NOTE: it's quite likely you don't need to know any of this.  but since
      you're here...

NOTE: there is currently no way to change a class's replication policy,
      except modifying the database, like so:

      ALTER TABLE class
          SET replpolicy='Union(MultipleHosts(3), OnDevice(13))'
          WHERE dmid=3 AND classid=1;

      Yes, we know this is sub-optimal.  A mogadm command should be
      done in the future.

About replication policies strings:

  -- if you don't have a colon in a policy, "MogileFS::ReplicationPolicy::"
     is prepended to the class name

  -- if a class doesn't have a replpolicy value, the historical default
     is used:  "MultipleHosts()".  see MogileFS::ReplicationPolicy::MultipleHosts
     for what that does.

  -- a replication policy of "MultipleHosts()" uses the class's "mindevcount"
     column.  but an explicit value, like "MultipleHosts(3)" will ignore the
     configured "mindevcount"

