Help ZFS FreeBSD 8.0 RC2 Write performance issue

[Dan Nelson]

In the last episode (Nov 11), Ivan Voras said:
> Sam Fourman Jr. wrote:
> > I am running FreeBSD 8.0RC2 and I dont understand why my ZFS/NFS is
> > acting weird on writes.  I get ~150mbit writes idk if this is good or
> > not?  but it paused for a few seconds every once and awhile.
> 
> You didn't give any "iostat" statistics - I suspect that if you 
> correlate ifstat and iostat output that you will see that network 
> "pauses" happen during spikes in IO. You should check for this and post 
> your results.

Yes, iostat would be useful here.  "iostat -zxC 2" will give you per-disk
stats plus CPU usage every 2 seconds (CPU may be a factor if you have
compression enabled).

On a Solaris box I admin, setting zfs_write_limit_override helped stuttering
while doing heavy writes.  It's not exported on FreeBSD, but it should be
easy to add it as a RW sysctl; it lives in dsl_pool.c and can be tweaked at
runtime.  Start big and tune it down so each write burst takes under a
second; it looks like you're writing solid for around 6-8 seconds now.  The
number will vary depending on your disk speed and how much ARC you have.

-- 
	Dan Nelson
	dnelson at allantgroup.com