Geek question: RAID array performance

[andygates]

We've got a big data store. It's currently built as a single max-size 2Tb RAID 5 array over 11 disks. It performs like a three-legged dog and each of the 300Gb disks has about 75Gb unused (because that would take it over the 2Tb SCSI RAID limit).

We need to rebuild it. We've been advised that volume sizes over 1.5Tb are dogs under SCSI RAID so we're keen not to do that. Our options are:

* RAID 10, 10 disks+hotswap, total volume space 1.5Tb, good write performance.

* RAID 5, 7 disks+hotswap, total volume 1.6Tb, but it feels wasteful of all these lovely disks.

* Two smaller RAID 5 volumes, 4+hotswap and 5+hotswap, giving us .9 and 1.2 Tb respectively. The volumes should behave better, because they're smaller and on fewer disks, but will this just move the bottleneck up to the servers' RAID controller?

Gurus, your wisdom is much appreciated!

Comments

[thudthwacker.livejournal.com]

What is the array being used for, and how much of the available capacity is currently being taken up?

[andygates.livejournal.com]

General file storage, just under 1Tb, and it's about 2:1 write:read from the activity monitors.

[thudthwacker.livejournal.com]

My personal tendency would be to go for the RAID 10 setup. Overall performance is better, and you don't have to worry about the possibility of misestimating what should go on which of the two smaller RAID volumes and having to shuffle stuff around.

In the interests of full disclosure, I'm irrationally touchy about having to move stuff between volumes. This is a holdover from several years back when we had an old SGI Challenge with a pile of differently-sized SCSI drives, and would regularly have to spend hours moving user groups off one and onto another when they ran out of space.

"...on the other hand..."

[thudthwacker.livejournal.com]

Of course, you get over half a TB more space if you set up two volumes, and that's not inconsiderable.

So, if you're worried that the controller will bottleneck if it has to handle two volumes, you can set up the 1.2TB volume first and put everything on it (since you're under a TB of use now), and check performance. You then set up the second volume and port some stuff over to it, and look for performance degradation. If it looks good, you can port more stuff over. If it looks bad, you can always do RAID 10. (He said cavalierly, as if he were the one who was going to be spending several hours shoveling bits around.)