Thursday, June 4, 2015

convert existing live btrfs root filesystem to raid1

Fuck it, we're doing it live!

All commands below make these assumptions:

  • sda = EXISTING boot drive
  • sda1 = EXISTING root fs
  • sdz = NEW boot drive
Modify the dev entries in the commands below according to your setup or bad things will happen!
  1. Copy the partition table from the old disk to the new one.  You probably don't want to get this backwards.

    yo@mama:# sfdisk -d /dev/sda | sfdisk /dev/sdz

    After doing this you may want to pull and then reinsert the drive, to refresh the entries in
    /dev.  Linux can be weird about this, partprobe doesn't always work.
  2. The disks I'm using are not the same size, the new one is a bit larger than the current one.  If the new one were smaller that would be a bit trickier, but since it isn't I won't really get into that.  (Basically you'd either need to recreate the partitions by hand on the new drive, or send the sfdisk output to file and modify that by hand before throwing on the new drive.)

    Instead, we'll just let that extra space be.
  3. Add the new disk.  Assuming your existing disk is mounted as the root filesystem, the command would go like this:

    yo@mama:# btrfs device add /dev/sdz1 /
    yo@mama:# btrfs balance start -dconvert=raid1 -mconvert=raid1 /

    If you've read the docs, you know there is also an -s flag for system chunks that can be passed to btrfs balance start.  You don't need to use -s here, according to various people on the internet, and I even found this comment that verifies it in the btrfs-progs sources:

     * allow -s only under --force, otherwise do with system chunks
     * the same thing we were ordered to do with meta chunks

    Also, for some reason balances happen in the foreground by default and output nothing.  Watch the progress by opening another terminal and running:

    yo@mama:# while btrfs balance status / ; do sleep 10; clear; done
  4. And that should be it.  You can make sure it went down as it was supposed to by running:

    yo@mama:# btrfs fi show /

    Which should give a listing with two devices.
Then, you can give it the balls-to-the-wall test by disconnecting your old boot drive. Make sure that the balance has finished before you do this!  If you're a sissy, run sync before pulling the drive.

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