On 08/24/2012 05:13 PM, Scott Aitken wrote:
> Hi all,
> I know the easiest answer to this question is "don't do it in the first
> place, and if you do, you should have a backup", however I'll ask it
> regardless.
> Is there a way to backup the ZFS metadata on each member device of a pool
> to another device (possibly non-ZFS)?
> I have recently read a discusson on this list regarding storing the working
> metadata on off-data devices (mirrored I assume).  Is there a way today to
> walk, and save, the metadata of an entire pool and save it somewhere?
> The main motivation for the question is that I recently ruined a large raidz
> pool buy overwriting the start and end of two member disks (and possibly some
> data).  I assume that if I could have restored the lost metadata I could
> have recovered most of the real data.

I don't think you've got a technical problem on your hands, so technical
solutions are likely to be of little use. Essentially, you're asking how
to protect your data from yourself. My suggestion: regular off-site backups.

ZFS metadata is already backed up. Labels are quadruple-redundant,
top-level metadata triple and normal metadata double-redundant.

The particular failure mode you had, namely that you directly erased all
labels from the affected devices, can only be protected against by doing
very regular (read: every 30-60s) backups of the first few MB from the
devices that are to be protected. Even so, this only protects you
against accidentally erasing *all* labels on a vdev's leaf devices. If
you only lost a few labels on, e.g. a raidz, you'd have to restore all
labels on all component disks of the raidz, and even so all hell might
break loose (due to inconsistent txg numbers, etc.). Another method
would be to try and reconstruct the label based on other component disk
labels - that might be slightly more workable, but requires a trained
specialist and cannot be done automatically.

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