Re: zfs pool behavior - is it ever freed?

[Mr Roooster <mrrooster%gmail.com@localhost>]

On Thu, 27 Jul 2023 at 19:28, Greg Troxel <gdt%lexort.com@localhost> wrote:
>
> Mike Pumford <mpumford%mudcovered.org.uk@localhost> writes:
>
[snip]
>
> > If I've read it right there needs to be a mechanism for memory
> > pressure to force ZFS to release memory. Doing it after all the
> > processes have been swapped to disk is way too late as the chances are
> > the system will become non-responsive by then. From memory this was a
> > problem FreeBSD had to solve as well.
>
> It would be interesting to read a description of what they did.  That
> seems easier than figuring it out from scratch.
>
I'm not sure they did a lot more than expose the ARC limit as a sysctl.

I moved to FreeBSD from Net a few years ago (mainly to get ZFS), and
have had similar issues under heavy load with a large ARC. It wouldn't
crash or hang, but it would always favour killing something over
flushing the ARC under pressure. I did a little bit of digging and got
the impression this was the way it was intended to work. (Although
reading this thread it may be a little more complex than that. :) )

Once I limited my ARC my problems went away. I limited mine to 16 gig
on a 96 gig system, but I was running some processes with high memory
usage. I've not had cause to increase it though, and the system runs
reliably. It has a few zpools, and I'm running a VM of an iSCSI
exposed ZVOL, so it get a decent amount of use.

(This is my home system, not a production system, however it does have
something like 10 HDDs in, so is often quite I/O loaded).

Cheers,

Ian