Re: xfs: very slow after mount, very slow at umount

Dave Chinner
2011-01-27T23:34:15Z

On Wed, Jan 26, 2011 at 10:53:17PM -0500, Mark Lord wrote:
> On 11-01-26 10:43 PM, Dave Chinner wrote:
> > On Wed, Jan 26, 2011 at 08:43:43PM -0500, Mark Lord wrote:
> >> On 11-01-26 08:22 PM, Mark Lord wrote:
> ..
> >> Thinking about it some more: the first problem very much appears as if
> >> it is due to a filesystem check happening on the already-mounted filesystem,
> >> if that makes any kind of sense (?).
> >
> > Not to me. You can check this simply by looking at the output of
> > top while the problem is occurring...
>
> Top doesn't show anything interesting, since disk I/O uses practically zero CPU.

My point is that xfs_check doesn't use zero cpu or memory - it uses
quite a lot of both, so if it is not present in top output while the
disk is being thrashed, it ain't running...

>
> >> running xfs_check on the umounted drive takes about the same 30-60 seconds,
> >> with the disk activity light fully "on".
> >
> > Well, yeah - XFS check reads all the metadata in the filesystem, so
> > of course it's going to thrash your disk when it is run. The fact it
> > takes the same length of time as whatever problem you are having is
> > likely to be coincidental.
>
> I find it interesting that the mount takes zero-time,
> as if it never actually reads much from the filesystem.
> Something has to eventually read the metadata etc.

Sure, for a clean log it has basically nothing to do - a few disk
reads to read the superblock, find the head/tail of the log, and
little else needs doing. Only when log recovery needs to be done
does mount do any significant IO.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com