ZFS performance help sought

[Mason Loring Bliss]

On Thu, Jan 21, 2016 at 06:55:45PM -0500, Mason Loring Bliss wrote:

> Here's what I set:
> 
> vfs.zfs.arc_max="4096M"
> vfs.zfs.arc_min="1024M"
> vfs.zfs.txg.timeout="3"
> vfs.zfs.write_limit_override="512M"
> 
> I'm not seeing any obvious way to verify the write_limit_override setting -
> it appears not to show up in sysctl output.
> 
> I'll wait for the current big transfer to finish and then I'll try it with
> prefetch disabled too.

Disabling prefetch doesn't do a thing here - the system is still painfully
overloaded, doing something that was simply unproblematic under Linux.

I'd be grateful for further debugging or tuning tips. Is it possible this has
nothing to do with ZFS and that I need to play with FreeBSD's scheduling
somehow?

Again, FreeBSD 10.2, ZFS tuned at noted above, and with prefetch disabled.
Eight gigs of RAM, pools less than 1TB. Doing a send/receive between pools on
different disks is bringing the system to its knees, where the literal same
hardware under Linux/ZoL doesn't break a sweat.

What else can I try?

-- 
Mason Loring Bliss   ((  "In the drowsy dark cave of the mind dreams
mason at blisses.org     ))  build  their nest  with fragments  dropped
http://blisses.org/  ((   from day's caravan." - Rabindranath Tagore