Re: nullfs and ZFS issues

Quoting Doug Ambrisko <ambrisko@ambrisko.com> (from Thu, 21 Apr 2022  
09:38:35 -0700):

> On Thu, Apr 21, 2022 at 03:44:02PM +0200, Alexander Leidinger wrote:
> | Quoting Mateusz Guzik <mjguzik@gmail.com> (from Thu, 21 Apr 2022
> | 14:50:42 +0200):
> |
> | > On 4/21/22, Alexander Leidinger <Alexander@leidinger.net> wrote:
> | >> I tried nocache on a system with a lot of jails which use nullfs,
> | >> which showed very slow behavior in the daily periodic runs (12h runs
> | >> in the night after boot, 24h or more in subsequent nights). Now the
> | >> first nightly run after boot was finished after 4h.
> | >>
> | >> What is the benefit of not disabling the cache in nullfs? I would
> | >> expect zfs (or ufs) to cache the (meta)data anyway.
> | >>
> | >
> | > does the poor performance show up with
> | > https://people.freebsd.org/~mjg/vnlru_free_pick.diff ?
> |
> | I would like to have all the 22 jails run the periodic scripts a
> | second night in a row before trying this.
> |
> | > if the long runs are still there, can you get some profiling from it?
> | > sysctl -a before and after would be a start.
> | >
> | > My guess is that you are the vnode limit and bumping into the 1  
> second sleep.
> |
> | That would explain the behavior I see since I added the last jail
> | which seems to have crossed a threshold which triggers the slow
> | behavior.
> |
> | Current status (with the 112 nullfs mounts with nocache):
> | kern.maxvnodes:               10485760
> | kern.numvnodes:                3791064
> | kern.freevnodes:               3613694
> | kern.cache.stats.heldvnodes:    151707
> | kern.vnodes_created:         260288639
> |
> | The maxvnodes value is already increased by 10 times compared to the
> | default value on this system.
>
> I've attached mount.patch that when doing mount -v should
> show the vnode usage per filesystem.  Note that the problem I was
> running into was after some operations arc_prune and arc_evict would
> consume 100% of 2 cores and make ZFS really slow.  If you are not
> running into that issue then nocache etc. shouldn't be needed.

I don't run into this issue, but I have a huge perf difference when  
using nocache in the nightly periodic runs. 4h instead of 12-24h (22  
jails on this system).

> On my laptop I set ARC to 1G since I don't use swap and in the past
> ARC would consume to much memory and things would die.  When the
> nullfs holds a bunch of vnodes then ZFS couldn't release them.
>
> FYI, on my laptop with nocache and limited vnodes I haven't run
> into this problem.  I haven't tried the patch to let ZFS free
> it's and nullfs vnodes on my laptop.  I have only tried it via

I have this patch and your mount patch installed now, without nocache  
and reduced arc reclaim settings (100, 1). I will check the runtime  
for the next 2 days.

Your mount patch to show the per mount vnodes count looks useful, not  
only for this particular case. Do you intend to commit it?

Bye,
Alexander.

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