I/O hang, possibly XFS, possibly general

[Dave Chinner]

On Thu, Jun 02, 2011 at 08:42:47PM -0400, Christoph Hellwig wrote:
> On Thu, Jun 02, 2011 at 10:42:46AM -0400, Paul Anderson wrote:
> > This morning, I had a symptom of a I/O throughput problem in which
> > dirty pages appeared to be taking a long time to write to disk.
> > 
> > The system is a large x64 192GiB dell 810 server running 2.6.38.5 from
> > kernel.org - the basic workload was data intensive - concurrent large
> > NFS (with high metadata/low filesize), rsync/lftp (with low
> > metadata/high file size) all working in a 200TiB XFS volume on a
> > software MD raid0 on top of 7 software MD raid6, each w/18 drives.  I
> > had mounted the filesystem with inode64,largeio,logbufs=8,noatime.
> 
> A few comments on the setup before trying to analze what's going on in
> detail.  I'd absolutely recommend an external log device for this setup,
> that is buy another two fast but small disks, or take two existing ones
> and use a RAID 1 for the external log device.  This will speed up
> anything log intensive, which both NFS, and resync workloads are lot.
> 
> Second thing if you can split the workloads into multiple volumes if you
> have two such different workloads, so thay they don't interfear with
> each other.
> 
> Second a RAID0 on top of RAID6 volumes sounds like a pretty worst case
> for almost any type of I/O.  You end up doing even relatively small I/O
> to all of the disks in the worst case.  I think you'd be much better
> off with a simple linear concatenation of the RAID6 devices, even if you
> can split them into multiple filesystems
> 
> > The specific symptom was that 'sync' hung, a dpkg command hung
> > (presumably trying to issue fsync), and experimenting with "killall
> > -STOP" or "kill -STOP" of the workload jobs didn't let the system
> > drain I/O enough to finish the sync.  I probably did not wait long
> > enough, however.
> 
> It really sounds like you're simply killloing the MD setup with a
> log of log I/O that does to all the devices.

And this is one of the reasons why I originally suggested that
storage at this scale really should be using hardware RAID with
large amounts of BBWC to isolate the backend from such problematic
IO patterns.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com