Re: RAID for the DB filesystem

On the performance argument, I wholeheartedly agree that RAID-5 is not where it's at.  Sequential I/O is on-par with other RAID types but when it comes to random I/O it's one of, if not, the worst of the bunch.

From a recoverability angle, losing a disk in a RAID-5 isn't the end of the world but your world will spin much, much slower than it did while it's recalculating all those parity blocks and while doing so you're at disk of data loss if a second drive goes.

There are units out there that allow for mirrored RAID-5, RAID-5+1, to protect from multiple disk failures however at that point RAID-10 is the route to go.

There are units that 'format' the RAID group only where the disk has been allocated.  In other words, if you have a 4 disk RAID-6 and 25% of it has been allocated to LUN(s) then the controller will have the parity calculated for only that 25% in use.  Makes recovery quicker in an underallocated situations but there is still a window with a RAID-5 recovery where a second disk failure kills the whole operation.  RAID-6 however is better in this case b/c it takes a third disk failure before data loss but you had better have a second spare waiting in the wings.

I don't believe RAID-10's are perfect either.  If your RAID-10 is really 2 RAID-0's mirrored, i.e. RAID-0+1, and you have 2 disks failure, one in each RAID-0, then that's a go-to-tape situation.  If your RAID-10 is really multiple mirrors striped, i.e. a true RAID-10 or RAID-1+0, you're just as susceptible to data loss except you must lose both sides of a single mirror.  Not as likely but still possible.

Recovery in either RAID-10 setups is simpler than the parity RAID's in that a disk must be copied only and no parity calculated.  This is still a window where a second disk failure could result in data loss.

I believe that regardless of your selection you must, must, must look at things as a 3 year solution, 5 years at the most.  As those disks spin and age the likelihood of multiple failures increase.  You may not have to replace a single disk in the first 3 years but should you lose power and those drives spin down and cool the odds of one or more not spinning back up are pretty good.  Trust me, I've experienced that many times.  Plan to replace aging units.

In the end, people will do what people will do and most likely the largest factor won't be performance, protection or recoverability but instead it will be money.  If you're lucky, money isn't an issue.

Greg