mirror_zfs/GIT

=========================== WHY USE GIT+TOPGIT? ==========================

Three major concerns were on our mind when setting up this project.

  o First we needed to structure the project in such a way that it would be
    easy to rebase all of our changes on the latest official ZFS release
    from Sun.  We absolutely need to be able to benefit from the upstream
    improvements and not get locked in to an old version of the code base.

  o Secondly, we wanted to be able to easily manage our changes in terms
    of a patch stack or graph.  This allows us to easily isolate specific
    changes and push them upstream for inclusion.  It also allows us to 
    easily update or drop specific changes based on what occurs upstream.

  o Thirdly we needed our DVCS to be integrated with the management of this
    patch stack or graph.  We have tried other methods in the past such as
    SVN+Quilt but have found managing the patch stack becomes cumbersome.
    By using Git+TopGit to more tightly integrate our patches in to the repo
    we expect several benefits.  One of the most important will be the
    ability to easily work on the patch's with a distributed development
    team, additionally the repo can track patch history, and we can utilize
    Git to merge patches and resolve conflicts.

TopGit is designed to specifically address these concerns by providing 
tools to simplify the handling of large numbers of interdependent topic 
branches.  When using a TopGit aware repo every topic branch represents
a 'patch' and that branch references its dependent branches.  The union
of all these branches is your final source base.

========================= SETTING UP GIT+TOPGIT ==========================

First off you need to install a Git package on your system.  For my
purposes I have been working on a RHEL5 system with git version 1.5.4.5
installed and it has been working well.  You will also need to go get
the latest version of TopGit which likely is not packaged nicely so you
will need to build it from source.  You can use Git to clone TopGit
from the official site here and your all set:

        > git clone http://repo.or.cz/w/topgit.git
        > make
        > make install    # Default installs to $(HOME)

========================== TOPGIT AND ZFS ================================

One you have Git and TopGit installed you will want to clone a copy of
the Linux ZFS repo.  While this project does not yet have a public home
it hopefully will some day.  In the meanwhile if you have VPN access to
LLNL you can clone the latest official repo here.  Cloning a TopGit
controlled repo is very similar to cloning a normal Git repo, but you
need to remember to use 'tg remote' to populate all topic branches.

        > git clone http://eris.llnl.gov/git/zfs.git zfs
        > cd zfs
        > tg remote --populate origin

Now that you have the Linux ZFS repo the first thing you will probably 
want to do is have a look at all the topic branches.  TopGit provides
a summary command which shows all the branches and a brief summary for
each branch obtained from the .topmsg files.

        > tg summary
 0      feature-branch                  [PATCH] feature-branch
        feature-commit-cb               [PATCH] feature commit cb
        feature-zap-cursor-to-key       [PATCH] feature zap cursor to key
        ...

By convention all TopGit branches are usually prefixed with 't/', however
I have chosen not to do this for simplicity.  A different convention I have
adopted is to tag the top most TopGit branch as 'top' for easy reference.
This provides a consistent label to be used when you need to reference the
branch which contains the union of all topic branches.

One thing you may also notice about the 'tg summary' command is it does
not show the branches in dependent order.  This is done because TopGit allows
each branch to express multiple dependencies as a DAC.  Initially this seemed
like an added complication which I planned to avoid by just implementing a
stack using the graph.   However, this ended up being problematic because
with a stack when a change was made to a branch near the base, it was a
very expensive operation to merge the change up to the top of the stack.
By defining the dependencies as a graph it is possible to keep the depth
much shallower thus minimizing the merging.  It has also proved insightful
as to each patches actual dependencies.

To see the dependencies you will need to use the --graphviz option and pipe
the result to dot for display.  The following command works fairly well for
me.  Longer term it would be nice to update this option to use a preferred
config options stored in the repo.

        > tg summary --graphviz | dot -Txlib -Nfontsize=8

========================= UPDATING A TOPIC BRANCH ========================

Updating a topic branch in TopGit is a pretty straight forward but there
are a few rules you need to be aware of.  The basic process involves 
checking out the relevant topic branch where the changes need to be made,
making the changes, committing the changes to the branch and then merging
those changes in to dependent branches.  TopGit provides some tools to make
this pretty easy, although it may be a little sluggish depending on how many
dependent branches are impacted by the change.  Here is an example:

        > git checkout modify-topic-branch  # Checkout the proper branch
        > ...update branch...               # Update the branch
        > git commit -a                     # Commit your changes
        > git checkout top                  # Checkout the top branch
        > tg update                         # Recursively merge in new branch

Assuming you change does not introduce any conflicts your done.  All branches
were dependent on your change will have had the changed merged in.  If your
change introduced a conflict you will need to resolve the conflict and then
continue on with the update.

========================== ADDING A TOPIC BRANCH =========================

Adding a topic branch in TopGit can be pretty straight forward.  If your 
adding a non-conflicting patch in parallel with other patches of the same
type, then things are pretty easy and TopGit does all the work.

        > git co existing-topic-branch      # Checkout the branch to add after
        > tg create new-topic-branch        # Create a new topic branch
        > ...update .topmsg...              # Update the branch message
        > ...create patch...                # Update with your changes
        > git commit -a                     # Commit your changes
        > git co dependent-topic-branch     # Checkout dependent branch
        > tg depend add new-topic-branch    # Update dependencies 
        > git checkout top                  # Checkout the top branch
        > tg update                         # Recursively merge in new branch

If you need to add your patch in series with another change things are
a little more complicated.  In this case TopGit does not yet support removing
dependencies so you will need to do it by hand, as follows.

        > git co existing-topic-branch      # Checkout the branch to add after
        > tg create new-topic-branch        # Create a new topic branch
        > ...update .topmsg...              # Update the branch message
        > ...create patch...                # Update with your changes
        > git commit -a                     # Commit your changes
        > git co dependent-topic-branch     # Checkout dependent branch
        > ...update .topdeps...             # Manually update dependencies
        > git commit -a                     # Commit your changes
        > tg update                         # TopGit update
        > git checkout top                  # Checkout the top branch
        > tg update                         # Recursively merge in new branch

Once your done, I find it is a good idea view the repo using the  
'tg summary --graphviz' command and verify the updated dependency graph.

========================= REMOVING A TOPIC BRANCH ========================

Removing a topic branch in TopGit is also currently not very easy.  To remove
a dependent branch the basic process is to commit a patch which reverts all
changes on the branch.  Then that reversion must be merged in to all dependent
branches, the dependencies manually updated and finally the branch removed.
If the branch is not empty you will not be able to remove it.

        > git co delete-topic-branch        # Checkout the branch to delete
        > tg patch | patch -R -p1           # Revert all branch changes
        > git commit -a                     # Commit your changes
        > git checkout top                  # Checkout the top branch
        > tg update                         # Recursively merge revert
        > git co dependent-topic-branch     # Checkout dependent branch
        > ...update .topdeps...             # Manually update dependencies
        > git commit -a                     # Commit your changes
        > tg delete delete-topic-branch     # Delete empty topic branch

Once your done, I find it is a good idea view the repo using the  
'tg summary --graphviz' command and verify the updated dependency graph.

============================ TOPGIT TODO =================================

TopGit is still a young package which seems to be under active development
by its author.  It provides the minimum set of commands needed but there
are clearly areas which simply have not yet been implemented.  My short
list of features includes:

  o 'tg summary --deps', option to display a text version of the topic
    branch dependency DAC.

  o 'tg depend list', list all topic branch dependencies.

  o 'tg depend delete', cleanly remove a topic branch dependency.

  o 'tg create', cleanly insert a topic branch in the middle
    of the graph and properly take care updating all dependencies.

  o 'tg delete', cleanly delete a topic branch in the middle
    of the graph and properly take care updating all dependencies.