Add Iohint in cephfs
This is the plan of BP "Add Iohint in Cephfs"(http://tracker.ceph.com/projects/ceph/wiki/Add_IOhint_in_CephFS).
By the following steps to implement this BP
a:add iohint in mds, mostly for mds journal. Like osd journal, most journal data don't read again and will overwrite. So w/ DONTNEED is make sense.
b:add iohint mount option. Different sub directory have different purpose. For example, one directory don't need cache data.
c:in cephfs client(kernel/client) add iohint. Hope the the posix_fadvise can effect the whole ceph cluster rather than client kernel page cache.
d:Maybe add iohint in cephfs repair.(Need review code).