ZFS hanging

[Dennis Glatting]

On Fri, 2012-07-13 at 17:06 +0200, Fabian Keil wrote:
> Lytochkin Boris <lytboris at gmail.com> wrote:
> 
> > On Thu, Jul 12, 2012 at 5:15 PM, Fabian Keil
> > <freebsd-listen at fabiankeil.de> wrote:
> > > fk at r500 ~ $zpool status
> > > load: 0.15  cmd: zpool 2698 [spa_namespace_lock] 543.23r 0.00u 0.12s 0% 2908k
> > 
> > This sounds familiar with http://www.freebsd.org/cgi/query-pr.cgi?pr=163770
> > Try playing with kern.maxvnodes.
> 
> Thanks for the suggestion, but the system is my laptop and I already
> set kern.maxvnodes=400000 which I suspect is more than I'll ever need.
> 
> Currently I uses less than a tenth of this, but I'll keep an eye on
> it the next time the issue occurs.
> 
> I usually reach this deadlock after losing the vdev in a single-vdev pool.
> My suspicion is that the deadlock is caused by some kind of "failure to
> communicate" between ZFS and the various geom layers involved.
> 
> I already know that losing vdevs with the pool configuration I use
> can cause http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/162010
> and http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/162036 and I
> suspect that the deadlock is just another symptom of the same issue.
> 

What is the math and constraints behind kern.maxvnodes and how would a
reasonable value be chosen?

On some of my systems (default):


iirc# sysctl -a | grep kern.maxvnodes
kern.maxvnodes: 1097048


bd3# sysctl -a | grep kern.maxvnodes
kern.maxvnodes: 587825


mc# sysctl -a | grep kern.maxvnodes
kern.maxvnodes: 2112911


btw# sysctl -a | grep kern.maxvnodes
kern.maxvnodes: 460985





> Fabian