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Dracut
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------
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This is the simple skeleton of a new initramfs infrastructure.
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Information about our goals and aims can be found at
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https://fedoraproject.org/wiki/Initrdrewrite
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Unlike existing initramfs's, this is an attempt at having as little as
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possible hard-coded into the initramfs as possible. The initramfs has
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(basically) one purpose in life -- getting the rootfs mounted so that
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we can transition to the real rootfs. This is all driven off of
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device availability. Therefore, instead of scripts hard-coded to do
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various things, we depend on udev to create device nodes for us and
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then when we have the rootfs's device node, we mount and carry on.
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This helps to keep the time required in the initramfs as little as
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possible so that things like a 5 second boot aren't made impossible as
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a result of the very existence of an initramfs. It's likely that
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we'll grow some hooks for running arbitrary commands in the flow of
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the script, but it's worth trying to resist the urge as much as we can
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as hooks are guaranteed to be the path to slow-down.
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Most of the initrd generation functionality in dracut is provided by a bunch
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of generator modules that are sourced by the main dracut script to install
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specific functionality into the initrd. They live in the modules subdirectory,
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and use functionality provided by dracut-functions to do their work.
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Some general rules for writing modules:
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* Use one of the inst family of functions to actually install files
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on to the initrd. They handle mangling the pathnames and (for binaries,
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scripts, and kernel modules) installing dependencies as appropriate so
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you do not have to.
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* Scripts that end up on the initrd should be POSIX compliant. dracut
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will try to use /bin/dash as /bin/sh for the initrd if it is available,
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so you should install it on your system -- dash aims for strict POSIX
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compliance to the extent possible.
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* Hooks MUST be POSIX compliant -- they are sourced by the init script,
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and having a bashism break your user's ability to boot really sucks.
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* Generator modules should have a two digit numeric prefix -- they run in
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ascending sort order. Anything in the 90-99 range is stuff that dracut
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relies on, so try not to break those hooks.
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* Generator modules and hooks must have a .sh extension.
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* We have some breakpoints for debugging your hooks. If you pass 'break'
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as a kernel parameter, the initramfs will drop to a shell just before
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switching to a new root. You can pass 'break=hookpoint', and the initramfs
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will break just before hooks in that hookpoint run.
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Also, there is an attempt to keep things as distribution-agnostic as
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possible. Every distribution has their own tool here and it's not
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something which is really interesting to have separate across them.
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So contributions to help decrease the distro-dependencies are welcome.
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The git tree can be found at
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git://fedorapeople.org/~katzj/dracut.git for now. See the TODO
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file for things which still need to be done and HACKING for some
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instructions on how to get started. There is also a mailing list that
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is being used for the discussion -- initramfs@vger.kernel.org. It is
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a typical vger list, send mail to majordomo@vger.kernel.org with body
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of 'subscribe initramfs email@host.com'
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Licensed under the GPLv2
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Copyright 2008, Red Hat, Inc. -- Jeremy Katz <katzj@redhat.com>
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