Thursday, February 09, 2006

Why do arrays have even numbers of disks?

Like it says: Why do arrays have even numbers of disks?

Most hardware disk arrays - certainly the ones that Sun sell - currently have an even number of drives in them. The StorEdge 3x00 series have 12, while the 6130 has 14.

The problem I have is that if you take away one drive to act as a hot spare, you're then left with 11 or 13. Not only is this an odd number, it's also prime.

So, what sort of sensible grouping of the drives can you come up with? I often punt and simply create a huge raid-5 volume spanning all the drives I've got left, which is simple. But there are cases when I really want to configure 2 identical sets of disks - either to mirror or to give to 2 hosts. To make this work, I have one drive left over (so I use it as a second hot spare).

Wouldn't it be neat to add an extra drive?


jamesd_wi said...

Because most paranoid sysadmins, beleve that you should have two hot swap drives, to handle two drives dieing in a short period in time.

Peter Tribble said...

Actually, with 2 hot spares you can handle 3 failures. If you believe that's a likely possibility, then I would start to worry about whether you've bought the right array!

Jaimec said...

Actually, I like to work with a Disk MTBF of 5 years. I believe the tests of the MTBF the manufacturers supply are extrapolations and, iff they were really sure of MTBFs of 15 years, they would increase the warranties they give you.
It's my way of working and, If you think that the average Storage Solution can live for way more than 5 years (you wouldn't believe the number orf SSA arrays we still have under contrat) the probability of 2 or 3 disks failing at once isn't that strange.
Also, I know who's making the disk configuration when the first box is sold, I don't know who will do the configuration when the second and third expantion are brought in. Assuring me that I have an "extra" Global Hot Spare there to cope with the second expansion that will have none makes perfect sense to me.
I always recomend creating 2 LUNs, each with a Global Hot Spare (well, a LUN doesn't have a global HS, but, you understand my meaning), you lose 4 disks in a Box but, you get much better resilience.