Booting from an lvm-cached volume

The following Debian Forum topic and blog post based on it has the missing information.

Outline

-1) Back up your LVM configuration and have a like CD ready.

0) Make sure you have a separate /boot partition (your cached root will only be available later). This can be a 200MB partition and can be part of the same VG as your cached root.

1) You need dm-cache in your kernel image (instead of module). check your config and make sure you have CONFIG_DM_CACHE=y. If it is a module (=m) you will need to recompile a kernel where this is set to y. It is probably a good idea to use menuconfig and set this option from there (it will make sure dm-cache's dependency chain is also =y).

    Device Drivers  --->

    Generic Driver Options  --->

--- Multiple devices driver support (RAID and LVM)
<*>   Device mapper support
<*>     Cache target (EXPERIMENTAL)

2) Install thin-provisioning-tools (will do fsck-like functions on the cache at boot-time).

3) Create a file in /etc/initramfs-tools/hooks with the following content. This will make sure the executable from step 2 and some dependencies are inside your init ramdisk image.

#!/bin/sh

PREREQ="lvm2"

prereqs()
{
    echo "$PREREQ"
}

case $1 in
prereqs)
    prereqs
    exit 0
    ;;
esac

if [ ! -x /usr/sbin/cache_check ]; then
    exit 0
fi

. /usr/share/initramfs-tools/hook-functions

copy_exec /usr/sbin/cache_check

manual_add_modules dm_cache dm_cache_mq

4) Run update-initramfs -u -k all to re-generate all your initrd images.

When using vbence's answer, also make the /etc/initramfs/hooks/ file from step 3 executable. Otherwise it won't get run by update-initramfs