ceph: capability management

ceph: capability management

The Ceph metadata servers control client access to data by issuing
capabilities granting clients permission to read and/or write to OSDs
(storage nodes).  Each capability consists of a set of bits indicating
which operations are allowed.

In the case of an EXCL (exclusive) or WR capabilities, the client is
allowed to change inode attributes (e.g., file size, mtime), noting
it's dirty state in the ceph_cap, and asynchronously flush that
metadata change to the MDS.

In the event of a conflicting operation (perhaps by another client),
the MDS will revoke the conflicting client capabilities.

A subset of capabilities (termed 'rdcaps') are opportunistically
issued by the MDS to grant the a client read lease on metadata, and
time out automatically.  Other capabilities (write capabilities, and
those that are "wanted" due to an open file) are explicitly released.