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-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.