In this post I will show you how you can migrate an existing Raspberry Pi installation to LVM.
The migration process can be summarized as follows:
- Backup you root partition
- Setup LVM
- Restore backup
- Adapt boot configurations
Preconditions
Before you begin, make sure the installation you want to migrate has the lvm2 package installed. Otherwise you will not be
able to boot after the migration!
Hardware Setup
For this post I used a Rapsberry Pi 3 with an USB card reader and a 16GB SD card. Here is what my setup looks like:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 1 14.9G 0 disk
├─sda1 8:1 1 41.5M 0 part
└─sda2 8:2 1 14.8G 0 part
mmcblk0 179:0 0 14.9G 0 disk
├─mmcblk0p1 179:1 0 41.5M 0 part /boot
└─mmcblk0p2 179:2 0 14.8G 0 part /
In this example mmcblk0 is the system disk and sda is the disk we will convert to use LVM.
Make sure there are no mounts points on ‘sda’!
IMPORTANT
Please resist the urge to use rpi-update for kernel updates. This will break LVM!
Backup
By default, there are 2 partitions. One for /boot and the other for /. We we will not alter ‘boot’. So we can leave
it alone and backup the root partition / only. I will use bcrm for this purpose,
but any backup with tar will suffice.
With bcrm it is as simple as providing the source disk and a destination folder. For this example we will
use /tmp as our workspace. So, let’s get started and create a backup:
cd /tmp
mkdir /tmp/backup
git clone https://github.com/Jeansen/bcrm.git
cd bcrm
./bcrm.sh -s /dev/sda -d /tmp/backup -i
Setup LVM
With a backup in place, we can now convert the root partition to a Physical Volume (PV).
pvcreate -y /dev/sda2
Then we need to setup a Volume Group (VG) which uses the previously created PV. For this example we will use the name ‘vg00’.
vgcreate vg00 /dev/sda2
Finally, we can create one or more Logical Volumes (LV). In this example we will simply use all of the available space.
lvcreate -n root -l100%FREE vg00
The commands ‘pvs’, ‘vgs’ and ‘lvs’ will give us a quick summary of what we have created. Let’s see, what we have:
root@raspberrypi:/tmp/bcrm# pvs
PV VG Fmt Attr PSize PFree
/dev/sda2 vg00 lvm2 a-- 14.79g 0
root@raspberrypi:/tmp/bcrm# vgs
VG #PV #LV #SN Attr VSize VFree
vg00 1 1 0 wz--n- 14.79g 0
root@raspberrypi:/tmp/bcrm# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
root vg00 -wi-a----- 14.79g
Finally we need to create a filesystem:
mkfs.ext4 /dev/mapper/vg00-root
Restoring root
With LVM in place, we first need to mount the newly created LV.
mkdir /media/root
mount /dev/mapper/vg00-root /media/root
Although we have a full system backup, we only need to restore the root partition. Looking into the backup folder /tmp/backup
reveals multiple files:
We need to extract the file that has sda2in its name:
tar xf /tmp/backup/*_sda2* -C /media/root
Adapt boot configuration
Since we now use LVM, we need a boot loader that can handle LVM volumes. Therefore we will need to create an ‘initramfs’ image and tell the system where to find it.
Let’s first mount the boot partition and add an ‘initramfs’ image.
mkdir /media/boot
mount /dev/sda1 /media/boot
mkinitramfs -o /media/boot/initramfs.gz
Now, we need to tell the system where to find it:
echo 'initramfs initramfs.gz followkernel' >> /media/boot/config.txt
In addition we will need to edit the kernel command line and filesystem table.
First we replace the value of root in /media/boot/cmdline.txt with /dev/mapper/vg00-root. Here is an example
of what the final result could look like:
dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mapper/vg00-root rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait
In the same way, we have to change /media/root/etc/fstab for /. Here is an example:
proc /proc proc defaults 0 0
PARTUUID=36c26eb6-01 /boot vfat defaults 0 2
/dev/mapper/vg00-root / ext4 defaults,noatime 0 1
Be prepared for new kernels
Finally, you’ll have to make sure initramfs is rebuild whenever there are kernel updates. One options is to always
do this after any APT upgrade sequence (done automatically by unattended-upgrades).
To have APT rebuild initramfs after every upgrade, you’ll have to create a configuration file for APT, and a little
script to select the right kernel version for initramfs.
Put the following in /etc/apt/apt.con.d/30initramfs
DPkg::Post-Invoke {"/usr/local/bin/update_initramfs"}
Then create the file /usr/local/bin/update_initramfs with the following content and make it executable:
#!/usr/bin/env bash
cmd="mkinitramfs -o /boot/initramfs.gz $(ls /lib/modules | grep $(uname -r | grep -o 'v7.*') | sort -V | tail -n 1)"
echo "$cmd"
$cmd || exit 1
exit 0
The catch here is to tell mkinitramfs to build for the new kernel and not the current one. This is necessary because after any kernel update the old one will be gone and initramfs would complain and fail because of missing folders in /lib/modules. This should be done without any user intervention, of course. Fortunately this is an easy task. With ls /lib/modules | grep $(uname -r | grep -o 'v7.*') | sort -V | tail -n 1you can select the latest “-v7” version and provide this to mkinitramfs.
Why 30initramfs?
You might have to change the number, but 30 in 30initramfs should be fine. This way we make sure the Post-Invoke hook is present before other tools like unattended-upgrades which by default use 50 or greater numbers. Also, note that the name initramfs as part of the filename is my personal preference.
What happens if I dont’ care for the kernel?
If you do not have initramfs rebuilt and you get a new kernel, then the new kernel will not be able to initiate LVM and mount your filesystems. You might see something similar to the following and be stuck in emergency mode:
WARNING: Failed to connect to lvmetad. Falling back to device scanning.
/dev/mapper/vg00-root: open failed: Permission denied
Failure to communicate with kernel device-mapper driver.
Incompatible libdevmapper 1.02.145 (2017-11-03) and kernel driver (unknown version).
All done
That’s it! Put this newly created SD card in your Raspberry Pi and see what will happen :-)