tuning vfs.zfs.vdev.max_pending and solving the issue of ZFS writes choking read IO

[Bob Friesenhahn]

On Wed, 24 Mar 2010, Dan Nelson wrote:
>
> I had similar problems on a 32GB Solaris server at work.  Note that with
> compression enabled, the entire system pauses while it compresses the
> outgoing block of data.  It's just a fraction of a second, but long enough
> for end-users to complain about bad performance in X sessions.  I had to
> throttle back to a 256MB write limit size to make the stuttering go away
> completely.  It didn't affect write throughput much at all.

Apparently this was a kernel thread priority problem in Solaris.  It 
is apparently fixed in recent versions of OpenSolaris.  The fix 
required adding a scheduling class which allowed the kernel thread 
doing the compression to be less than the priority of normal user 
processes (such as the X11 server).

Bob
--
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/