Subject: Re: Playing with dkwedge
To: Manuel Bouyer <>
From: Bill Studenmund <>
List: tech-kern
Date: 08/24/2005 13:09:48
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On Wed, Aug 24, 2005 at 09:49:35PM +0200, Manuel Bouyer wrote:
> On Wed, Aug 24, 2005 at 12:37:51PM -0700, Bill Studenmund wrote:
> >=20
> > Yes and no. That's what softdeps uses snapshots for, or one of the thin=
> > it uses them for. However snapshots are more for being able to make=20
> > self-consistent backups and for simple "undelete" (deleted something by=
> > mistake? Chances are it's in the snapshot, so just bring it back).
> I meant snapshot are made to take a fixed, consistent image of a live
> filesystem. So it's possible to run fsck on this to detect problems.
> Of course there are other uses too :)

True. But I think we really only need to do that every few weeks at best.

> > If you (or I) really care about the data, we should be using a RAID 5 o=
> > better. And we should have a program that verifies parity. Not just rea=
> > the whole disk, but verifies each stripe's parity. Run it say once a we=
> > on the whole array, and things are good.
> Yes, that would be the best choise. Unless you're using a hardware RAID,
> in which case you can't do this check (and if the hardware controller doe=
> you have to trust it).

Yes. We also would need some sort of ioctl to do this. Even software raid=
will, I expect, not trigger parity verification on a good stripe read. So=
we need a way to tell the unit, verify this.

> And, we can't afford to use ECC memory and RAID everywhere. A periodic fs=
> helps detect hardware problems (not talking about software bugs :), and
> it would be a shame to loose this.

I don't think that wedges will lose this. And a snapshot may be a good way=
to do periodic verification. For a snapshot to be useful in a wedge world,=
it has to show up somewhere other than the main location (for any use of a=
snapshot). So you could just fsck that.

> > > BTW, we should probably add the -x and -X options to fsck, similar to
> > > dump(8).
> >=20
> > What options are those? I do not see them in our dump(8).
> >From a 3.0_BETA system:
>      -x snap-backup
>              Use a snapshot with snap-backup as backup for this dump.  See
>              fss(4) for more details.  Snapshot support is experimental. =
>              sure you have a backup before you use it.
>      -X      Similar to -x but uses a file system internal snapshot on the
>              file system to be dumped.

Interesting. As I actually looked at the man pages on for=20
"NetBSD-current" and didn't find these options documented.

Take care,


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