On 27/11/2012 14:09, Roy Sigurd Karlsbakk wrote:
One way to handle this at md block level would be to have an
option for raid arrays to always do a full stripe read and
consistency check whenever a block is read. If the consistency
check fails (without any errors being indicated from the drives),
the array should simply return a read error - it should /not/
attempt to recover the data (since it can't tell which parts are
the real problem). If arrays with this option are used as
first-level arrays, with a "normal" md raid array (raid1, raid5,
etc.) on top, then the normal raid recovery process will replace
the bad data and initiate a new write to correct the undetected
read error. I think this would perhaps give you the level of
reliability you are looking for, and be suitable for big arrays
(indeed, it would be unsuitable for small arrays as you need at
least two levels).
If this system is running RAID-6, recovery should be possible to
check both parity chunks, right?
Yes, of course. (And if anyone ever needs it, it is possible to extend
raid6 to 3 parity chunks. I've done the maths, but it is not
implemented - there doesn't seem to be a big need for it.) But - again
referring back to Neil's blog - if the low-level raid spots a
consistency error, it still cannot correct it reliably even with 2
parity chunks, and should pass on a read error to the higher level raid.
Using raid6 at the low level would let you do a good consistency check
even in the case of a failed drive (or a known read error on a drive) -
or two simultaneous undetected read errors. And raid6 on the higher
level raid would let you correct such errors, even when there are other
errors around. You'd soon reach the point where it is more likely for
your disks to spontaneously turn into a bowl of petunias than for read
errors to be undetected or unrecoverable.
Vennlige hilsener / Best regards
roy -- Roy Sigurd Karlsbakk (+47) 98013356 roy@xxxxxxxxxxxxx
http://blogg.karlsbakk.net/ GPG Public key:
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