Raid 5

[Luis Hernandez_5]

What happens when you lose 2 disks of Raid 5 device?

[Steven Clementi]

Depends on how the device is configured and which storage system your using....


Simple put... a standard RAID5 array can not withstand a 2 drive failure, so your data would be hosed.

If you had a hot spare, as long as the second drive did not fail while the first one was rebuilding, you would be safe, theoretically. If they fail at the same time, even with a hot spare, you probably will suffer data loss.

If you have RAID ADG, which is essentially RAID5 with a second parity stripe, you would be ok.


Think of this scenario...

You have 3 drives configure as RAID5 w/ Parity. One drive failes and your operating in a degraded state. This is because regular data is being read from both drives, as well as the data that was on the third drive which is stripped across the 2 good drives. If you lost the second drive, the parity can not be read since it needs at parity stripe from the first failed drive to rebuid the data.

If you have VRAID5, then there is a bit of difference in the theory, especially when dealing with the bigger, more robust array systems like an XP or an EVA.


Hope this helps.

Steven

[Greg Carlson]

Luis,

Here is a tutorial with some illustrations describing how the parity is distributed.

http://www.acnc.com/04_01_05.html

A good resource for learning the very basics of RAID. As stated before however, standard raid 5 can only lose 1 hard drive at a time.

Ciao,
Greg