Question

19

Creating btrfs subvolume like @ or @home

rated 0 times [ 19] [ 0] / answers: 1 / hits: 31811 / 2 Years ago, wed, october 19, 2022, 8:17:27

Being intrigued by btrfs I decided to toy around with it in a VM. Installation works great as long as I remember to put /boot on a separate partition and it all seems to work fine.

What I would like to do is create a separate subvolume for bulk data with different settings than things like @ and @home. btrfs subvolume create data will do that for me, but I'm curious why it keeps naming it @/data (or similar things when I try /data or @data). Is there a special way to create the subvolumes like @ and @home so my data subvolume will show up as @data? (and: what is the exact difference between the naming schemes?)

Additional information:

ubuntu 12.04.2, running as a KVM guest on mint15 host;

kernel 3.8.0-27-generic from ubuntu-x-swat (raring backport);

btrfs using two devices, /dev/sda and /dev/sdb

/dev/sda and /dev/sdb are partitions on a disk not used by the host OS

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rstride

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3 Years ago

answered 2 Years ago enytidge · Accepted Answer

Sigh, seems I posted a bit too soon, with a bit of help of the almighty Zosma on IRC, we found out how this is supposed to work.

Subvolumes @ and @home are children of the btrfs root, so / is not actually the root of the file system. The UUID of the btrfs root is supplied in /etc/fstab twice, providing the mounts for / and /home. Mounting the actual root of the file system shows the subvolumes created by the installer:

root@box:~# mkdir /mnt/btrfs

root@box:~# mount UUID=f8963df3-1320-4bc0-a125-62be185b029e /mnt/btrfs

root@box:~# ls /mnt/btrfs

@  @home

So that's where they went! Creating a new subvolume in the exact way the ubuntu installer did is now easy:

root@box:~# btrfs subvolume create /mnt/btrfs/@data

Create subvolume '/mnt/btrfs/@data'

Now, as we've created a subvolume in the btrfs root instead of in the root filesystem for the install, /data/ won't show up by itself. Luckily, it's trivial to copy the trick used by the installer by examining /etc/fstab and add another mount for /data:

UUID=f8963df3-1320-4bc0-a125-62be185b029e /data  btrfs  defaults,subvol=@data 0 2

Last step is to create the mount point: mkdir /data. After a reboot, we'll have a /data directory mounted from a separate btrfs subvolume.

… Profit!