EqualLogic Snapshots

I recently implemented my first EqualLogic SAN recently and I must say that I am now a bit of a fan of the product. That’s quite a statement coming from me as I have never been a fan of DELL. If I am being honest I probably only disliked them because it’s cool not to like DELL.

It is dead easy to setup and configure and the management console (web based) is responsive and intuitive.

However there is one major flaw as I see it and that is there snapshot implementation. Yes they are easy to setup, manage and can be used to do application consistent snapshots (using host integration tools) but they have one fatal flaw. The Equallogic arrays use a 16Mb page no matter how much data has changed when using snapshots.

 As an example of why this is bad, consider an Exchange 2007 server that needs to make 10 x 8k random writes to the database. Assuming none of the 8k blocks reside in the same 16Mb page you will need a 160Mb snapshot (10 x 16Mb) to allow for 80k worth of writes. You are going to need alot of spare space on your array if you plan on using snapshots.

This (I assume) is most likely why snap shot reservation is set to 100% by default meaning a 100Gb LUN needs 200Gb of available space. Add RAID partiy and host spares into the mix and you are looking at less than 50% usable space!

I would still consider using it for none database LUNs like a file server as writes are more likely to be sequential and thus won’t accentuate the issue as much.

Despite this flaw, I still like to product as long as you don’t plan on making extensive use of snapshots.

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