Re: mount prob: "log inconsistent or not a log"

[Timothy Shimmin <tes@sgi.com>]

Jonathan C. Detert wrote:
> * Timothy Shimmin <tes@sgi.com> [071220 18:49]:
>> Jonathan C. Detert wrote:
>>> I just want to clarify - I mean to ask:
>>>
>>> 	Is it possible that an xfs filesystem which is in use by an o.s.,
>>> 	last mounted by the o.s. months ago, could have a corrupted log, and
>>> 	yet keep functioning until such time as a remount of it is attempted?
>>>
>> Yes.
>> The log is read on every mount. I don't believe we read it otherwise.
>> On a mount we look for the log head and an unmount record etc. to decide
>> whether to do a replay or not.
>> So it could be corrupted and we would not know and we don't really care
>> until the next mount.
>> However, if we are modifying metadata in the filesystem (with some 
>> exceptions)
>> then we will be continuing to write to the log and we will continue to
>> wrap the log. And hence if someone corrupted the log at a point in the past,
>> we may very well be able to overwrite the log and effectively lose that
>> corruption.
>>
>> Do you understand the basic idea?
> 
> I think so.
> 
>> We just write to the log while the file-system is mounted and we are writing
>> to the filesystem (particularly changing metadata); and
>> we read from the log during (just prior) mounting of the filesystem.
>>
>>> 	I.e. if a log gets corrupted while the f.s. is in use, will anyone
>>> 	notice, until such time as an attempt to remount the f.s. is made?
>> No I don't think anyone will notice.
>> No-one has cause to read it, so noone cares.
>>
>> If you unmount it cleanly. Run repair on it before mounting it again.
>> Then repair may find corruption when it tries to find the log head etc...
>> to work out if the log is clean or not.
>> i.e. in this case repair will try to read the log before you've tried
>> to do a mount.
> 
> So, how do you recover from a situation like i have, namely:
> 
> 1) the xfs file system is not currently mounted
> 2) an attempt to mount the xfs fs fails, complaining about log being
>    inconsistent or not a log
> 
> You say noone cares if the log is corrupt, but it seems to be a
> fatal problem if you want to mount the fs.
> 
Noone cares until you need to read it and it is still corrupt
and then you care when you want to mount it.
So yes, you care ;-)

> Bottom line: how do you recover when trying to remount an xfs fs whose
> log happens to be corrupt?

Without doing anything fancy, you are stuffed.
You need to clear the log and repair it using "xfs_repair -L"
(the -L will zero the log and for linux write a log record in there).

I was wondering that if only the end of the log was corrupted and that
was not between the tail and the head, then one could write
0x00000255 (597 dec) at the start of each sector (which is the previous
cycle#) where the corruption is.
Then the recovery code would likely think that this was old
data as it has the old cycle# (and hence ignore it).
However, it is xmas breakup day and I'm too tired to try it out
at the moment. :)
Was thinking of something like:
xfs_io -c 'pwrite -b 512 -S 0x00000255 38889s 31s' sdb-xfs-log.fixup
But that's not right. Sorry.
And then I'm still not convinced about the tail numbers in many
of the log records (particularly the head) anyway.

--Tim