Large LUN vs more luns

[Tim D Fulford]

I seem to be ad odds with some of the above replies. I would go for 24x 64GB LUNs and extent stripe. If you go for 2x 768GB you will get enourmouse disk queues. BTW you may also need to up the max_scsi_queue_depth kernel parameter to 64 or more (default 8).

I suspect you will be trying to get something like 20,000 IO/s If you have 2 LUNs you will be trying to get 10,000 IO/s down each LUN. This will mean an effective service time for EACH LUN to be 0.1 ms. With 24 LUNs you will be asking for a more reasonable 1.2 ms (still pritty impressive).

The other main reason for going with more resonably size LUNs is the disk queue, assuming you do get 0.1 ms per LUN you will still be queuing 12 times more IOs. This will appear on the HP-UX system. OK it will munch through the queue quickly, but when you consider queues build up with a power of 2 (square, something I remembered from a stats lecure) you will be getting vast queues very quickly.

The last thing, you may want to consider reducing your VG size from 1.5GB. The reason I say this is LVM has a hard limit of 255 LV per VG. This means that you will need to create LVs of just over 6GB each. We use Informix which means we are limited to 2GB LVs/chunks, thus the biggest VG we can have is 0.5 TB. If you have a similar limit or will be creating LVs smaller than 6GB you will end up with unusable space that you CANNOT recoup (OK backup/destroy/restore will do it)!

Just my advice based on experience

Tim