On Wed, 19 Mar 2008, Craig Ringer wrote:
david@xxxxxxx wrote:
you can? I've never seen a raid 5 setup with more then a single parity
dirve (or even the option of having more then one drives worth of
redundancy). you can have hot-spare drives, but thats a different thing.
With RAID 4, where the "parity drives" are in fact dedicated to parity
information, the controller could just store the parity data mirrored on more
than one drive. Unfortunately write performance on RAID 4 is absolutely
horrible, and a second or third parity disk would not help with that.
I suppose there's nothing stopping a controller adding a second disk's worth
of duplicate parity information when striping a four or more disk RAID 5
array, but I thought that's basically what RAID 6 was.
just duplicating the Raid 4 or 5 pairity information will not help you if
the parity drive is not one of the drives that fail.
raid 6 uses a different pairity algorithm so that any two drives in the
array can fail with no data loss.
even this isn't completely error proof. I just went through a scare with a
15 disk array where it reported 3 dead drives after a power outage. one of
the dead drives ended up being the hot-spare, and another drive that acted
up worked well enough to let me eventually recover all the data (seek
errors), but it was a very scary week while I worked through this.
David Lang
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