Re: btrfsck: backpointer mismatch (and multiple other errors)

[Kai Krakow <hurikhan77@gmail.com>]

Am Mon, 4 Apr 2016 04:34:54 +0000 (UTC)
schrieb Duncan <1i5t5.duncan@cox.net>:

> Meanwhile, putting bcache into write-around mode, so it makes no
> further changes to the ssd and only uses it for reads, is probably
> wise, and should help limit further damage.  Tho if in that mode
> bcache still does writeback of existing dirty and cached data to the
> backing store, some further damage could occur from that.  But I
> don't know enough about bcache to know what its behavior and level of
> available configuration in that regard actually are.  As long as it's
> not trying to write anything from the ssd to the backing store, I
> think further damage should be very limited.

bcache has 0 for dirty data most of the time for me - even in write
back mode. It does write back during idle time and at reduced rate,
usually that finishes within a few minutes.

Switching the cache to write-around initiates instant write-back of all
dirty data, so within seconds it goes down to zero and the cache
becomes detachable.

I'll go test the soon-to-die SSD as soon as it replaced. I think it's
still far from failing with bitrot. It was overprovisioned by 30% most
of the time, with the spare space trimmed. It certainly should have a
lot of sectors for wear levelling. In addition, smartctl shows no
sector errors at all - except for one: raw_read_error_rate. I'm not
sure what all those sensors tell me, but that one I'm also seeing on
hard disks which show absolutely no data damage.

In fact, I see those counters for my hard disks. But dd to /dev/null of
the complete raw hard disk shows no sector errors. It seems good. But
well, counting 1+1 together: I currently see data damage. But I guess
that's unrelated.

Is there some documentation somewhere what each of those sensors
technically mean and how to read the raw values and thresh values?

I'm also seeing multi_zone_error_rate on my spinning rust.

According to smartctl health check and smartctl extended selftest,
there's no problems at all - and the smart error log is empty. There
has never been an ATA error in dmesg... No relocated sectors... From my
naive view the drives still look good.

-- 
Regards,
Kai

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