this from fs(5):
The fs_minfree element gives the minimum acceptable percentage of file
     system blocks that may be free.  If the freelist drops below this level
     only the super-user may continue to allocate blocks.  The fs_minfree ele‐
     ment may be set to 0 if no reserve of free blocks is deemed necessary,
     however severe performance degradations will be observed if the file sys‐
     tem is run at greater than 90% full; thus the default value of fs_minfree
     is 10%.

The element fs_optim specifies whether the file system should try to min‐
     imize the time spent allocating blocks, or if it should attempt to mini‐
     mize the space fragmentation on the disk.  If the value of fs_minfree
     (see above) is less than 10%, then the file system defaults to optimizing
     for space to avoid running out of full sized blocks.  If the value of
     minfree is greater than or equal to 10%, fragmentation is unlikely to be
     problematical, and the file system defaults to optimizing for time.


this from newfs(8):
     -m free-space
             The percentage of space reserved from normal users; the minimum
             free space threshold.  The default value used is defined by
             MINFREE from <ufs/ffs/fs.h>, currently 8%.  See tunefs(8) for
             more details on how to set this option.

-o optimization
             (space or time).  The file system can either be instructed to try
             to minimize the time spent allocating blocks, or to try to mini‐
             mize the space fragmentation on the disk.  If the value of min‐
             free (see above) is less than 8%, the default is to optimize for
             space; if the value of minfree is greater than or equal to 8%,
             the default is to optimize for time.  See tunefs(8) for more
             details on how to set this option.

this from tunefs:
     -m minfree
             Specify the percentage of space held back from normal users; the
             minimum free space threshold.  The default value used is 8%.
             Note that lowering the threshold can adversely affect perfor‐
             mance:
     
             ·   Settings of 5% and less force space optimization to always be
                 used which will greatly increase the overhead for file
                 writes.
             
             ·   The file system's ability to avoid fragmentation will be
                 reduced when the total free space, including the reserve,
                 drops below 15%.  As free space approaches zero, throughput
                 can degrade by up to a factor of three over the performance
                 obtained at a 10% threshold.
             
             If the value is raised above the current usage level, users will
             be unable to allocate files until enough files have been deleted
             to get under the higher threshold.