kernel - SWAP CACHE part 14/many - Add more features, man page

kernel - SWAP CACHE part 14/many - Add more features, man page

* Implement write clustering.  Swapcache attempts to cluster writes
  for optimal matching between swap and the buffer cache.  This
  also reduces the IOPS for writes by a factor 16.  The SSD should
  be able to do write combining and erasing more optimally as well.

* Add vm.swapcache.minburst

  This ensures that curburst is allowed to recover sufficiently that
  a nice good write burst can be done, once curburst hits 0.  Otherwise
  swapcache winds up doing tiny bursts which tend to fragment the cache.

* Add vm.swapcache.maxfilesize

  If set to non-zero prevents swapcache from caching files larger than
  the specified size.  That is, swapcache will only cache smaller files.
  This is experimental because there are issues caching small files
  anyway (the vnodes get recycled too quickly).

* Allow vm.swapcache.curburst to be manually set larger than
  vm.swapcache.maxburst, so the initial load-in can be different
  from the maximum reburst.

* Adjust the code which deals with write errors on swap to ensure
  that the backing store is destroyed (because it isn't a clean copy).