There are now two multiqueues for in cache blocks.  A clean one and a
dirty one.

writeback_work comes from the dirty one.  Demotions come from the clean
one.

There are two benefits:
- Performance improvement, since demoting a clean block is a noop.
- The cache cleans itself when io load is light.
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NHeinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

A simple cache policy that writes back all data to the origin.

This is used to decommission a dm cache by emptying it.
Signed-off-by: NHeinz Mauelshagen <mauelshagen@redhat.com>
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

A cache policy that uses a multiqueue ordered by recent hit
count to select which blocks should be promoted and demoted.
This is meant to be a general purpose policy.  It prioritises
reads over writes.
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>