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