3510 Config with Expansion Units

Question

Hello,
I'm reconfiguring a 3510 with 2 controllers and 1 expansion unit. It will be the storage portion of a Veritas cluster, with 4 hosts directly attached and redundant paths managed by DMP.
The best practices guide states:
"For best performance and RAS, create logical drives across expansion units."
I interpret that to mean, create the logical disks with a mixture of physical disks from both units. This definitely makes sense when you start getting into huge logical disks and want to maximize usage of the 2 Gb fiber channel bandwidth. I understand the performance part of the sentance in the documentation.
But I'm having trouble with the availability part of their statement. If I create logical disks spanning multiple expansion units, and for whatever reason I lose an entire expansion unit, my entire logical drive is then unavailble (unless I am missing something here).
I know this is a remote possibility since nearly everything in the 3510 is redundant, but I wondered if anyone had any thoughts. Short of mirroring an entire logical disk using software, which seems a tad extreme, does anyone have thought on this type of thing? Would you do it differently, trying to keep a single logical disk within a single enclosure?
Thanks for reading and any input.

Answer 1

Your assesment is correct. If you place components of the LUN across the trays then, a tray failure, will take out the LUN. Your performance may increase if you do that, it really depends on how much performance you need, size of the lun, etc. but for general cases it's a good thing.