I need some advise on snapmanager for exchange and fractional reserve (FR). We have a volumes, and every volume has a LUN. We use exchange 2007 which is in a windows cluster setup. We have about 20 storage groups , and each storage group is mapped to each LUN, so we have 20 luns which are in 20 volumes. The FR is set to 100%. The problems we have is that the luns and volumes are running out of space which in turn affects the snapmanager for exchange backups.
So we are looking into changing the fractional reserve to a lesser value. I wanted to find out if this will save space changing the FR to for example 80 %. I wanted to know the impact of this, and will it cause issues to the LUNs if the FR is changed to a different setting.
If fractional reserve is set below 100%, it is possible that volume will run out of space for new writes to LUN from host. Most operating systems do not expect that disk can suddenly run out of space and do not handle this case gracefully.
So you will need to monitor your system for available space and decide in advance what you will do if space becomes dangerously low. Increase volume, delete some snapshots etc.
Which amount can be considered “dangerously low” depends on your environment – how heave IO is, what change rate is, possible peaks of activity etc.
NetApp best practice has, for some years now, been to set FR to zero percent on Exchange FlexVols. If you find that you have an issue whereby the Exchange database is exploding your storage is the least of your problems; your Exchange is the problem and will dismount before the volume chokes. Average change rate on a database is in the high single digits. Obviously YMMV.
If you want some buffer spend some weeks slowly ratcheting down your FR towards zero that's fine. Assess the change rate on the volume and gain the confidence that the volume size is what you want it to be for the change rate and number of days snaps to be retained.