From 489bea7ee8e1dbecfa517b8415568044ab57c73a Mon Sep 17 00:00:00 2001 From: Michael Tokarev Date: Sat, 20 Dec 2014 08:48:44 +0000 Subject: mdadm (3.3.2-5) unstable; urgency=medium * use-tempnode-not-devnode.patch: change udev rules file to use $tempnode which works both on wheezy and jessie udev, instead of $devnode which only works in jessie. At this stage it is better to make rules file compatible with old version instead of adding versioned dependency. Should be removed for jessie+1. (Closes: #770883) * fix Closes: list in previous entry (Closes: #771852) # imported from the archive --- README.initramfs | 123 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 123 insertions(+) create mode 100644 README.initramfs (limited to 'README.initramfs') diff --git a/README.initramfs b/README.initramfs new file mode 100644 index 00000000..8f9b8ddf --- /dev/null +++ b/README.initramfs @@ -0,0 +1,123 @@ +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' or 'mdassemble' (which is a +trimmed-down 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. -- cgit v1.2.3