Is it possible to convert btrfs to ext4 without losing data? I'm experiencing a very low speed on read/write operations on btrfs.
I'm just going on the available tools, not experience or documentation. You might want to test this with a partition (even just an image) you don't mind losing if it all goes wrong. Don't attempt this without taking a backup.
btrfs-convert has a roll-back function to undo a conversion. I'm not sure if this works for partitions that weren't converted to
Start by unmounting the filesystem. If it's essential to the system, boot into a LiveCD.
sudo apt-get install btrfs-tools
Roll back the conversion
sudo btrfs-convert -r /dev/sdXn
I did this using a system with multiboot distros (Ubuntu on ext4, Xubuntu on btrfs). Running Ubuntu, I have taken the Xubuntu btrfs home partition, and archived it using fsarchiver. I was then able to restore it, again using fsarchiver, to a different partition and specify ext4 filesystem type. That seemed to work okay.
Still in Ubuntu, I mounted the Xubuntu root partition on /mnt, and edited /mnt/@/etc/fstab to change the mount for /home to point to the new ext4 partition, changing both the UUID and the fs type, and deleting the subvolume data. I saved the file, and rebooted into Xubuntu.
There was an error in booting, and booting halted. The error was that I had another partition I wanted to mount to my ~/Documents directory which failed. I selected to manually fix the problem. It turns out that my root partition mounted okay as ext4, but when I did a list command I got
#ls /home @home
So the original btrfs subvolume structure was still there in the ext4 filesystem. I issued the command
mount --bind /home/@home /home
I then pressed ctrl-D to continue booting, and Xubuntu continued booting and everything came up fine. I haven't done it yet, as I'm still on this initial boot, but I'm certain I could put that bind command in my fstab file. Or perhaps I could have left the subvolume info in the fstab entry, I don't know.
What I will do instead, however (this is off-topic), is take a more mature instance of a /home (ext4) partition for Xubuntu from another computer and use that instead. I've already proven that this will work.
**The solution that works in all cases, is of course to copy everything from the root partition to a directory on another partition (assuming you have one with enough free space), reformat
ext4, copy back. I just did that, and hit a few issues so I thought I'd write a little howto:
- Boot on a rescue USB key, mount both partitions (let's say
/mntfor the real root partition, and
/destfor the partition serving as a temporary destination)
- Beware that you should mount the BTRFS subvolumes too (I didn't do that, and I lost all of
/root...). The best way is probably to do the usual
mount --bind /dev /mnt/dev; mount --bind /sys /mnt/sys; mount --bind /proc /mnt/proc; chroot /mnt) and then
mount -a -t btrfsso that the subvolumes described in
/etc/fstabare all mounted.
- Exit the chroot or use another TTY (Ctrl+Alt+F2), and launch the copy, like
cp -a /mnt /dest
- Reformat the partition:
mount /dev/*the_root_device* /mnt
- Copy back the data with
cp -a /dest/mnt/* /mntAnd now (in the chroot) don't forget to
/etc/fstabto remove all subvolumes, change the filesystem type from
ext4, and update the
UUID(see the output of
blkid, or switch to a label like
tune2fs -L rootfs /dev/*the_root_device*)
dracut, and recreate the bootloader (I use another distro so I don't have the exact command for this step on ubuntu). Cross fingers and reboot. Finally, remember to clean up the /dest copy in the temporary destination.**