Ceph » rbd

The ceph messenger supports a pagelist for its data in
addition to the trail (which is also a pagelist).

The trail is only used by the osd client in support of
object class method calls, and was added to allow the
method to provide input parameters to the method and
supply output data in a different data field (the message's
pages field is used for output in this case).

In reworking the messenger code to allow multiple data
sources it has become clear that we can now eliminate the
trail. An osd client now records input and output
data for a request separately, so it's no longer necessary
to use distinct fields in the message to describe the
locations for incoming and outgoing data (i.e., the
pages and trail).

Furthermore, there is in place a pagelist in a message
that's used in the file system code. So what had been
put into the trail can instead just be stored as a
"normal" pagelist for the outgoing data of the request.

I'm calling this a bug because that trail notion made
some code more complicated than it needed to be. :)