configuration choices with Dell CERC (adaptec 2610SA)

[Kenneth D. Merry]

On Sat, Nov 26, 2005 at 16:59:56 -0500, user wrote:
> 
> First off, thank you very much for your help.
> 
> On Sat, 26 Nov 2005, Scott Long wrote:
> 
> > The problem is not with filesystem corruption, it's with parity 
> > corruption.  It's possible to get a situation where a stripe is
> > partially written out and then interrupted by a power failure, leaving
> > the parity inconsistent with the stripe.  You won't notice this until
> > it's time to rebuild a failed drive from the parity, and then you'll
> > get corruption.
> 
> 
> I will only be creating mirrors.  Is a mirror impervious to this, or are
> the potential problems even worse ?  (my guess is impervious .. ?)

Mirrors are not impervious to the problem.  You still have the same issue;
you don't know whether any outstanding data got written to all drives, some
drives or no drives.  If you resync the mirror, you're basically choosing
one drive as "good" and the other as "suspect", regardless of what the
actual situation is.

> What's the worst thing that could happen to a mirror, with write caching
> turned on for the array (not for the disks - I'll take your advice on not
> turning on write caching on the disks) that suffers a power failure ?

Assuming you don't have a battery backed cache on the controller, you may
wind up with parts of your filesystem corrupted.  Hopefully fsck will be
able to make sense of it, but you never know.

> > It would be better to let the rebuild happen as fast as possible.  Disks
> > have a bad habit of failing in groups, and you don't want to increase
> > the chance of a multi-disk failure by making recovery slow.
> 
> So again ... only using mirrors ... and in a two-drive chassis, I can't
> have a hot spare, so I am not sure if it even really matters what I set
> that to ... comments ?

If you only have a two drive chassis, your best bet is to replace a failed
drive as soon as you can and let the rebuild complete as soon as you can.

Ken
-- 
Kenneth Merry
ken at kdm.org