Some libraries that Ceph uses are incorporated into the build tree
through a technique known as git subtrees. This is an alternative to
git submodules, that is also used in Ceph.

One such library is the dmclock library. Here are some basic notes on
the use of git subtrees.

When a subtree is added to the repo, commands such as these were run
from the top-level ceph directory:

    git subtree add --prefix src/dmclock \
	git@github.com:ceph/dmclock.git master --squash

That essentially brings in a full copy of the library into the
subdirectory src/dmclock, but squashes all the commits into a single
one.

If in time the library is updated and you'd like to bring the updates
in, you could run:

    git subtree pull --prefix src/dmclock \
        git@github.com:ceph/dmclock.git master --squash

WARNINGS

If you'd like to modify the library contained in a subtree it's likely
best to modify it in the library's repo and then pull in the changes
with "git subtree pull ..." (see above).

IMPORTANT: If you modify the library within the ceph tree, then do NOT
combined changes within the subtree and outside the subtree in a
single commit. Each commit should either only contain changes within
the subtree or outside the subtree. That gives you the option to
cleanly push those changes back to the library's repo.

IMPORTANT: If you do a rebase and the commits that are redone include
the commits that encapsulate a "git subtree add ..." or "git subtree
pull ..." you have to be VERY CAREFUL. Those commits lose the prefix
and try to apply the changes to the wrong paths! Instead, you
essentially have to do the rebase interactively, remove the commits
for the subtree add/pull, and manually re-do them between the other
commits. Some developers prefer not to even do a "git rebase ..." and
instead cherry-pick the commits and manually do the subtree
adds/pulls.