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diff --git a/man/dracut.asc b/man/dracut.asc new file mode 100644 index 0000000..c806ab2 --- /dev/null +++ b/man/dracut.asc @@ -0,0 +1,182 @@ +dracut {mainversion} +==================== +:author: Harald Hoyer +:email: harald@profian.com +:revnumber: {version} +:language: bash + += Introduction +This section is a modified version of +http://en.wikipedia.org/wiki/Initrd which is licensed under the +Creative Commons Attribution/Share-Alike License. + +== Definition +An _initial ramdisk_ is a temporary file system used in the boot process of the +Linux kernel. _initrd_ and _initramfs_ refer to slightly different schemes for +loading this file system into memory. Both are commonly used to make +preparations before the real root file system can be mounted. + +== Rationale +Many Linux distributions ship a single, generic kernel image that is intended to +boot as wide a variety of hardware as possible. The device drivers for this +generic kernel image are included as loadable modules, as it is not possible to +statically compile them all into the one kernel without making it too large to +boot from computers with limited memory or from lower-capacity media like floppy +disks. + +This then raises the problem of detecting and loading the modules necessary to +mount the root file system at boot time (or, for that matter, deducing where or +what the root file system is). + +To further complicate matters, the root file system may be on a software RAID +volume, LVM, NFS (on diskless workstations), or on an encrypted partition. All +of these require special preparations to mount. + +Another complication is kernel support for hibernation, which suspends the +computer to disk by dumping an image of the entire system to a swap partition or +a regular file, then powering off. On next boot, this image has to be made +accessible before it can be loaded back into memory. + +To avoid having to hardcode handling for so many special cases into the kernel, +an initial boot stage with a temporary root file system +—now dubbed early user space— is used. This root file system would contain +user-space helpers that would do the hardware detection, module loading and +device discovery necessary to get the real root file system mounted. + +== Implementation +An image of this initial root file system (along with the kernel image) must be +stored somewhere accessible by the Linux bootloader or the boot firmware of the +computer. This can be: + +* The root file system itself +* A boot image on an optical disc +* A small ext2/ext3/ext4 or FAT-formatted partition on a local disk + (a _boot partition_) +* A TFTP server (on systems that can boot from Ethernet) + +The bootloader will load the kernel and initial root file system image into +memory and then start the kernel, passing in the memory address of the image. + +Depending on which algorithms were compiled statically into it, the kernel can +currently unpack initrd/initramfs images compressed with gzip, bzip2 and LZMA. + +== Mount preparations +dracut can generate a customized initramfs image which contains only whatever is +necessary to boot some particular computer, such as ATA, SCSI and filesystem +kernel modules (host-only mode). + +dracut can also generate a more generic initramfs image (default mode). + +dracut's initramfs starts only with the device name of the root file system (or +its UUID) and must discover everything else at boot time. A complex cascade of +tasks must be performed to get the root file system mounted: + +* Any hardware drivers that the boot process depends on must be loaded. All +kernel modules for common storage devices are packed onto the initramfs and then +udev pulls in modules matching the computer's detected hardware. + +* On systems which display a boot rd.splash screen, the video hardware must be +initialized and a user-space helper started to paint animations onto the display +in lockstep with the boot process. + +* If the root file system is on NFS, dracut does then: +** Bring up the primary network interface. +** Invoke a DHCP client, with which it can obtain a DHCP lease. +** Extract the name of the NFS share and the address of the NFS server from the +lease. +** Mount the NFS share. + +* If the root file system appears to be on a software RAID device, there is no +way of knowing which devices the RAID volume spans; the standard MD utilities +must be invoked to scan all available block devices with a raid signature and +bring the required ones online. + +* If the root file system appears to be on a logical volume, the LVM utilities +must be invoked to scan for and activate the volume group containing it. + +* If the root file system is on an encrypted block device: +** Invoke a helper script to prompt the user to type in a passphrase and/or +insert a hardware token (such as a smart card or a USB security dongle). + +* Create a decryption target with the device mapper. + +dracut uses udev, an event-driven hotplug agent, which invokes helper programs +as hardware devices, disk partitions and storage volumes matching certain rules +come online. This allows discovery to run in parallel, and to progressively +cascade into arbitrary nestings of LVM, RAID or encryption to get at the root +file system. + +When the root file system finally becomes visible: + +* Any maintenance tasks which cannot run on a mounted root file system +are done. +* The root file system is mounted read-only. +* Any processes which must continue running (such as the rd.splash screen helper +and its command FIFO) are hoisted into the newly-mounted root file system. + +The final root file system cannot simply be mounted over /, since that would +make the scripts and tools on the initial root file system inaccessible for any +final cleanup tasks. On an initramfs, the initial root file system cannot be +rotated away. Instead, it is simply emptied and the final root file system +mounted over the top. + +If the systemd module is used in the initramfs, the ordering of the services +started looks like <<dracutbootup7>>. + +== Dracut on shutdown + +On a systemd driven system, the dracut initramfs is also used for the shutdown +procedure. + +The following steps are executed during a shutdown: + +* systemd switches to the shutdown.target +* systemd starts + $prefix/lib/systemd/system/shutdown.target.wants/dracut-shutdown.service +* dracut-shutdown.service executes /usr/lib/dracut/dracut-initramfs-restore + which unpacks the initramfs to /run/initramfs +* systemd finishes shutdown.target +* systemd kills all processes +* systemd tries to unmount everything and mounts the remaining read-only +* systemd checks, if there is a /run/initramfs/shutdown executable +* if yes, it does a pivot_root to /run/initramfs and executes ./shutdown. + The old root is then mounted on /oldroot. + /usr/lib/dracut/modules.d/99shutdown/shutdown.sh is the shutdown executable. +* shutdown will try to unmount every /oldroot mount and calls the various + shutdown hooks from the dracut modules + +This ensures, that all devices are disassembled and unmounted cleanly. + += User Manual + +:leveloffset: 1 +include::dracut.8.asc[] + +:leveloffset: 1 +[[dracutconf5]] +include::dracut.conf.5.asc[] + +[[dracutcmdline7]] +include::dracut.cmdline.7.asc[] + +[[lsinitrd1]] +include::lsinitrd.1.asc[] + += Developer Manual + +:leveloffset: 1 +[[dracutmodules7]] +include::dracut.modules.7.asc[] + +[[dracutbootup7]] +include::dracut.bootup.7.asc[] + +:leveloffset: 0 +[appendix] +License +------- +This work is licensed under the Creative Commons Attribution/Share-Alike +License. To view a copy of this license, visit +http://creativecommons.org/licenses/by-sa/3.0/ or send a letter to Creative +Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. + |