Have an introductory README.md that makes sense
Skip the "how do we build" for now.
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Hacktree
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========
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Problem statement
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-----------------
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Hacking on the core operating system is painful - this includes most
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of GNOME from upower and NetworkManager up to gnome-shell. I want a
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system that matches these requirements:
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0. Does not disturb your existing OS
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1. Is not terribly slow to use
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2. Shares your $HOME - you have your data
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3. Allows easy rollback
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4. Ideally allows access to existing apps
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Comparison with existing tools
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------------------------------
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- Virtualization
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Fails on points 2) and 3).
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- Rebuilding OS packages
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Fails on points 1) and 4). Is also just very annoying.
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- "sudo make install"
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Now your system is in an undefined state - it's very possble left over files here
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will come back later to screw you.
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- jhbuild + OS packages
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The state of the art in GNOME - but can only build non-root things -
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this means you can't build NetworkManager, and thus are permanently
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stuck on whatever the distro provides.
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Who is hacktree for?
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------------------------------
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First - operating system developers and testers. I specifically keep
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a few people in mind - Dan Williams and Eric Anholt, as well as myself
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obviously. For Eric Anholt, a key use case for him is being able to
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try out the latest gnome-shell, and combine it with his work on Mesa,
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and see how it works/performs - while retaining the ability to roll
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back if one or both breaks.
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The rollback concept is absolutely key for shipping anything to
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enthusiasts or knowledable testers. With a system like this, a tester
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can easily perform a local rollback - something just not well
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supported by dpkg/rpm. (Why not Conary? AIUI Conary is targeted at
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individual roots, so while you could roll back a given root, it would
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use significantly more disk space than hacktree)
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Also, distributing operating system trees (instead of packages) gives
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us a sane place to perform automated QA **before** we ship it to
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testers. We should never be wasting these people's time.
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Even better, this system would allow testers to bisect across
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operating system builds efficiently.
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The core idea - chroots
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-----------------------
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chroots are the original lightweight "virtualization". Let's use
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them. So basically, you install a mainstream distribution (say
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Debian). It has a root filesystem with a regular layout, /etc, /usr,
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/lib etc.
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Now, what we can do is have a system that installs chroots, like:
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/gnomeos/root-3.0-opt/{usr,etc,var,...}
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/gnomeos/root-3.2-opt/{usr,etc,var,...}
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These live in the same root filesystem as your regular distribution
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(Note though, the root partition should be reasonably sized, or
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hopefully you've used just one big partition).
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You should be able to boot into one of these roots. Since hacktree
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lives inside a distro created partition, a tricky part here is that we
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need to know how to interact with the installed distribution's grub.
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This is an annoying but tractable problem.
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Hacktree will allow efficiently parallel installing and downloading OS
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builds.
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An important note here is that we explicitly link /home in each root
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to the real /home. This means you have your data. This also implies
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we share uid/gid, so /etc/passwd will have to be in sync. Probably
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what we'll do is have a script to pull the data from the "host" OS.
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Making this efficient
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---------------------
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One of the first things you'll probably ask is "but won't that use a
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lot of disk space"? Indeed, it will, if you just unpack a set of RPMs
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or .debs into each root.
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Besides chroots, there's another old Unix idea we can take advantage
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of - hard links. These allow sharing the underlying data of a file,
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with the tradeoff that changing any one file will change all names
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that point to it. This mutability means that we have to either:
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1. Make sure everything touching the operating system breaks hard links
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This is probably tractable over a long period of time, but if anything
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has a bug, then it corrupts the file effectively.
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2. Make the core OS read-only, with a well-defined mechanism for mutating
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under the control of hacktree.
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I chose 2.
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A userspace content-addressed versioning filesystem
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---------------------------------------------------
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At its very core, that's what hacktree is. Just like git. If you
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understand git, you know it's not like other revision control systems.
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git is effectively a specialized, userspace filesystem, and that is a
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very powerful idea.
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At the core of git is the idea of "content-addressed objects". For
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background on this, see <http://book.git-scm.com/7_how_git_stores_objects.html>
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Why not just use git? Basically because git is designed mainly for
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source trees - it goes to effort to be sure it's compressing text for
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example, under the assumption that you have a lot of text. Its
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handling of binaries is very generic and unoptimized.
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In contrast, hacktree is explicitly designed for binaries, and in
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particular one type of binary - ELF executables (or it will be once we
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start using bsdiff).
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Another big difference versus git is that hacktree uses hard links
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between "checkouts" and the repository. This means each checkout uses
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almost no additional space, and is *extremely* fast to check out. We
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can do this because again each checkout is designed to be read-only.
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So we mentioned above the
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/gnomeos/root-3.0-opt
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/gnomeos/root-3.2-opt
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There is also a "repository" that looks like this:
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.ht/objects/17/a95e8ca0ba655b09cb68d7288342588e867ee0.file
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.ht/objects/17/68625e7ff5a8db77904c77489dc6f07d4afdba.meta
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.ht/objects/17/cc01589dd8540d85c0f93f52b708500dbaa5a9.file
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.ht/objects/30
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.ht/objects/30/6359b3ca7684358a3988afd005013f13c0c533.meta
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.ht/objects/30/8f3c03010cedd930b1db756ce659c064f0cd7f.meta
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.ht/objects/30/8cf0fd8e63dfff6a5f00ba5a48f3b92fb52de7.file
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.ht/objects/30/6cad7f027d69a46bb376044434bbf28d63e88d.file
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Each object is either metadata (like a commit or tree), or a hard link
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to a regular file.
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Atomic upgrades, rollback
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-------------------------
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Hacktree is designed to atomically swap operating systems - such that
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during an upgrade and reboot process, you either get the full new
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system, or the old one. There is no "Please don't turn off your
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computer". We do this by simply using a symbolic link like:
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/gnomeos -> /gnomeos-e3b0c4429
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Where /gnomeos-e3b0c4429/ has the full regular filesystem tree with
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usr/ etc/ directories as above. To upgrade or rollback (there is no
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difference internally), we simply check out a new tree into
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/gnomeos-b90ae4763 for example, then swap the symbolic link, then
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remove the old tree.
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Configuration Management
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------------------------
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By now if you've thought about this problem domain before, you're wondering
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about configuration management. In other words, if the OS is read only,
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how do I edit /etc/sudoers?
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Well, have you ever been a system administrator on a zypper/yum
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system, done an RPM update, which then drops .rpmnew files in your
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/etc/ that you have to go and hunt for with "find" or something, and
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said to yourself, "Wow, this system is awesome!!!" ? Right, that's
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what I thought.
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Configuration (and systems) management is a tricky problem, and I
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certainly don't have a magic bullet. However, one large conceptual
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improvement I think is defaulting to "rebase" versus "merge".
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This means that we won't permit direct modification of /etc - instead,
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you HAVE to write a script which accomplishes your goals. To generate
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a tree, we check out a new copy, then run your script on top.
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If the script fails, we can roll back the update, or drop to a shell
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if interactive.
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What about "packages"?
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----------------------
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Basically I think they're a broken idea. There are several different
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classes of things that demand targeted solutions:
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1. Managing and upgrading the core OS (hacktree)
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2. Managing and upgrading desktop applications (gnome-shell, glick?)
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3. System extensions - these are arbitrary RPMs like say the nVidia driver.
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We apply them after constructing each root. Media codecs also fall
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into this category.
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How one might install say Apache on top of hacktree is an open
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question - I think it probably makes sense honestly to ship services
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like this with no configuration - just the binaries. Then admins can
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do whatever they want.
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Downloads
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---------
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I'm pretty sure hacktree should be significantly better than RPM with
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deltarpms. Note we only download changed objects. This means that if
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OpenOffice is rebuilt and just the binary changes, but no data files,
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we don't redownload ANY of that data. And bsdiff is supposedly very
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good for binaries.
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Upstream branches
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----------------
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Note that this system will make it easy to have multiple *upstream* roots too.
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For example, something like:
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- builds
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A filesystem tree generated after every time an artifact is built.
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- fastqa
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After each root is built, a very quick test suite is run in it;
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probably this is booting to GDM. If that works, a new commit is
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made here. Hopefully we can get fastqa under 2 minutes.
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- dailyqa
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Much more extensive tests, let's say they take 24 hours.
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RPM Compatibility
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-----------------
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We should be able to install LSB rpms. This implies providing "rpm".
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The tricky part here is since the OS itself is not assembled via RPMs,
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we need to fake up a database of "provides" as if we were. Even
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harder would be maintaining binary compatibilty with any arbitrary
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%post scripts that may be run.
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Note these RPMs act like local configuration - they would be
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reinstalled every time you switch roots.
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Other systems
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-------------
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I've spent a long time thinking about this problem, and here are some
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of the other possible solutions out there I looked at, and why I
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didn't use them:
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- Git: <http://git-scm.com/>
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Really awesome, and the core inspiration here. But like I mentioned
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above, not at all designed for binaries - we can make different tradeoffs.
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- bup: <https://github.com/apenwarr/bup>
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bup is cool. But it shares the negative tradeoffs with git, though it
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does add positives of its own. It also inspired me.
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- NixOS: <http://nixos.org/>
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The NixOS people have a lot of really good ideas, and they've definitely
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thought about the problem space. However, their approach of checksumming
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all inputs to a package is pretty wacky. I don't see the point, and moreover
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it uses gobs of disk space.
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- Conary: <http://wiki.rpath.com/wiki/Conary:Updates_and_Rollbacks>
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If rpm/dpkg are like CVS, Conary is closer to Subversion. It's not
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bad, but hacktree is better than it for the exact same reasons git
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is better than Subversion.
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- Jhbuild: <https://live.gnome.org/Jhbuild>
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What we've been using in GNOME, and has the essential property of allowing you
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to "fall back" to a stable system. But hacktree will blow it out of the water.
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