Historically when the kernel boots it used a mechanism called 'autodetect' to identify partitions (marked as partition type 'fd') which are used in RAID arrays. It then attempts to automatically assemble and start these arrays.
This approach can cause problems in several situations (see various mailing list archives for the debates) and kernel autodetect is deprecated.
The recommended approach now is to use 'initramfs'. This approach allows a great deal of flexibility in preparing the kernel for booting. Essentially the normal kernal boot image is extended to contain an initram filesystem containing a variety of data and scripts. These scripts can do almost anything. A typical usage is to include kernel modules and userspace configuration tools (such as mdadm). This is of interest to us because mdadm could, for example, examine the partitions and only assemble arrays with a defined UUID.
This script was written by Nix and posted to the linux-raid list:
It has a number of improvements over the initramfs embedded in the script that comes with mdadm:
- It handles LVM2 as well as md (obviously if you boot off RAID you still have to boot off RAID1, but /boot can be a RAID1 filesystem of its own now, with / in LVM, on RAID, or both at once)
- It fscks / before mounting it
- If anything goes wrong, it drops you into an emergency shell in the rootfs, from where you have all the power of ash without hardly any builtin commands, lvm and mdadm to diagnose your problem :) you can't do *that* with in-kernel array autodetection!
- it supports arguments `rescue', to drop into /bin/ash instead of init after mounting the real root filesystem, and `emergency', to drop into a shell on the initramfs before doing *anything*.
- It supports root= and init= arguments, although for arcane reasons to do with LILO suckage you need to pass the root argument as `root=LABEL=/dev/some/device', or LILO will helpfully transform it into a device number, which is rarely useful if the device name is, say, /dev/emergency-volume-group/root ;) right now, if you don't pass root=, it tries to mount /dev/raid/root after initializing all the RAID arrays and LVM VGs it can.
- it doesn't waste memory. initramfs isn't like initrd: if you just chroot into the new root filesystem, the data in the initramfs *stays around*, in *nonswappable* kernel memory. And it's not gzipped by that point, either!
- it needs a very new busybox, from Subversion after the start of this year: I'm using svn://busybox.net/trunk/busybox revision 14406, and a 2.6.12+ kernel with sysfs and hotplug support; this is because it populates /dev with the `mdev' mini-udev tool inside busybox, and switches root filesystems with the `switch_root' tool, which chroots only after erasing the entire contents of the initramfs (taking *great* care not to recurse off that filesystem!)
- if you link against uClibc (recommended), you need a CVS uClibc too (i.e., one newer than 0.9.27).
- it doesn't try to e.g. set up the network, so it can't do really whizzy things like mount a root filesystem situated on a network block device on some other host: if you want to do something like
that you've probably already written a script to do it long ago
- the init script's got a few too many things hardwired still, like the type of the root filesystem. I expect it's short enough to easily hack up if you need to :)
- you need an /etc/mdadm.conf and an /etc/lvm/lvm.conf, both taken by default from the system you built the kernel on: personally I'd recommend a really simple one with no device= lines, like
DEVICE partitions ARRAY /dev/md0 UUID=some:long:uuid:here ARRAY /dev/md1 UUID=another:long:uuid:here ARRAY /dev/md2 UUID=yetanother:long:uuid:here ...
So here is a usr/init for your kernel...
#!/bin/sh # # init --- locate and mount root filesystem # By Nix <firstname.lastname@example.org>. # # Placed in the public domain. # export PATH=/sbin:/bin /bin/mount -t proc proc /proc /bin/mount -t sysfs sysfs /sys CMDLINE=`cat /proc/cmdline` # Populate /dev from /sys /bin/mount -t tmpfs tmpfs /dev /sbin/mdev -s INIT_ARGS="$@" # If there is a forced root filesystem or init, accept the forcing for param in $CMDLINE; do case "$param" in init=*) eval "$param";; rescue) echo "Rescue boot mode: invoking ash."; init=/bin/ash; INIT_ARGS="-";; emergency) echo "Emergency boot mode. Dropping to a minimal shell."; echo "Reboot with Ctrl-Alt-Delete."; exec /bin/sh;; root=LABEL=*) root="`echo $param | cut -d= -f3-`";; esac done # Assemble the RAID arrays. /sbin/mdadm --assemble --scan --auto=md --run FAILED= # Scan for volume groups. /sbin/lvm vgscan --ignorelockingfailure --mknodes && /sbin/lvm vgchange -ay --ignorelockingfailure [[ -z $root ]] && root=/dev/raid/root fsck -a $root if [[ $? -eq 4 ]]; then echo "Filesystem errors left uncorrected." echo echo "Dropping to a minimal shell. Reboot with Ctrl-Alt-Delete." exec /bin/sh fi if [[ -n $root ]]; then /bin/mount -o rw -t ext3 $root /new-root fi if /bin/mountpoint /new-root >/dev/null; then :; else echo "No root filesystem given to the kernel or found on the root RAID array." echo "Append the correct 'root=' boot option." echo echo "Dropping to a minimal shell. Reboot with Ctrl-Alt-Delete." exec /bin/sh fi if [[ -z "$init" ]]; then init=/sbin/init fi # Unmount everything and switch root filesystems for good: # exec the real init and begin the real boot process. /bin/umount -l /proc /bin/umount -l /sys /bin/umount -l /dev echo "Switching to /new-root and running '$init'" exec switch_root /new-root $init $INIT_ARGS And usr/initramfs (will need adjustment for your system): # # Files needed for early userspace. # Placed in the public domain. # dir /bin 0755 0 0 file /bin/busybox /usr/i686-pc-linux-uclibc/bin/busybox 0755 0 0 slink /bin/sh /bin/busybox 0755 0 0 slink /bin/msh /bin/busybox 0755 0 0 slink /bin/[ /bin/busybox 0755 0 0 slink /bin/[[ /bin/busybox 0755 0 0 slink /bin/test /bin/busybox 0755 0 0 slink /bin/mount /bin/busybox 0755 0 0 slink /bin/umount /bin/busybox 0755 0 0 slink /bin/cat /bin/busybox 0755 0 0 slink /bin/ls /bin/busybox 0755 0 0 slink /bin/mountpoint /bin/busybox 0755 0 0 slink /bin/echo /bin/busybox 0755 0 0 slink /bin/false /bin/busybox 0755 0 0 slink /bin/true /bin/busybox 0755 0 0 slink /bin/mkdir /bin/busybox 0755 0 0 dir /sbin 0755 0 0 slink /sbin/mdev /bin/busybox 0755 0 0 slink /sbin/fsck /bin/busybox 0755 0 0 slink /sbin/e2fsck /bin/busybox 0755 0 0 slink /sbin/fsck.ext2 /bin/busybox 0755 0 0 slink /sbin/fsck.ext3 /bin/busybox 0755 0 0 slink /sbin/switch_root /bin/busybox 0755 0 0 file /sbin/mdadm /usr/i686-pc-linux-uclibc/sbin/mdadm 0755 0 0 file /sbin/lvm /usr/i686-pc-linux-uclibc/sbin/lvm 0755 0 0 file /init usr/init 0755 0 0 # supporting directories dir /proc 0755 0 0 dir /sys 0755 0 0 dir /new-root 0755 0 0 dir /etc 0755 0 0 dir /etc/lvm 0755 0 0 file /etc/lvm/lvm.conf /etc/lvm/lvm.conf 0644 0 0 file /etc/mdadm.conf /etc/mdadm.conf 0644 0 0 # initial device files required (mdev creates the rest) dir /dev 0755 0 0 nod /dev/console 0600 0 0 c 5 1 nod /dev/null 0666 0 0 c 1 3
And the busybox config file I used for all this --- you *will* need to change the CROSS_COMPILER_PREFIX and the EXTRA_CFLAGS_OPTIONS, and you might want to build in more tools as well for use when things go wrong, in emergency mode:
HAVE_DOT_CONFIG=y CONFIG_FEATURE_BUFFERS_USE_MALLOC=y CONFIG_FEATURE_DEVPTS=y CONFIG_STATIC=y CONFIG_LFS=y USING_CROSS_COMPILER=y CROSS_COMPILER_PREFIX="/usr/bin/i686-pc-linux-uclibc-" EXTRA_CFLAGS_OPTIONS="-march=pentium3 -fomit-frame-pointer" CONFIG_INSTALL_NO_USR=y CONFIG_INSTALL_APPLET_SYMLINKS=y PREFIX="./_install" CONFIG_CAT=y CONFIG_CUT=y CONFIG_ECHO=y CONFIG_FALSE=y CONFIG_LS=y CONFIG_MKDIR=y CONFIG_TEST=y CONFIG_TRUE=y CONFIG_FEATURE_AUTOWIDTH=y CONFIG_E2FSCK=y CONFIG_FSCK=y FDISK_SUPPORT_LARGE_DISKS=y CONFIG_MDEV=y CONFIG_MOUNT=y CONFIG_SWITCH_ROOT=y CONFIG_UMOUNT=y CONFIG_MOUNTPOINT=y CONFIG_FEATURE_SH_IS_MSH=y CONFIG_MSH=y CONFIG_FEATURE_SH_EXTRA_QUIET=y CONFIG_FEATURE_SH_STANDALONE_SHELL=y CONFIG_FEATURE_COMMAND_EDITING=y CONFIG_FEATURE_COMMAND_EDITING_VI=y CONFIG_FEATURE_COMMAND_HISTORY=15 CONFIG_FEATURE_COMMAND_TAB_COMPLETION=y CONFIG_FEATURE_IPC_SYSLOG_BUFFER_SIZE=0 CONFIG_MD5_SIZE_VS_SPEED=2