Re: ZFS tunable parameters

[Simon Burge <simonb%NetBSD.org@localhost>]

Hi Sotiris,

Sotiris Lamprinidis wrote:

> 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.
>
> Here are the new sysctl values on boot on a 8G system:
>
>     vfs.zfs.arc_min = 392903680
>     vfs.zfs.arc_max = 3143229440
>     vfs.zfs.arc_average_blocksize = 8192
>     vfs.zfs.arc_shrink_shift = 7
>     vfs.zfs.compressed_arc_enable = 1
>     vfs.zfs.arc_meta_limit = 785807360
>     vfs.zfs.arc_meta_min = 0
>     vfs.zfs.arc_free_target = 682

A FreeBSD 13 VM shows these sysctls:

   vfs.zfs.arc_max: 0
   vfs.zfs.arc_min: 0
   vfs.zfs.arc.average_blocksize: 8192
   vfs.zfs.arc.shrink_shift: 0
   vfs.zfs.compressed_arc_enabled: 1
   vfs.zfs.arc.meta_limit: 0               (different name - arc.meta.XXX)
   vfs.zfs.arc.meta_min: 0                 (different name - arc.meta.XXX)
   vfs.zfs.arc_free_target: 5285

One day we'll sync up with FreeBSD/OpenZFS, but in the meantime I
think it's ok to add more knobs that match the current FreeBSD names.
Michael's patch from a follow-up reply introduces a vfs.zfs_arc which is
different again.

At first glance, all the extra #ifdef stuff looks a little messy; I
don't know if that can be somehow done cleaner?  I'll look at your patch
and Michael's patch this weekend to see I can knock together.

Cheers,
Simon.