Re: "parent transid verify failed" and mount usebackuproot does not seem to work

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

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On 2020/7/2 上午5:36, Illia Bobyr wrote:
> On 7/1/2020 3:48 AM, Qu Wenruo wrote:
>> On 2020/7/1 下午6:16, Illia Bobyr wrote:
>>> On 6/30/2020 6:36 PM, Qu Wenruo wrote:
>>>> On 2020/7/1 上午3:41, Illia Bobyr wrote:
>>>>> [...]
>>>> Looks like some tree blocks not written back correctly.
>>>>
>>>> Considering we don't have known write back related bugs with 5.6, I
>>>> guess bcache may be involved again?
>>> A bit more details: the system started to misbehave.
>>> Interactive session was saying that the main file system became read/only.
>> Any dmesg of that RO event?
>> That would be the most valuable info to help us to locate the bug and
>> fix it.
>>
>> I guess there is something wrong before that, and by somehow it
>> corrupted the extent tree, breaking the life keeping COW of metadata and
>> screwed up everything.
> 
> After I will restore the data, I will check the kernel log to see if
> there are any messages in there.
> Will post here if I will find anything.
> 
>>> [...]
>>>> In this case, I guess "btrfs ins dump-super -fFa" output would help to
>>>> show if it's possible to recover.
>>> Here is the output: https://pastebin.com/raw/DtJd813y
>> OK, the backup root is fine.
>>
>> So this means, metadata COW is corrupted, which caused the transid mismatch.
>>
>>>> Anyway, something looks strange.
>>>>
>>>> The backup roots have a newer generation while the super block is still
>>>> old doesn't look correct at all.
>>> Just in case, here is the output of "btrfs check", as suggested by "A L
>>> <mail@lechevalier.se>".  It does not seem to contain any new information.
>>>
>>> parent transid verify failed on 16984014372864 wanted 138350 found 131117
>>> parent transid verify failed on 16984014405632 wanted 138350 found 131127
>>> parent transid verify failed on 16984013406208 wanted 138350 found 131112
>>> parent transid verify failed on 16984075436032 wanted 138384 found 131136
>>> parent transid verify failed on 16984075436032 wanted 138384 found 131136
>>> parent transid verify failed on 16984075436032 wanted 138384 found 131136
>>> Ignoring transid failure
>>> ERROR: child eb corrupted: parent bytenr=16984175853568 item=8 parent
>>> level=2 child level=0
>>> ERROR: failed to read block groups: Input/output error
>> Extent tree is completely screwed up, no wonder the transid error happens.
>>
>> I don't believe it's reasonable possible to restore the fs to RW status.
>> The only remaining method left is btrfs-restore then.
> 
> There are no more available SATA connections in the system and there is
> a lot of data in that FS (~7TB).
> I do not immediately have another disk that would be able to hold this much.
> 
> At the same time this FS is RAID0.
> I wonder if there is a way to first check if restore will work should I
> will disconnect half of the disks, as each half contains all the data.
> And then if it does, I would be able to restore by reusing the space on
> of the mirrors.

Yes, there is.

We have the out-of-tree rescue mount options patchset.
It allows you to mount the fs RO, with extent tree completely corrupted.

It's in David's misc-next branch already:
https://github.com/kdave/btrfs-devel/tree/misc-next

Then you can try to mount the fs with "-o
ro,rescue=skipbg,rescue=nologreplay" and do your tests on what can be
salvaged and what can not as if your fs is still alive.

This should provide a more flex solution compared to btrfs-restore, but
it needs to compile the kernel.

> 
> I see "-D: Dry run" that can be passed to "btrfs restore", but, I guess,
> it would not really do a full check of the data, making sure that the
> restore would really succeed, does it?

It would only check the metadata, but that should cover most of the risks.

Thanks,
Qu
> 
> Is there a way to perform this kind of check?
> Or is "btrfs restore" the only option at the moment?
> 


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