On Tue, Mar 16, 2021 at 7:39 PM Qu Wenruo <quwenruo.bt...@gmx.com> wrote:
> > Using that restore I was able to restore approx. 7 TB of the
> > originally stored 22 TB under that directory.
> > Unfortunately nearly all the files are damaged. Small text files are
> > still OK. But every larger binary file is useless.
> > Is there any possibility to fix the filesystem in a way, that I get
> > the data less damaged?
>  From the result, it looks like the on-disk data get (partially) wiped out.
> I doubt if it's just simple controller failure, but more likely
> something not really reaching disk or something more weird.

Hey Qu, thanks for the reply.

So it's not clear until further downthread that it's bcache in
writeback mode with an SSD that failed. And I've probably
underestimated the significance of how much data (in this case both
Btrfs metadata and user data) and for how long it can stay *only* on
the SSD with this policy.

https://bcache.evilpiepirate.org/ says it straight up, if using
writeback, it is not at all safe for the cache and backing devices to
be separated. If the cache device fails, everything on it is gone. By
my reading, for example, if the writeback percent is 50%, and the
cache device is 128G, at any given time 64G is *only* on the SSD.
There's no idle time flushing to the backing device that eventually
makes the backing device possibly a self sufficient storage device on
its own, it always needs the cache device.

Chris Murphy

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