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

[gedhrel.livejournal.com]

For large raid for fileservers, we tend to use 4+1 blocks of disk. The point about this is that (1) a recreate is relatively painless; (2) compared to, say, 14+1, the odds of getting two failures in one raid-5 group are that much smaller (it still happens, alas).

In the absence of any specifics I'd suggest 2x (4+1 raid 5) and a pool spare. Amalgamate the resulting volumes however you like (at the server level if necesasry). Raid-5 is fine (some would say great) for a stock fileserver. Depending on the size of the files in question, you might want to drop the stripe size to give the array a chance to do whole-stripe writes - a lot depends on the capabilities of your raid kit (what is it?) where that is concerned.

[gedhrel.livejournal.com]

Incidentally for the 500GB disks in our high-density kit we've tended to allocate 2 pool spares per 14-disk row; mostly because of the pain assocaited with a physical replacement.

[thudthwacker.livejournal.com]

Amalgamate the resulting volumes however you like (at the server level if necesasry).

[smacks self in forehead]

I've even had my coffee.