EMC – CX4 Configuration – a few things I’d forgotten

I’ve been commissioning some new CX4-960s recently (it’s a long story), and came across a few things that I’d forgotten about for some reason. If you’re running older disks, and they get replaced by EMC, there’s a good chance they’ll be a higher capacity. In our case I was creating a storage pool with 45 300GB FC disks and kept getting the following error.

This error was driving me nuts for a while, until I realised that one of the 300GB disks had, at some point, been replaced with a 450GB drive. Hence the error.

The other thing I came across was the restriction that Private LUNs (Write Intent Log, Reserved LUN Pool, MetaLUN Components) have to reside on traditional RAID Groups and can’t live in storage pools. Not a big issue, but I hadn’t really planned to use RAID Groups on these arrays. If you search for emc254739 you’ll find a handy KB article on WIL performance considerations, including this nugget “Virtual Provisioning LUNs are not supported for the WIL; RAID group-based LUNs or metaLUNs should be used”. Which clarifies why I was unable to allocate the 2 WIL LUNs I’d configured in the pool.

*Edit* I re-read the KB article and realised it doesn’t address the problem I saw. I had created thick LUNs on a storage pool, but these weren’t able to be allocated as WIL LUNs. Even though the article states “[The WIL LUNs] can either be RAID-group based LUNs, metaLUNs or Thick Pool LUNs”. So I don’t really know. Maybe it’s a VNX vs CX4 thing. Maybe not.