metaslab: expose zfs_metaslab_condense_pct and zfs_metaslab_sm_blksz_* on Linux

[chrislongros]

Motivation and Context

zfs_condense_pct controls how aggressively per-metaslab on-disk space maps are rewritten in compact form.
zfs_metaslab_sm_blksz_with_log controls Block size for space map in pools with log space map enabled.
They are already tunables on FreeBSD but had no Linux equivalents.

Description

• Register zfs_condense_pct and zfs_metaslab_sm_blksz_* with ZFS_MODULE_PARAM in module/zfs/metaslab.c, alongside the existing metaslab tunables.
• Add a matching entry in man/man4/zfs.4.

How Has This Been Tested?

Tested on an Arch Linux VM with the patched module.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.

[amotin]

Good catch. But I see there also zfs_metaslab_sm_blksz_*, which are also not OS-specific. And may be while at it zfs_condense_pct could be renamed to not collide with zpool condense in #16747.

[chrislongros]

Good catch. But I see there also zfs_metaslab_sm_blksz_*, which are also not OS-specific. And may be while at it zfs_condense_pct could be renamed to not collide with zpool condense in #16747.

What about zfs_spacemap_condense_pct as a new alternative name?

[amotin]

What about zfs_spacemap_condense_pct as a new alternative name?

I'd think all 3 should go same way for uniformity. We don't have "spacemap" node, but we do have "metaslab" node, where the other two are living now.