User:Kaij/Migrating to bcachefs

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Here is some quick help if you're trying to migrate from btrfs (or some other FS, in theory) to bcachefs.

My recommendation is to setup a new device as bcachefs first, move partitions/subvolumes to that incrementally, and then eventually replace the root.

All this can be done in a live system, except maybe moving root.

Assumptions

  • You have some bcachefs setup, and mounted at /mnt/bcachefs-raw (/dev/sda in the example)
  • You want to create independently named subvolumes from the paths, in this case /live/subvolume. This isn't strictly necessary, but the reason for the X-mount.subdir=... trick.
  • You have several manageable partitions/subvolumes that can be migrated. That way the interruption time is doable. Migrating several terrabytes of data in one go should probably be done offline.
  • !! You should already know how to modify your /etc/fstab, have a general idea of your system layout, and be comfortable figuring things out. This is merely here to shorten the journey.
  • !! bcachefs is experimental.
  • !! Fiddling with important and in-use mount points is dangerous.
  • !! You need to know what you're doing

Technique

The following should be done for each partition/subvolume. Before doing this, you may want to create a root subvolume & mount it at /mnt/temp-root. We will later switch into that root and reconfigure. 

   `# bcachefs subvolume create /mnt/bcachefs-raw/live/root && mount /dev/sda /mnt/temp-root -o noatime,X-mount.subdir=live/root`

The basic idea is to take existing paritions/subvolumes, 1. remount them as readonly (to prevent anything changing while migrating: 

   `# mount -f -o remount,existing options,ro /path`

2. create the bcachefs subvolume: 

   `# bcachefs subvolume create /mnt/bcachefs-raw/live/my-subvolume`

3. mount the subvolume appropriately: 

   `# mount /dev/sda /mnt/temp-root/path -o noatime,X-mount.subdir=live/my-subvolume`

4. copy all files, including attributes etc: 

   `# rsync -avxHAXWS --numeric-ids --info=progress2 /path/ /mnt/temp-root/path/`

5. replace in live system: (adjust fstab if necessary) 

   `# umount /path && mount /dev/sda /path -o noatime,X-mount.subdir=live/my-subvolume`

6. profit

Partition Order

If like me, you have several (nested) partitions/subvolumes that are non-critical, do those first. To figure out what partitions/subvolumes are "critical" check them using 

   `# fuser -vm /path`

This list should only contain processes you can stop normally. Additionally, some partitions, like /home, are inherently critical to the systems functionality. You'll need to reboot at a minimum to get these to work.

This is usually not a big deal, make sure you have copied everything over, just adjust /etc/fstab, and reboot. Things should just work. Depending on the partition services may complain about /home (or whatever folder you're moving) being readonly, but this is generally fine.

Depending on your setup, x-systemd.requires-mounts-for may come in handy. For example, I have a subvolume /home/kaij/.steam, which I want to mount after /home, hence I add `x-systemd.requires-mounts-for=/home` to the /home/kaij/.steam entry.

Notably btrfs seems to be more relaxed with this kind of thing, while bcachefs will complain sometimes.

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