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

["Jonathan C. Detert" <Jonathan.Detert@xxxxxxxx>]

* Timothy Shimmin <tes@xxxxxxx> [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.

Bottom line: how do you recover when trying to remount an xfs fs whose
log happens to be corrupt?
-- 
Jon Detert
IT Systems Administrator, Milwaukee School of Engineering
1025 N. Broadway, Milwaukee, Wisconsin 53202, U.S.A.
--
"Solitary trees, if they grow at all, grow strong."
~ Winston Churchill