autotrim = on | off
Automatic TRIM happens continuously in the background and operates solely on recently freed blocks (ms_trim not ms_allocatable). ... While the automatic TRIM process is highly effective it is more likely than a manual TRIM to encounter tiny ranges. Ranges less than or equal to 'zfs_trim_extent_bytes_min' (32k) are considered too small to efficiently TRIM and are skipped. This means small amounts of freed space may not be automatically trimmed.