Remove GIT notes
These notes describe how to use TopGit with this repo. As of this point TopGit is no longer used so the notes have been removed. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
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Brian Behlendorf
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=========================== WHY USE GIT+TOPGIT? ==========================
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Three major concerns were on our mind when setting up this project.
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o First we needed to structure the project in such a way that it would be
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easy to rebase all of our changes on the latest official ZFS release
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from Sun. We absolutely need to be able to benefit from the upstream
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improvements and not get locked in to an old version of the code base.
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o Secondly, we wanted to be able to easily manage our changes in terms
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of a patch stack or graph. This allows us to easily isolate specific
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changes and push them upstream for inclusion. It also allows us to
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easily update or drop specific changes based on what occurs upstream.
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o Thirdly we needed our DVCS to be integrated with the management of this
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patch stack or graph. We have tried other methods in the past such as
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SVN+Quilt but have found managing the patch stack becomes cumbersome.
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By using Git+TopGit to more tightly integrate our patches in to the repo
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we expect several benefits. One of the most important will be the
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ability to easily work on the patch's with a distributed development
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team, additionally the repo can track patch history, and we can utilize
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Git to merge patches and resolve conflicts.
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TopGit is designed to specifically address these concerns by providing
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tools to simplify the handling of large numbers of interdependent topic
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branches. When using a TopGit aware repo every topic branch represents
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a 'patch' and that branch references its dependent branches. The union
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of all these branches is your final source base.
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========================= SETTING UP GIT+TOPGIT ==========================
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First off you need to install a Git package on your system. For my
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purposes I have been working on a RHEL5 system with git version 1.5.4.5
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installed and it has been working well. You will also need to go get
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the latest version of TopGit which likely is not packaged nicely so you
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will need to build it from source. You can use Git to clone TopGit
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from the official site here and your all set:
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> git clone git://repo.or.cz/topgit.git
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> make
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> make install # Default installs to $(HOME)
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========================== TOPGIT AND ZFS ================================
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One you have Git and TopGit installed you will want to clone a copy of
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the Linux ZFS repo. While this project does not yet have a public home
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it hopefully will some day. In the meanwhile if you have VPN access to
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LLNL you can clone the latest official repo here. Cloning a TopGit
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controlled repo is very similar to cloning a normal Git repo, but you
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need to remember to use 'tg remote' to populate all topic branches.
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> git clone git://eris.llnl.gov/zfs.git
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> cd zfs
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> tg remote --populate origin
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Alternately, if you don't want to worry about using TopGit on your system
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you can simply clone the repo and then checkout the origin/top branch. At
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this point you can simply create a new branch to track your changes. You
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can then feed your patch back to the official repo maintainers to be merged
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in to the proper topic branches.
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> git clone git://eris.llnl.gov/zfs.git
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> cd zfs
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> git checkout origin/top
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> git checkout -b <new_branch_name>
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Now that you have the Linux ZFS repo the first thing you will probably
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want to do is have a look at all the topic branches. TopGit provides
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a summary command which shows all the branches and a brief summary for
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each branch obtained from the .topmsg files.
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> tg summary
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0 feature-branch [PATCH] feature-branch
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feature-commit-cb [PATCH] feature commit cb
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feature-zap-cursor-to-key [PATCH] feature zap cursor to key
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...
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By convention all TopGit branches are usually prefixed with 't/', however
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I have chosen not to do this for simplicity. A different convention I have
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adopted is to tag the top most TopGit branch as 'top' for easy reference.
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This provides a consistent label to be used when you need to reference the
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branch which contains the union of all topic branches.
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One thing you may also notice about the 'tg summary' command is it does
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not show the branches in dependent order. This is done because TopGit allows
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each branch to express multiple dependencies as a DAC. Initially this seemed
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like an added complication which I planned to avoid by just implementing a
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stack using the graph. However, this ended up being problematic because
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with a stack when a change was made to a branch near the base, it was a
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very expensive operation to merge the change up to the top of the stack.
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By defining the dependencies as a graph it is possible to keep the depth
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much shallower thus minimizing the merging. It has also proved insightful
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as to each patches actual dependencies.
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To see the dependencies you will need to use the --graphviz option and pipe
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the result to dot for display. The following command works fairly well for
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me. Longer term it would be nice to update this option to use a preferred
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config options stored in the repo.
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> tg summary --graphviz | dot -Txlib -Nfontsize=8
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========================= UPDATING A TOPIC BRANCH ========================
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Updating a topic branch in TopGit is a pretty straight forward but there
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are a few rules you need to be aware of. The basic process involves
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checking out the relevant topic branch where the changes need to be made,
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making the changes, committing the changes to the branch and then merging
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those changes in to dependent branches. TopGit provides some tools to make
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this pretty easy, although it may be a little sluggish depending on how many
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dependent branches are impacted by the change. Here is an example:
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> git checkout modify-topic-branch # Checkout the proper branch
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> ...update branch... # Update the branch
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> git commit -a # Commit your changes
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> git checkout top # Checkout the top branch
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> tg update # Recursively merge in new branch
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Assuming you change does not introduce any conflicts your done. All branches
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were dependent on your change will have had the changed merged in. If your
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change introduced a conflict you will need to resolve the conflict and then
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continue on with the update.
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========================== ADDING A TOPIC BRANCH =========================
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Adding a topic branch in TopGit can be pretty straight forward. If your
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adding a non-conflicting patch in parallel with other patches of the same
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type, then things are pretty easy and TopGit does all the work.
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> git co existing-topic-branch # Checkout the branch to add after
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> tg create new-topic-branch # Create a new topic branch
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> ...update .topmsg... # Update the branch message
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> ...create patch... # Update with your changes
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> git commit -a # Commit your changes
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> git co dependent-topic-branch # Checkout dependent branch
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> tg depend add new-topic-branch # Update dependencies
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> git checkout top # Checkout the top branch
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> tg update # Recursively merge in new branch
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If you need to add your patch in series with another change things are
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a little more complicated. In this case TopGit does not yet support removing
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dependencies so you will need to do it by hand, as follows.
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> git co existing-topic-branch # Checkout the branch to add after
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> tg create new-topic-branch # Create a new topic branch
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> ...update .topmsg... # Update the branch message
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> ...create patch... # Update with your changes
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> git commit -a # Commit your changes
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> git co dependent-topic-branch # Checkout dependent branch
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> ...update .topdeps... # Manually update dependencies
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> git commit -a # Commit your changes
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> tg update # TopGit update
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> git checkout top # Checkout the top branch
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> tg update # Recursively merge in new branch
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Once your done, I find it is a good idea view the repo using the
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'tg summary --graphviz' command and verify the updated dependency graph.
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========================= REMOVING A TOPIC BRANCH ========================
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Removing a topic branch in TopGit is also currently not very easy. To remove
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a dependent branch the basic process is to commit a patch which reverts all
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changes on the branch. Then that reversion must be merged in to all dependent
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branches, the dependencies manually updated and finally the branch removed.
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If the branch is not empty you will not be able to remove it.
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> git co delete-topic-branch # Checkout the branch to delete
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> tg patch | patch -R -p1 # Revert all branch changes
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> git commit -a # Commit your changes
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> git checkout top # Checkout the top branch
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> tg update # Recursively merge revert
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> git co dependent-topic-branch # Checkout dependent branch
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> ...update .topdeps... # Manually update dependencies
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> git commit -a # Commit your changes
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> tg delete delete-topic-branch # Delete empty topic branch
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Once your done, I find it is a good idea view the repo using the
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'tg summary --graphviz' command and verify the updated dependency graph.
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============================ TOPGIT TODO =================================
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TopGit is still a young package which seems to be under active development
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by its author. It provides the minimum set of commands needed but there
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are clearly areas which simply have not yet been implemented. My short
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list of features includes:
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o 'tg summary --deps', option to display a text version of the topic
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branch dependency DAC.
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o 'tg depend list', list all topic branch dependencies.
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o 'tg depend delete', cleanly remove a topic branch dependency.
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o 'tg create', cleanly insert a topic branch in the middle
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of the graph and properly take care updating all dependencies.
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o 'tg delete', cleanly delete a topic branch in the middle
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of the graph and properly take care updating all dependencies.
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