Disk array

[farhi]

Anyone knows which array configuration can stand the loss of 2 disk mech. and still allows the customers to access 100% to their data ?

[federico_3]

You can find something following the link below:

http://www.enterprisestorage.hp.com/pdfs/idc_autoraid.pdf


Federico

[Carol Garrett]

Any disk array where you mirror the disks will be able to survive 2 disks failing at the same time.

For RAID the large disk arrays, EMC, XP256/512 have multiple spares so they can also survive 2 disks failing at once. Smaller disk arrays like Nike/Clarions are usually broken into luns of around 5 disks (1 is the spare) so only if you lose 2 disks in 1 lun would it be a disaster.

Some large sites use multiple XP256/512's or EMC's and then mirror them to a second one in another building - just to be safe. OK if you can afford it.

[Victor BERRIDGE]

No,
I know (I have some) only of a spare per controller, or in cabs where you can have 4-6 arrays, 2 spares, so you could configure that one with 2 arrays with 2 spares...

Usually the external subsystems are built in such a way, the case of 2 disks failure such never occur unless you havent noticed the first failure that happened some (quite) time ago...

[Victor BERRIDGE]

And of course you can mirror your arrays, some subsystems offers some soft facilities like HDS/shadowimage, XP256-512 business copy XP, EMc'sequivalent or IBM-ESS flash copy...

[Dave Wherry]

As you can see from the previous posts the real answer is yes and no. It depends on the array.
Smaller arrays like an AutoRaid or FC-30 can, I believe only designate one hot spare. If you lose a second disk you're going down.
Larger arrays like an XP256/512 or EMC can have multiple hot spares. You could lose more than one disk and survive, or maybe not.
In an XP your array group consists of 4 disk drives. In a Raid-1 configuration you have 2 usable drives and each has a mirror. If 1 drive fails a hot spare will go active and rebuild from the remaining good drive. If you lose the primary and mirror from a pair the hot spare would have nothing to rebuild from. You're down.
If you lost 1 disk from a pair and a disk from another pair and had 2 hot spares you would be OK, and so on and so on. The key is you need 1 disk from a pair still good for the hot spare to rebuild from.
If you are using Raid-5 you have a similar situation. Raid-5 uses all 4 disks in the array group. Essentially 3 for data and 1 for parity. If you lose 1 disk the hot spare will rebuild the group from the remaining 3 disks. If you lose 2 disks from a group you do not have enough to rebuild from.
So, the answer is yes and no. It depends on the configuration and the extent of disk failures. I think it was Victor who said losing 2 disks is unlikely, although possible. Losing more than one disk would most likely occur after physically moving a disk subsystem. You move it to another building or room. These things are bullet resistant, not bullet proof.