libcephfs exposes a "snapdiff" API that lets programmatic users query for the list of files changed between two snapshots. This identifies the changed files, but does not identify the changed regions within those files.

RADOS exposes primitives that let you identify the regions which have changed between two snapshots. We should extend the CephFS snapdiff API to additionally let users query for the file diff, by querying the underlying RADOS objects. This should dramatically reduce the amount of data read and transmitted when trying to maintain consistent copies over time.