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.  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.  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 patch stack in to the repo
    we expect several benefits.  One of the most important will be the
    ability to easily work on the patch stack with a distributed developer
    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      t/LAST                          [PATCH] LAST
        t/feature-commit-cb             [PATCH] zfs commit callbacks
        t/fix-clock-wrap                [PATCH] fix clock wrap
        t/fix-dnode-cons                [PATCH] fix dnode constructor
        ...

By convention all TopGit branches are prefixed with 't/', and the Linux
ZFS repo also introduces the convention that the top most development 
branch be called 't/LAST".  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.  While this project only expresses
a single dependency per branch TopGit implements dependencies as a DAC just
like Git.  To see the dependencies you will need to use the --graphviz
option and pipe the result to dot for display.  The following command while
long 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 if they exist.

        > tg summary --graphviz | dot -Txlib -Nfontsize=8 -Eminlen=0.01 \
        -Grankdir=LR -Nheight=0.3 -Nwidth=2 -Nfixedsize=true

========================= 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.  Here is an example:

        > git checkout t/feature-commit-cb  # Checkout the proper branch
        > ...update branch...               # Update the branch
        > git commit -a                     # Commit your changes
        > git checkout t/LAST               # Checkout the LAST 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 is a little more complicated.  When adding
a new branch to the end of the patch graph things are pretty easy and TopGit
does all the work.  However, I expect out common case to be adding patches
to the middle of the graph.  TopGit will allow you to do this but you must
be careful to manually update the dependency information in the .topdeps
file.

        > git co t/existing-topic-branch    # Checkout the branch to add after
        > tg create t/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 t/dependent-topic-branch   # Checkout dependent branch
        > ...update .topdeps...             # Manually update dependencies
        > git commit -a                     # Commit your changes
        > tg update                         # TopGit update
        > git checkout t/LAST               # Checkout the LAST branch
        > tg update                         # Recursively merge in new branch

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

============================ 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.