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diff --git a/README.initramfs b/README.initramfs deleted file mode 100644 index c5fa668..0000000 --- a/README.initramfs +++ /dev/null @@ -1,122 +0,0 @@ -Assembling md arrays at boot time. ---------------------------------- -December 2005 - -These notes apply to 2.6 kernels only and, in some cases, -to 2.6.15 or later. - -Md arrays can be assembled at boot time using the 'autodetect' functionality -which is triggered by storing components of an array in partitions of type -'fd' - Linux Raid Autodetect. -They can also be assembled by specifying the component devices in a -kernel parameter such as - md=0,/dev/sda,/dev/sdb -In this case, /dev/md0 will be assembled (because of the 0) from the listed -devices. - -These mechanisms, while useful, do not provide complete functionality -and are unlikely to be extended. The preferred way to assemble md -arrays at boot time is using 'mdadm'. To assemble an array which -contains the root filesystem, mdadm needs to be run before that -filesystem is mounted, and so needs to be run from an initial-ram-fs. -It is how this can work that is the primary focus of this document. - -It should be noted up front that only the array containing the root -filesystem should be assembled from the initramfs. Any other arrays -should be assembled under the control of files on the main filesystem -as this enhanced flexibility and maintainability. - -A minimal initramfs for assembling md arrays can be created using 3 -files and one directory. These are: - -/bin Directory -/bin/mdadm statically linked mdadm binary -/bin/busybox statically linked busybox binary -/bin/sh hard link to /bin/busybox -/init a shell script which call mdadm appropriately. - -An example init script is: - -============================================== -#!/bin/sh - -echo 'Auto-assembling boot md array' -mkdir /proc -mount -t proc proc /proc -if [ -n "$rootuuid" ] -then arg=--uuid=$rootuuid -elif [ -n "$mdminor" ] -then arg=--super-minor=$mdminor -else arg=--super-minor=0 -fi -echo "Using $arg" -mdadm -Acpartitions $arg --auto=part /dev/mda -cd / -mount /dev/mda1 /root || mount /dev/mda /root -umount /proc -cd /root -exec chroot . /sbin/init < /dev/console > /dev/console 2>&1 -============================================= - -This could certainly be extended, or merged into a larger init script. -Though tested and in production use, it is not presented here as -"The Right Way" to do it, but as a useful example. -Some key points are: - - /proc needs to be mounted so that /proc/partitions can be accessed - by mdadm, and so that /proc/filesystems can be accessed by mount. - - The uuid of the array can be passed in as a kernel parameter - (rootuuid). As the kernel doesn't use this value, it is made available - in the environment for /init - - If no uuid is given, we default to md0, (--super-minor=0) which is a - commonly used to store the root filesystem. This may not work in - all situations. - - We assemble the array as a partitionable array (/dev/mda) even if we - end up using the whole array. There is no cost in using the partitionable - interface, and in this context it is simpler. - - We try mounting both /dev/mda1 and /dev/mda as they are the most like - part of the array to contain the root filesystem. - - The --auto flag is given to mdadm so that it will create /dev/md* - files automatically. This is needed as /dev will not contain - and md files, and udev will not create them (as udev only created device - files after the device exists, and mdadm need the device file to create - the device). Note that the created md files may not exist in /dev - of the mounted root filesystem. This needs to be deal with separately - from mdadm - possibly using udev. - - We do not need to create device files for the components which will - be assembled into /dev/mda. mdadm finds the major/minor numbers from - /proc/partitions and creates a temporary /dev file if one doesn't already - exist. - -The script "mkinitramfs" which is included with the mdadm distribution -can be used to create a minimal initramfs. It creates a file called -'init.cpio.gz' which can be specified as an 'initrd' to lilo or grub -(or whatever boot loader is being used). - - - - -Resume from an md array ------------------------ - -If you want to make use of the suspend-to-disk/resume functionality in Linux, -and want to have swap on an md array, you will need to assemble the array -before resume is possible. -However, because the array is active in the resumed image, you do not want -anything written to any drives during the resume process, such as superblock -updates or array resync. - -This can be achieved in 2.6.15-rc1 and later kernels using the -'start_readonly' module parameter. -Simply include the command - echo 1 > /sys/module/md_mod/parameters/start_ro -before assembling the array with 'mdadm'. -You can then echo - 9:0 -or whatever is appropriate to /sys/power/resume to trigger the resume. |