167 lines
7.1 KiB
HTML
167 lines
7.1 KiB
HTML
<article>
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<h1>fedostree</h1>
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<p>An instance
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of <a href="https://github.com/cgwalters/rpm-ostree">rpm-ostree</a>
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for Fedora. This project takes multiple RPM package sets from
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Fedora, assembles them on the build server side, and stores these
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trees in
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an <a href="http://live.gnome.org/Projects/OSTree">OSTree</a>
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repository. Client systems can them atomically upgrade and switch
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between these trees.
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</p>
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<h3>Trying it out</h3>
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<p>See <a href="#installation">installation</a>.</p>
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<h3>Background</h3>
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<p>Fedora today is an extremely flexible system. One can find
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Fedora builds running on everything from hobbyist ARM devices,
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to workstations, to testing servers.
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</p>
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<p>
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This flexibility derives in large part from the fact that from a
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technological point of view, Fedora is a collection of packages.
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While pre-assembled "deliverables" such as the Live CDs are
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distributed by the project, they are only a temporary state.
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</p>
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<p>
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For example, as soon as they are installed, upgrading involves
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having a package manager that dynamically reassemble the system
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from newer parts in the Fedora package collection. One cannot
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file a bug against the "default offering" as a whole - a package
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must be chosen.
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</p>
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<p>
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Furthermore, nearly every aspect of the Fedora infrastructure
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(and documentation) is structured in terms of packages, from
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user-facing tools such as Bugzilla and Bodhi, to developer tools
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such as Koji. The announced security updates are based on package
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names.
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</p>
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<p>
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In contrast for example, ChromeOS is delivered and updated as an
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pre-assembled atomic unit. ChromeOS is (compared to Fedora)
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completely inflexible, but fulfills a targeted role clearly
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well.
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</p>
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<h3>How OSTree allows a middle ground</h3>
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<p>
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Fundamentally, packages are partial filesystem trees with
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metadata - they are assembled by a package manager into a complete
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bootable tree. It's important to emphasize that it is only
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these <emphasis>complete</emphasis> trees that users run.
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</p>
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<p>
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OSTree allows an OS distributor to
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ship <emphasis>multiple</emphasis> complete bootable filesystem
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trees, and furthermore, client machines can atomically switch
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between them, or even track multiple trees independently.
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</p>
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<p>
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This allows a middle ground between the two extremes of a
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combinatorial explosion of packages, and a singular OS.
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</p>
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<p>For example, these are some of the trees the current prototype generates:
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<ul>
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<li><tt>fedostree/20/x86_64/base/minimal</tt> - Just <tt>@core</tt>.</li>
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<li><tt>fedostree/20/x86_64/server/docker-io</tt> - This tree contains <tt>@standard</tt> plus <tt>docker-io</tt>.</li>
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<li><tt>fedostree/20/x86_64/server/freeipa-server</tt> - This tree contains <tt>@standard</tt> plus FreeIPA.</li>
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<li><tt>fedostree/20/x86_64/workstation/gnome-core</tt> - The GNOME workstation with no applications.</li>
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<li><tt>fedostree/20/x86_64/workstation/gnome-default</tt> - The above, with default applications.</li>
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<li><tt>fedostree/20/x86_64/workstation/gnome-development-and-virt</tt>
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- The above, with development tools, and virtualization client
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and server side.</li>
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</ul>
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</p>
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<h3>Initial goals</h3>
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<p>
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The first goal of this project is to be an <i>additional</i>
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deployment option built in the Fedora infrastructure. In this
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phase, developers and testers can use OSTree to replicate and
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atomically transition between the pre-assembled trees produced by
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this build server.
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</p>
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<p>
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In this phase, no common mechanism for additional software
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installation is provided. That said, individual trees can do so;
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for example <tt>server/docker-io</tt> tree can use Docker to
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install and run server container applications independent of
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OSTree.
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</p>
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<p>
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This phase does include basic integration testing on the build
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server side, which will be a major benefit to the Fedora project
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and its downstreams.
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</p>
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<h3>Required changes in Fedora/RPM for initial deployment</h3>
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<p>
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A change to
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include <a href="https://sourceware.org/bugzilla/show_bug.cgi?id=16142">/usr/lib/passwd</a>
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is required; this is implemented currently by
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the <tt>nss-altfiles</tt> package. See
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also <a href="http://fedorapeople.org/~walters/Use-usr-lib-passwd-for-system-users-if-it-exists.patch">this
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patch</a> for shadow-utils.
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</p>
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<p>
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All RPMs should stop shipping files and directories
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in <tt>/var</tt>.
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See <a href="https://people.gnome.org/~walters/ostree/doc/layout.html">this
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section</a> of the OSTree documentation.
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</p>
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<p>
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RPM should cope with its database living
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in <tt>/usr/share/rpm</tt> and being immutable.
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</p>
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<h3>Development area: OSTree Layering</h3>
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<p>
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This phase would be allowing "layering" of trees. For example,
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if one installs the <tt>base/minimal</tt> tree, one could imagine
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taking the <tt>strace</tt> package, and computing a new
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filesystem tree which is the union of the two.
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</p>
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<p>
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While plain standalone ELF executables would work with no
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modification, a generalization of this kind of dynamic layering
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implies a higher level above OSTree that is aware of things
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like <tt>ldconfig</tt> and <tt>gtk-update-icon-cache</tt> and how
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to trigger them when layers are combined
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</p>
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<p>
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Conceptually, this is a step back towards combinatorics. For example,
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if libvirt is a layer that could be applied on top of the base server
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layer as well as the workstation layer, then there would need to be
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some notion of dependencies.
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</p>
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<h3>Development area: Local package assembly</h3>
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<p>
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There is absolutely no reason one could not just use the package
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manager on the client side to download and assemble packages -
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but rather than operating live on your current root, OSTree
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allows setting up the chosen tree for the next boot
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atomically.
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</p>
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<p>
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The problem is making this sort of thing efficient and scalable;
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it would require careful integration of the local OSTree repository
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and the package manager caching to operate at a speed comparable to
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traditional package management.
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</p>
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<h3>Development area: Live updates</h3>
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<p>
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If one is using OSTree in a model without a separate application
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mechanism (as is the case for rpm-ostree), it is potentially
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painful to reboot just to upgrade applications.
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</p>
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<p>
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It would be quite easy to do a bind mount of the new /usr over
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top of the old. This would avoid some of the problems dpkg/rpm
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have in creating an <emphasis>partial</emphasis> view. But even
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this model would still break many real world desktop applications
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such as Firefox. Few applications handle files from an arbitrary
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new version replacing their current ones.
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</p>
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<p>
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Starting from an <emphasis>safe</emphasis> basis, it should be
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possible to engineer userspace so that many classes of upgrades
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can be applied both live and safely, without race conditions.
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</p>
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</article>
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