[Linux-cluster] GFS2: debugging I/O issues

[Daniel Dehennin]

Steven Whitehouse <swhiteho at redhat.com> writes:

> If you are worried about read I/O, then I'd look carefully at the
> fragmentation using filefrag on a few representative files to see how
> they are laid out on disk.

A running qcow2 image, using a backing file:

- running qcow2 is 822MB with 3002 extents

- backing file is 2.2GB with 2893 extents

Another saved images, used as read-only backing file for running VMs is
7.5GB with 9640 extents.

> There are other possible causes of
> performance issues too - do you have the fs mounted noatime (which we
> recommend for most use cases) for example?

Right, I missed that one, I need to planify a down time.

> Running a filesystem which is close to the capacity limit can generate
> fragmentation over time, 80% would usually be ok, and more recent
> versions of GFS2 are better than older ones at avoiding fragmentation
> in such circumstances,

It's running on Ubuntu Trusty a 3.13 kernel and gfs2-utils 3.1.6.

Thanks.
-- 
Daniel Dehennin
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