zfs: Emit warning upon use of legacy sysctl

[emaste]

Summary

PR:             266374

[emaste]

sys/contrib/openzfs/module/os/freebsd/zfs/sysctl_os.c

754

AFAICT min_auto_ashift/max_auto_ashift did not use arg1/arg2 before, I imagine this was just done without being required.

[jlduran]

sys/contrib/openzfs/module/os/freebsd/zfs/sysctl_os.c

167

I would rather take an old, new tuple with the full sysctl instead, as it is not a 1:1 translation:

old	new
vfs.zfs.arc_max	vfs.zfs.arc.max
vfs.zfs.arc_min	vfs.zfs.arc.min
vfs.zfs.arc_free_target	vfs.zfs.arc.free_target
vfs.zfs.arc_no_grow_shift	vfs.zfs.arc.no_grow_shift
vfs.zfs.max_auto_ashift	vfs.zfs.vdev.max_auto_ashift
vfs.zfs.min_auto_ashift	vfs.zfs.vdev.min_auto_ashift

[jlduran]

sys/contrib/openzfs/module/os/freebsd/zfs/sysctl_os.c

167

Sans vfs.zfs..

[emaste]

correct mapping per @jlduran

[jlduran]

Sorry to bother, but I have the following doubt:
There are still a few renamed LEGACY sysctls that were removed (and reverted):

old	new
vfs.zfs.l2arc_write_max	vfs.zfs.l2arc.write_max
vfs.zfs.l2arc_write_boost	vfs.zfs.l2arc.write_boost
vfs.zfs.l2arc_headroom	vfs.zfs.l2arc.headroom
vfs.zfs.l2arc_headroom_boost	vfs.zfs.l2arc.headroom_boost
vfs.zfs.l2arc_feed_secs	vfs.zfs.l2arc.feed_secs
vfs.zfs.l2arc_feed_min_ms	vfs.zfs.l2arc.feed_min_ms
vfs.zfs.l2arc_noprefetch	vfs.zfs.l2arc.noprefetch
vfs.zfs.l2arc_feed_again	vfs.zfs.l2arc.feed_again
vfs.zfs.l2arc_norw	vfs.zfs.l2arc.norw

[emaste]

There are still a few renamed LEGACY sysctls that were removed (and reverted):

Indeed, unfortunately they're not handled by a SYSCTL_PROC and there's not a convenient/trivial way to add a warning.

SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, l2arc_write_max,
        CTLFLAG_RWTUN, &l2arc_write_max, 0,
        "Max write bytes per interval (LEGACY)");

I could add a SYSCTL_UQUAD wrapper that emits a warning but I didn't think it was worth doing. The auto_ashift sysctl was important to warn on for a few reasons:

• Setting it has a persistent effect, because it applies for the lifetime of a newly created vdev.
• The installer set it so it existed on a great number of systems.

In comparison I think the l2arc knobs are tuning parameters that, if legacy names are ignored, may have a performance impact but not a functional or persistent effect.

[jlduran]

In D53033#1213779, @emaste wrote:

There are still a few renamed LEGACY sysctls that were removed (and reverted):

Indeed, unfortunately they're not handled by a SYSCTL_PROC and there's not a convenient/trivial way to add a warning.

SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, l2arc_write_max,
        CTLFLAG_RWTUN, &l2arc_write_max, 0,
        "Max write bytes per interval (LEGACY)");

I could add a SYSCTL_UQUAD wrapper that emits a warning but I didn't think it was worth doing. The auto_ashift sysctl was important to warn on for a few reasons:

• Setting it has a persistent effect, because it applies for the lifetime of a newly created vdev.
• The installer set it so it existed on a great number of systems.

In comparison I think the l2arc knobs are tuning parameters that, if legacy names are ignored, may have a performance impact but not a functional or persistent effect.

I agree.
It would be ideal if someone from ZFS could review this patch, but in the meantime I'll approve the changes.

[jlduran]

When/if this patch lands, I think I should also land D34598 (in separate chunks) as well.

[emaste]

In D53033#1213831, @jlduran wrote:

When/if this patch lands, I think I should also land D34598 (in separate chunks) as well.

Oh yeah, I forgot about that patch. I've just clicked accept on it now.

[emaste]

It would be ideal if someone from ZFS could review this patch, but in the meantime I'll approve the changes.

Indeed. I have pushed this now to support the 15.0 release process but would still appreciate post-commit ZFS review.