Sent: Fri Dec 17 2010 14:24:37 GMT-0700 (Mountain Standard Time)
From: Robin Hill <robin@xxxxxxxxxxxxxxx>
To: linux-raid@xxxxxxxxxxxxxxx
Subject: Re: integrity verification on raid-5?
On Fri Dec 17, 2010 at 02:08:16PM -0700, Patrick H. wrote:
Is there a way to do integrity verification on a raid-5 array? I'm
working on building a storage system on SSDs under raid-5 and want to be
able to perform periodic integrity checks. Basically just check the
drives to make sure that they match what the parity drive has.
After a bit of googling I saw other people wanting the same thing but
nobody with any result. I dont see why this cant be done, but is there
any tool to do so?
There's built-in functionality to do this. To start the check, run:
echo check > /sys/block/mdX/md/sync_action
You can check progress by catting /proc/mdstat, and the number of errors
is reported at the end in /sys/block/mdX/md/mismatch_cnt. To rewrite
the parity data for any mismatches, use "repair" instead of "check" in
the first command.
Currently, there's no easy way to find out what file(s) are affected by
the mismatches though.
The docs say that for both raid 5 & 6 it the repair function simply
rewrites the parity drive(s). For raid-5 I can understand this as
there's no way to tell if the data is incorrect, or if the parity is
incorrect since there's only 1 parity. And while I dont know the details
of the algorithms involved in raid-6, couldnt you do something like:
Calculate replacement data for both parity drives
If one of the 2 parity drives doesnt match its replacement data
assume that drive is bad
Else if both parity drives dont match their replacement data
one of the data drives must be bad
calculate replacement data for each data drive and find the one that
doesnt match
If more than 1 data drive doesnt match its replacement data
we have multiple-drive failure (could be any combination of
parity & data drives) and cant determine which ones
Else
the world is ok
Its probably a heck of a lot more computationally expensive, but it can
isolate which drive is the bad one. But again, I'm not knowledgeable on
the the internal details of raid-6 and might just be completely off my
rocker.
-Patrick
Cheers,
Robin
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
