Re: ZFS tunable parameters

[Sotiris Lamprinidis<sotiris%lamprinidis.com@localhost>]

> From: mlelstv%serpens.de@localhost (Michael van Elst)
> Date: Wed, 24 Jun 2026 16:28:25 -0000 (UTC)
>
> Yes. I'm using a similar patch but haven't found what knobs were
> actually useful. But I have some other adjustments to help
> memory usage.

That's great, I'm going to try your adjustments and see how they help.

One comment, arc_average_blocksize and comressed_arc_enabled are read-only on
FreeBSD (RDTUN). I'm also wondering, in sysctl_createv do we actually have to
pass sizeof(var) in newlen for primary types?

I am running some benchmarks on three systems, with 12, 8, and 2 gigs of ram,
running MKUPDATE=no build while periodically running find -type f on all zfs
filesystems and Greg's stress-zfs. I tried halving arc_max and _min for testing.

In general, when running find, the metadata usage explodes irregardless of
arc_max or meta_limit [2;3], but [1] setting arc_max works. And In all cases
c is respecting the c_max limit.


> From: Greg Troxel <gdt%lexort.com@localhost>
> Date: Wed, 24 Jun 2026 13:44:51 -0400
>
> It would be great to at least get sysctls to control arc size into the
> tree, vs a number of copies uncommitted.

I think arc_max, arc_min, and arc_free_target could be useful.

There is a difference between Michael's style, rewriting entire functions for
NetBSD, or my approach with many #ifdefs, I guess mine will be shorter but
not sure what is better.

S.

[1]: https://lamprinidis.com/pub/zfs/2g_sys.png
[2]: https://lamprinidis.com/pub/zfs/8g_sys.png
[3]: https://lamprinidis.com/pub/zfs/12g_sys.png