Re: trouble mounting btrfs filesystem....

[Hans van Kranenburg <hans.van.kranenburg@mendix.com>]

On 08/14/2018 07:09 PM, Andrei Borzenkov wrote:
> 14.08.2018 18:16, Hans van Kranenburg пишет:
>> On 08/14/2018 03:00 PM, Dmitrii Tcvetkov wrote:
>>>> Scott E. Blomquist writes:
>>>>  > Hi All,
>>>>  > 
>>>>  > [...]
>>>
>>> I'm not a dev, just user.
>>> btrfs-zero-log is for very specific case[1], not for transid errors.
>>> Transid errors mean that some metadata writes are missing, if
>>> they prevent you from mounting filesystem it's pretty much fatal. If
>>> btrfs could recover metadata from good copy it'd have done that.
>>>
>>> "wanted 2159304 found 2159295" means that some metadata is stale by 
>>> 9 commits. You could try to mount it with "ro,usebackuproot" mount
>>> options as readonly mount is less strict. If that works you can try
>>> "usebackuproot" without ro option. But 9 commits is probably too much
>>> and there isn't enough data to rollback so far.
>>
>> Keep in mind that a successful mount with usebackuproot does not mean
>> you're looking at a consistent filesystem. After each transaction
>> commit, disk space that is no longer referenced is immediately freed for
>> reuse.
>>
> 
> With all respect - in this case btrfs should not even pretend it can do
> "usebackuproot". What this option is for then?

I have no idea. As the Dutch say: "schiet mij maar in de kerstboom". The
commit that introduces it contains no explanation at all.

The only thing I can think of is when you're a developer and writing
buggy code that writes out corrupted tree root blocks, which makes the
filesystem crash in some way directly after a transaction ends while no
other new writes are done again yet, combined with getting fed up of
having to mkfs and put your testdata back all the time.

>> So, even if you can mount with usebackuproot, you have to hope that none
>> of the metadata blocks that were used back then have been overwritten
>> already, even the ones in distant corners of trees. A full check / scrub
>> / etc would be needed to find out.

-- 
Hans van Kranenburg