On Wed, 26 Dec 2007, Mark Mielke wrote:
david@xxxxxxx wrote:
On Wed, 26 Dec 2007, Mark Mielke wrote:
Florian Weimer wrote:
seek/read/calculate/seek/write since the drive moves on after the
read), when you read you must read _all_ drives in the set to check
the data integrity.
I don't know of any RAID implementation that performs consistency
checking on each read operation. 8-(
Dave had too much egg nog... :-)
Yep - checking consistency on read would eliminate the performance
benefits of RAID under any redundant configuration.
except for raid0, raid is primarily a reliability benifit, any performance
benifit is incidental, not the primary purpose.
that said, I have heard of raid1 setups where it only reads off of one of
the drives, but I have not heard of higher raid levels doing so.
What do you mean "heard of"? Which raid system do you know of that reads all
drives for RAID 1?
Linux dmraid reads off ONLY the first. Linux mdadm reads off the "best" one.
Neither read from both. Why should it need to read from both? What will it do
if the consistency check fails? It's not like it can tell which disk is the
right one. It only knows that the whole array is inconsistent. Until it gets
an actual hardware failure (read error, write error), it doesn't know which
disk is wrong.
yes, the two linux software implementations only read from one disk, but I
have seen hardware implementations where it reads from both drives, and if
they disagree it returns a read error rather then possibly invalid data
(it's up to the admin to figure out which drive is bad at that point).
no, I don't remember which card this was. I've been playing around with
things in this space for quite a while.
David Lang
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