Re: ZFS tunable parameters

[Greg Troxel <gdt%lexort.com@localhost>]

Brad Spencer <brad%anduin.eldar.org@localhost> writes:

> Sotiris Lamprinidis<sotiris%lamprinidis.com@localhost> writes:
>
>> I looked into exposing ZFS tunables through sysctl and it seems
>> straightforward. Despite being under "dist" I am not sure if this is true 
>> upstream, so the patch is directly on arc.c (and I think it'd be tricky to
>> implement otherwise).
>>
>> I tested arc_min and arc_max, and it seems like changes do have an effect
>> (e.g. on c, c_max). Additionally tested arc_free_target and it does have
>> an effect also.
>
> Thank you very much for this patch.

I'm not sure where we are on merging this patch.  I think it's useful
for people to be able to set and observe these tunables.

It seems to be that patching arc.c is fine.  The concept of upstream is
a bit messy.

> I have been struggling with ZFS on a 8 to 16GB DOMU build system for
> quite a while and with this patch and setting the values to what you
> have listed I was able to perform my abuse builds without any problems
> at all.  100% workable with 4 parallel builds all using the 2 CPUs I
> give to the DOMU.  Before this I could do one or two full system
> builds before I would have to reboot the build box and I was pretty
> much unable to do more than 2 parallel builds at a time without
> locking the system up.  Clamping the size of the ARC seems to be the
> key.  One work around I was trying to use was setting the maxvnodes to
> a very low value and that work around could be removed as well.

> I am running this on a -current as of 2026-07-15.  This find is very
> much appreciated.  Thanks again.

Interesting that you are still having lockups on current.  It has a lot
of locking fixes (also in 11, not in 10).

My guess is that reducing arc is a workaround, not a fix, and is
avoiding the locking bugs by leaving more free.  Still, not crashing is
good....   I don't mean to say we shouldn't merge the patch, or that it
isn't helpful to be able to set/observe -- only that there is deeper
trouble.