Re: Unrecoverable fs corruption?

[Christoph Anton Mitterer <calestyo@scientia.net>]

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On Fri, 2016-01-01 at 08:13 +0000, Duncan wrote:
> you can also try a read-only scrub
OT: I just wondered, would a balance include everything a scrub
includes (i.e. read+verify all data and rebuild an errors on different
devices / block copies)... of course in addition to also copying all
"good" data... and perhaps with the difference, that you don't get that
detailed information as in scrub but only the kernel log messages about
errors?

> In this case, 
> you'll need to recover from the degraded-mount working device as if
> the 
> second one had entirely failed.
> 
> What I'd do in this case, if you haven't done so already, is that
> read-
> only btrfs scrub, just to see where you are in terms of corruption on
> the 
> remaining device.
I don't think that this is the best order of the steps - at least not
when it's about precious data.

Doing a scrub at this phase, would just read all data, telling you the
status,... but first you should try to copy as much as possible (just
in case the remaining good drive fails as well) and *then* do the scrub
to see what's actually good or not.


Alternatively the first step could be backing up to another drive in
the sense of dd-copy (beware of the problem of UUID collisions in
btrfs: you MUST make sure here that the kernel doesn't see[0] devices
with the same IDs, which is of course the case with dd, unless you
write to e.g. an image file and not a device)

This has advantages and disadvantages:
- btrfs rebuild would only rebuild those block that are actually
used... so you need to do less reads from a possibly soon-to-be-dying
device
- OTOH, you only copy the blocks which btrfs thinks are actually
used,... and if later it would turn out that there are filesystem
corruptions in these, you don't have any other areas (with possibly
older data) where you could try some last-resort-recoveries..



Cheers,
Chris.

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