This is really a quick post to discuss how RAID 6 can be a bit of a pain to work with when you’re trying to combine traditional CLARiiON / VNX DAEs and Storage Pool best practices. It’s no secret that EMC strongly recommend using RAID 6 when you’re using SATA-II / NL-SAS drives that are 1TB or greater. Which is a fine and reasonable thing to recommend. However, as you’re no doubt aware, the current implementation of FAST VP uses Storage Pools that require homogeneous RAID types. So you need multiple tools if you want to run both RAID 1/0 and RAID 6. If you want a pool that can leverage FAST to move slices between EFD, SAS, and NL-SAS, it all needs to be RAID 6. There are a couple of issues with this. Firstly, given the price of EFDs, a RAID 6 (6+2) of EFDs is going to feel like a lot of money down the drain. Secondly, if you stick with the default RAID 6 implementation for Storage Pools, you’ll be using 6+2 in the private RAID groups. And then you’ll find yourself putting private RAID groups across backend ports. This isn’t as big an issue as it was with the CX4, but it still smells a bit ugly.
What I have found, however, is that you can get the CLARiiON to create non-standard sized RAID 6 private RAID groups. If you create a pool with 10 spindles in RAID 6, it will create a private RAID groups in a 8+2 configuration. This seems to be the magic number at the moment. If you add 12 disks to the pool it will create 2 4+2 private RAID groups, and if you use 14 disks it will do a 6+2 and a 4+2 RAID group. Now, the cool thing about 10 spindles in a private RAID group is that you could, theoretically (I’m extrapolating from the VNX Best Practices document here), split the 8+2 across two DAEs in a 5+5. In this fashion, you can increase the rebuild times slightly in the event of a disk failure, and you can also draw some sensible designs that fit well in a traditional DAE4P. Of course, creating your pools in increments of 10 disks is going to be a pain, particularly for larger Storage Pools, and particularly as there is no re-striping of data done after a pool expansion. But I’m sure EMC are focussing on this issue in the future, as a lot of customers have had a problem with the initial approach. The downside to all this, of course, is that you’re going to suffer a capacity and, to a lesser extent, performance penalty by using RAID 6 across the board. In this instance you need to consider whether FAST VP is going to give you the edge over split RAID pools or traditional RAID groups.
I personally like the idea of Storage Pools, and I’m glad EMC have gotten on-board with them in their midrange stuff. I’m also reasonably optimistic that they’re working on addressing a lot of issues that have come up in the field. I just don’t know when that will be.