scrub was Re: RAID5 Problem - $1000 reward for help

scrub was Re: RAID5 Problem - $1000 reward for help

[Dexter Filmore]

Am Sonntag, 17. September 2006 13:36 schrieben Sie:
> On 9/17/06, Ask Bjørn Hansen <ask@develooper.com> wrote:
> > > It's recommended to use a script to scrub the raid device regularly,
> > > to detect sleeping bad blocks early.
> >
> > What's the best way to do that?  dd the full md device to /dev/null?
>
> echo check >/sys/block/md?/md/sync_action
>
> Distros may have cron scripts to do this "right".
>
> And you need a fairly recent kernel.

Does this test stress the discs a lot, like a resync? 
How long does it take? 
Can I use it on a mounted array?

-- 
-----BEGIN GEEK CODE BLOCK-----
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w--(---) !O M+ V- PS+ PE Y++ PGP t++(---)@ 5 X+(++) R+(++) tv--(+)@ 
b++(+++) DI+++ D- G++ e* h>++ r* y?
------END GEEK CODE BLOCK------

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Re: scrub was Re: RAID5 Problem - $1000 reward for help

[Tuomas Leikola]

On 9/17/06, Dexter Filmore <Dexter.Filmore@gmx.de> wrote:
> > > > It's recommended to use a script to scrub the raid device regularly,
> > > > to detect sleeping bad blocks early.
> > >
> > > What's the best way to do that?  dd the full md device to /dev/null?
> >
> > echo check >/sys/block/md?/md/sync_action
> >
> > Distros may have cron scripts to do this "right".
> >
> > And you need a fairly recent kernel.
>
> Does this test stress the discs a lot, like a resync?
> How long does it take?
> Can I use it on a mounted array?
>

yup.
long. think resync.
yup.

It's practically "read everything, verify checksum, report bad blocks
or inconsistencies".

echo repair>sync_action causes md to fix redundancy blocks if they're
out of sync (but at that point you already have another problem like
flakey hardware or so)

Re: scrub was Re: RAID5 Problem - $1000 reward for help

[Rob Bray]

> Am Sonntag, 17. September 2006 13:36 schrieben Sie:
>> On 9/17/06, Ask Bjørn Hansen <ask@develooper.com> wrote:
>> > > It's recommended to use a script to scrub the raid device regularly,
>> > > to detect sleeping bad blocks early.
>> >
>> > What's the best way to do that?  dd the full md device to /dev/null?
>>
>> echo check >/sys/block/md?/md/sync_action
>>
>> Distros may have cron scripts to do this "right".
>>
>> And you need a fairly recent kernel.
>
> Does this test stress the discs a lot, like a resync?
> How long does it take?
> Can I use it on a mounted array?
>

I'd like to add to this question -- does 'check' action on a RAID5 array
verify the accuracy of parity data, blindly read back all data, or only
verify readability of data blocks?

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Re: scrub was Re: RAID5 Problem - $1000 reward for help

[linux]

>> Does this test stress the discs a lot, like a resync?
>> How long does it take?
>> Can I use it on a mounted array?

> I'd like to add to this question -- does 'check' action on a RAID5 array
> verify the accuracy of parity data, blindly read back all data, or only
> verify readability of data blocks?

The former: it reads all data and verifies the parity.

It's a bit hard to see buried in drivers/md/raid5.c:handle_stripe5(),
which is more than a little intricate.  If you search for
MD_RECOVERY_CHECK in there, you can see where it rewrites the newly
added disk if syncing or just notes the problem if checking.