the size of those snaps. And if the source is sized to accommodate those calculations, then the destination will abide.

Sorry, I may not be understanding the question...

[ianaforbes]

Hi Mike. Thanks for the responses. By rolling snapshots I mean a method to aid in a better recovery point objective (RPO) while not placing too much stress on the system by continuoulsy performing snapmanager backups:

http://now.netapp.com/NOW/knowledge/docs/SnapManager/relsmsql211/html/software/admin/mirrbkp3.htm

A maximumof two rolling snapshots are ever retained so it doesn't look like space will be a concern.