CLARiiON Virtual LUN Technology and Layered Applications

I recently had one of those head-slapping moments where a ridiculously simple thing had me scratching my head and sifting through release notes to find out how to do something that I’d done many times before, but couldn’t get working this time. I have a client with some EMC CLARiiON AX4-5 arrays running full Navisphere and MirrorView/Asynchronous. He was running out of space on a NetWare fileserver LUN and needed to urgently expand the volume. As there was some space on another Raid Group, and he was limited by the size of the secondary image he could create on the DR array, we came up with a slightly larger size and I suggest using the LUN Migration tool to perform the migration. EMC calls this “Virtual LUN Technology”, and, well, it’s a pretty neat thing to have access to. I think it came in with FLARE Release 16 or 19, but I’m not entirely sure.

In any case, we went through the usual steps of creating a larger LUN on the Raid Group, removing the MirrorView relationship, but, for some reason, we couldn’t see the newer, larger LUN. I did some testing and found that we could migrate to a LUN that was the same size, but not a larger LUN. This was strange, as I thought we’d removed the MirrorView relationship and freed the LUN from any obligations it may have felt to the CLARiiON’s Layered Applications. To wit, the latest FLARE release notes refer to this limitation, which also applies to the CX4 – “If a layered application is using the source LUN, the source LUN can be migrated only to a LUN of the same size”. What I didn’t realise, until I’d spent a few hours on this, was that the SAN Copy sessions I’d created originally to migrate the LUNs from the CX200 to the AX4-5 were still there. Even though they weren’t active (the CX200 is no long gone), Navisphere wasn’t too happy about the idea that the LUN in question would be bigger than it was originally. Removing the stale SAN Copy sessions allowed me to migrate the LUN to the larger destination, and from a NetWare perspective things went smoothly. Of course, recreating the secondary image on the DR array required a defrag of the RAID Group to make enough space for the larger image, but that’s a story for another time.