129 lines
4.2 KiB
Text
129 lines
4.2 KiB
Text
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SOME IMPORTANT NOTES ABOUT HOW DRACUT WORKS
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===========================================
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Will Woods <wwoods@redhat.com>
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v1.0, August 2012
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// vim: set syntax=asciidoc textwidth=78:
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File locations / installation
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-----------------------------
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The files are installed into the initramfs according to the instructions in
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`module-setup.sh`. So this line:
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inst_hook cmdline 25 "$moddir/parse-anaconda-options.sh"
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means that the file `parse-anaconda-options.sh` will be installed at priority
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`25` in the `cmdline` hook. In practice, this means that it will be located at
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/lib/dracut/hooks/cmdline/25-parse-anaconda-options.sh
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inside the initramfs.
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Hooks and script ordering
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-------------------------
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The hooks run in the following order:
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cmdline::
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This is where you parse (and _only_ parse) the boot commandline. Just
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set up config files and do sanity checks; the real action is later.
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pre-udev::
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This is where you write out udev rules (before udev starts).
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pre-trigger::
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At this point udev is running but *kernel modules haven't been loaded*.
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If you need to set udev environment variables, set them here.
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initqueue::
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This is the mainloop, where initramfs tries to find/fetch rootfs.
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Scripts in this hook will run _repeatedly_ until finished or timeout (see
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below). Runs at 0.5-second intervals.
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initqueue/settled::
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This part of the mainloop only runs once _udev is settled_, i.e. once all
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devices have been found.
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initqueue/online::
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This hook runs _every time a network device goes online_.
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initqueue/finished::
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If all the scripts here return success dracut exits the mainloop, _even if
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some initqueue scripts have not yet run_.
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To put it another way, if you want dracut to wait for something to happen,
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you need a script in `initqueue/finished` that returns non-zero _until_
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the thing you're waiting for happens.
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pre-mount::
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Runs _once_ before trying to mount rootfs.
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mount::
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Each script in this hook runs in order, until one of them mounts rootfs at
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`$NEWROOT`. May run multiple times.
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pre-pivot::
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This is where you (e.g.) copy files into `$NEWROOT` before switching over.
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cleanup::
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Clean up after your other hooks.
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The scripts _within_ each hook run according to the numeric priority given in
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the `inst_hook` lines in `module-setup.sh`.
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Variables, scope, sharing data between scripts
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----------------------------------------------
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Each script in a hook gets sourced by the same `bash` interpreter. If you
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define a function or variable in a script, each subsequent script _in the same
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hook_ can see and use it, but *it won't be visible to other hooks.*
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If you export a variable, it will be available to all subsequent scripts. You
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can, of course, also share data by writing files to `/tmp`.
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*NOTE:* You can break _other_ modules by accidentally overwriting their
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variables. Avoid the following variable names in your own code.
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Exported Variables
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~~~~~~~~~~~~~~~~~~
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==== Special dracut variables ====
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$root:: The root device. Must be set by the end of the `cmdline` hook.
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Might not actually be a device (e.g. "nfs").
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$rflags:: Mount flags for the root device.
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$fstype:: The fstype of the root filesystem. Usually `auto`.
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$netroot:: The network root location. Syntax depends on type of network root.
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==== Read-only dracut variables ====
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$NEWROOT:: Mountpoint for the root filesystem. Usually `/sysroot`.
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$hookdir:: Location of the dracut hooks. Usually `/lib/dracut/hooks`.
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$RDRETRY:: Number of loops to try before giving up. Usually *60* (=30 seconds).
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$main_loop:: Counter for the current mainloop iteration.
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$DRACUT_QUIET:: Whether dracut should operate quietly; `yes` or `no`.
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(Don't worry about this; just use `info()` or `warn()` instead)
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$UDEVVERSION:: Self-explanatory.
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==== Variables from other modules ====
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$resume, $splash:: Used by `95resume`.
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$CURL_HOME:: Exported by `45url-lib`.
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==== Anaconda ====
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$kickstart:: Anaconda-style URL for the kickstart.
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$anac_updates:: Anaconda-style URL for `updates.img`.
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$ksdevice:: Network device to use for fetching kickstart/stage2/etc.
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Further Reading
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---------------
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Dracut documentation:
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http://www.kernel.org/pub/linux/utils/boot/dracut/dracut.html
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My other set of dracut notes:
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http://wwoods.fedorapeople.org/doc/dracut-notes.html
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