Restore FreeBSD sysctl processing for arc.min and arc.max

[allanjude]

Motivation and Context

Before OpenZFS 2.0, trying to set the FreeBSD sysctl vfs.zfs.arc_max
to a disallowed value would return an error.
Since the switch, it instead only generates WARN_IF_TUNING_IGNORED

Also lost, was the ability to set vfs.zfs.arc_max to a value less
than the default vfs.zfs.arc_min at boot time. Restore this as well.

Description

How Has This Been Tested?

BEFORE:

loader.conf:

vfs.zfs.arc.max=402653184

After boot, the values on the running system (notice c_max is higher than requested by the user):

vfs.zfs.arc.max: 402653184
vfs.zfs.arc.min: 0
kstat.zfs.misc.arcstats.c_max: 33122279424
kstat.zfs.misc.arcstats.c_min: 1068625664

Attempting to set the arc.max to less than arc.min:
# sysctl vfs.zfs.arc.max=402653184

vfs.zfs.arc.max: 402653184 -> 402653184
[on the console only] kernel: Solaris: WARNING: ignoring tunable zfs_arc_max (using 33122279424 instead)

Resulting values (again, notice that the users request was ignored, and outside of inspecting dmesg, they have no way of knowing):

vfs.zfs.arc.max: 402653184
kstat.zfs.misc.arcstats.c_max: 33122279424

AFTER:

loader.conf:

vfs.zfs.arc.max=402653184

After boot, the values on the running system:

vfs.zfs.arc.max: 402653184
vfs.zfs.arc.min: 0
kstat.zfs.misc.arcstats.c_max: 402653184
kstat.zfs.misc.arcstats.c_min: 201326592

Attempting to set the arc.max to less or equal to arc.min:
# sysctl vfs.zfs.arc.max=201326592

vfs.zfs.arc.max: 402653184
sysctl: vfs.zfs.arc.max=201326592: Invalid argument

Resulting values:

vfs.zfs.arc.max: 402653184
kstat.zfs.misc.arcstats.c_max: 402653184

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[allanjude]

This is causing arcstats_runtime_tuning to fail, as it tries to set vfs.zfs.arc.min back to 0, which is less than the minimum, 64MB.

Do we want to support 0 as a special value? what would it mean, currently, it would just be ignored by arc_tuning_update() because it fails the first condition

[tonynguien]

This is causing arcstats_runtime_tuning to fail, as it tries to set vfs.zfs.arc.min back to 0, which is less than the minimum, 64MB.

Do we want to support 0 as a special value? what would it mean, currently, it would just be ignored by arc_tuning_update() because it fails the first condition

I believe the current behavior is that a 0 value implies tunable is not set and arc_c_min remains as-is.

I think removing the below sysctl update (for both param_set_arc_min and param_set_arc_max) would restore the behavior.

	/* Update the sysctl to the tuned value */
	zfs_arc_min = arc_c_min;

[allanjude]

I believe the current behavior is that a 0 value implies tunable is not set and arc_c_min remains as-is.

I think removing the below sysctl update (for both param_set_arc_min and param_set_arc_max) would restore the behavior.

	/* Update the sysctl to the tuned value */
	zfs_arc_min = arc_c_min;

I think we still want the update, to restore the older illumos ZFS FreeBSD behaviour, but, we can make it conditional on 'val' not being 0, to resolve the test and the expected '0' functionality.

[tonynguien]

I believe the current behavior is that a 0 value implies tunable is not set and arc_c_min remains as-is.
I think removing the below sysctl update (for both param_set_arc_min and param_set_arc_max) would restore the behavior.

	/* Update the sysctl to the tuned value */
	zfs_arc_min = arc_c_min;

I think we still want the update, to restore the older illumos ZFS FreeBSD behaviour, but, we can make it conditional on 'val' not being 0, to resolve the test and the expected '0' functionality.

Ah, OK. That works.

[tonynguien]

LGTM.

[ghost]

• defined MIN_ARC_MAX
• rebased