hammer2 - Limit bulkfree cpu and SSD I/O
* Limit resource utilization when running bulkfree. The default is 5000
tps (meta-data blocks per second) and can be changed via the
vfs.hammer2.bulkfree_tps sysctl.
* Designed primarily to limit cpu utilization when meta-data is cached,
and to limit SSD utilization otherwise. This feature generally cannot
be used to limit HDD utilization because it cannot currently distinguish
between cached and uncached I/O. Setting a low a number to accomodate
a HDD will cause bulkfree to take way too long to run.