[Solved] BTRFS migration (from ext4) of root filesystem

Not fully sure if this is the right place to share some conclusions of a day of migration experiments, saving others some time. Happy to be redirected to a more appropriate place.

I needed to migrate the root filesystem of a server running Debian from ext4 to btrfs, for which I found https://wiki.debian.org/Btrfs%20migration However, installing a second OS as suggested there did seem rather complicated to me (e.g. wanting to save me the trouble of partition re-layouting and of bootloader adjustments).

Goal was to perform the migration booting from a USB stick. The steps I followed in the end are as follows (credits also to https://forum.level1techs.com/t/moving- ... -fs/210037):

Data migration

• Boot from rescue system (I was using systemrescue from USB, using their Xfce)
• rsynced ext4 content from root filesystem (let's say sda1) to another partition (rsync -aPX)
• Created btrfs on sda1 (you can probably also use btrfs-convert, I wanted a clean start)
• btrfs subvolume create @rootfs (on sda1)
• rsync data back, into @rootfs

BTW: There are several guides that suggest adding further subvolumes next to @rootfs, such as @log, in order to reduce the size of the rootfs-subvolume for backup purposes.

Grub update + re-building the initrd image

• mount -o subvol=@rootfs /dev/sda1 /mnt
• mount --bind /dev /mnt/dev
• mount --bind /proc /mnt/proc
• mount --bind /sys /mnt/sys
• chroot /mnt
• fix /etc/fstab with new blkid (and others if changed)* see example below, using results from 'blkid /dev/sda1'
• dir /lib/modules (and look for the latest kernel version in there)
• dracut -f '' <latest kernel version>
• update-grub
• grub-install /dev/sda1
• exit
• reboot

The step that I had a hard time to find documented on the net was the dracut part. This was required because the old initrd does not include the btrfs module.

example /etc/fstab:

Statistics: Posted by KomtGoed — 2025-01-16 21:37 — Replies 0 — Views 8

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