Subject: Re: Bad sectors vs RAIDframe
To: Stephen Borrill <firstname.lastname@example.org>
From: Thor Lancelot Simon <email@example.com>
Date: 06/08/2005 13:00:51
On Wed, Jun 08, 2005 at 05:40:57PM +0100, Stephen Borrill wrote:
> On Mon, 6 Jun 2005, Thor Lancelot Simon wrote:
> >We got a bad run of Samsung Spinpoint drives that we unfortunately
> >installed in NetBSD Foundation servers about a year ago. I have had
> >to recover several of them (all in 2-way RAIDframe mirrors) by using
> >dd to copy the data from the corresponding sectors on one drive over
> >the bad sectors of the other, often doing this in both directions to
> >recover from multi-drive failures within a set. Since then, RAIDframe
> >has been changed so that it retries on disk error before failing a
> >component, and never fails components from non-redundant sets -- so a
> >newer kernel may let you get somewhere with data recovery, too.
> I'm guessing these changes (or at least the second half) are:
I don't think that's all of it -- it looks too early. Greg?
I think the changes are in the 2.0 branch _now_ but I don't think
they were in it when 2.0 was built and released.
> With 1.6.2, a read error causes component failure. As the read is not
> retried a successfully ECC corrected sector will not be spotted. If you
> spot this in time, initiating a rewrite will generally be OK as upon a
> write failure it'll map in a new sector. It will happily fail all
> components in an array and then panic. In this respect, having a
> RAIDframe RAID 1 mirrored set is actually significantly worse than
> having a single disc (if you fail to spots failures quickly).
That's true for 1.6.2, at least. Actually, with the change to never
fail a component of a non-redundant set due to disk error, simply
telling the set to rebuild will issue the sector writes necessary
to fix the problem -- unless the rebuild fails because it can only
move the data one way, and it can't read some of it from the "from"
component (whichever read errored last); working by hand you can do
the right thing, which is immediately upon read error dd the data
from the _other_ half of the mirror back to the half that errored.
RAIDframe could clearly automatically DTRT in almost every case
like this -- "regenerate the data from parity and write-back" is
the same as "read from other half of mirror and write-back" but
it's hard to see exactly how to make it do so. The internals of
RAIDframe scare me.