Re: Massive filesystem corruption since kernel 5.2 (ARCH)

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2019/8/27 下午2:13, Swâmi Petaramesh wrote:
> Le 27/08/2019 à 07:06, Swâmi Petaramesh a écrit :
>>
>> Now the machine looks stable so far with a 5.2, albeit more recent, Arch
>> kernel : 5.2.9-arch1-1-ARCH.
>
> As my 1st machine looks fairly stable now, I just upgraded to 5.2
> another one that had always been running <= 5.1 before.
>
> So I keep an eye on the syslog.
>
> Right after reboot in 5.2 I see :
>
> kernel: BTRFS warning (device dm-1): block
> group 34390147072 has wrong amount of free space
> kernel: BTRFS warning (device dm-1):
> failed to load free space cache for block group 34390147072, rebuilding
> it now
>
> So it seems that the 5.2 kernel finds and tries to fix minor
> inconsistencies that were unnoticed in previous kernel versions ?

It means something wrong is already done in previous kernel.

V1 space cache use regular-file-like internal structures to record used
space. V1 space cache doesn't use btrfs' regular csum tree, but uses its
own inline csum to protect its content.

If free space cache is invalid but passes its csum check, it's
completely *possible* to break metadata CoW, thus leads to transid mismatch.

You can go v2 space cache which uses metadata CoW to protect its space
cache, thus in theory it should be a little safer than V1 space cache.

Or you can just disable space cache using nospace_cache mount option, as
it's just an optimization. It's also recommended to clean existing cache
by using "btrfs check --clear-space-cache v1".

I'd prefer to do a "btrfs check --readonly" anyway (which also checks
free space cache), then go nospace_cache if you're concerned.

Thanks,
Qu

>
> I wonder if such things could be the cause of the corruption issues I
> got : finding some inconsistencies with new checks right after a kernel
> upgrade, trying to fix them and creating a mess instead ?
>
> (This 2nd machine has been rebooted twice after this and still looks
> happy...)
>
> Kind regards.
>
> ॐ
>