Ubuntu – Unable to mount secondary BTRFS filesystem to specific subvolume

btrfsfilesystemmountsnapshot

So I'm trying to wrap my head around BTRFS and set up my fileserver using BTRFS filesystems. Here is my setup:

  • /dev/sda1 : BTRFS filesystem, contains @, @home subvolumes mounted to
    / and /home.

  • /dev/sdb1 and /dev/sdc1 : BRTFS pooled filesystem, 2x3GB HDD's,
    contains all my media files. Currently mounted to /mnt/media

I have them setup this way because I do not want any of my media files taking up space on /dev/sda1 which is a 120GB SSD; I'd like to only reserve that for system files. I'd like to mount this secondary BTRFS filesystem to a top-level subvolume labeled @media so I can do snapshots. However, every time I try to mount either /dev/sdb1 or /dev/sdc1 using subvolid, I get this error:

peetipablo@flexo:/$ sudo mount -t btrfs -o subvolid=354 /dev/sdb1 /mnt/media
mount: mount(2) failed: No such file or directory

Here's the subvolume list:

peetipablo@flexo:/$ sudo btrfs subvolume list /
ID 257 gen 142548 top level 5 path @
ID 258 gen 142527 top level 5 path @home
ID 310 gen 113715 top level 5 path @apt-snapshot-release-upgrade-utopic-2015-04-30_08:11:14
ID 311 gen 113766 top level 5 path @apt-snapshot-release-upgrade-vivid-2015-04-30_08:39:50
ID 312 gen 134632 top level 257 path var/lib/machines
ID 313 gen 142500 top level 257 path root_snapshot_5_13_15
ID 354 gen 142517 top level 5 path @media

If I try to make @media the default subvolume for directory /mnt/media, I am able to mount sdb1 without the -o subvolid, but when I try to snapshot I get an error:

peetipablo@flexo:/$ sudo btrfs subvolume set-default 354 /mnt/media
peetipablo@flexo:/$ sudo btrfs subvolume get-default /mnt/media
ID 354 gen 142517 top level 5 path @media
peetipablo@flexo:/$ sudo mount -t btrfs /dev/sdb1 /mnt/media
peetipablo@flexo:/$ sudo btrfs subvolume snapshot @media /mnt/media_snap
ERROR: error accessing '@media'
peetipablo@flexo:/$ sudo btrfs subvolume snapshot /mnt/media /mnt/media_snap
Create a snapshot of '/mnt/media' in '/mnt/media_snap'
ERROR: cannot snapshot '/mnt/media' - Invalid cross-device link

I have a feeling that I'm not understanding something or am using this newer filesystem type in a way that it's not meant to be used. Should I just merge all 3 devices into one BTRFS filesystem and not worry about filling up my SSD with media? Is there a way to achieve my end goal using a different method than the one I'm currently using?

Best Answer

Btrfs filesystems do not interact with each other, so you cannot mount the @media subvolume on /dev/sd{b,c}1 because the @media subvolume is currently on /dev/sda1, and you get a cross-device error when you try to snapshot /mnt/media to /mnt/media_snap because /mnt/media is on /dev/sd{b,c}1 whereas /mnt/media_snap is on /dev/sda1.

Instead of having the @media subvolume on the /dev/sda1 btrfs filesystem, you want to create an @media subvolume on the btrfs filesystem on /dev/sd{b,c}1:

sudo mount /dev/sdb1 /mnt/media/
sudo btrfs subvol create /mnt/media/@media
sudo umount /mnt/media/
sudo mount -o subvol=@media /dev/sdb1 /mnt/media/

You can create snapshots of /mnt/media/, but you can place the snapshot only on the /dev/sd{b,c}1 filesystem. So you can create a read-only snapshot like this:

sudo btrfs subvol snap -r /mnt/media/ /mnt/media/<snapshot_name>

But I recommend placing each snapshot of a subvolume outside the subvolume itself, because this simplifies things when you need to revert to a snapshot. To do this, you will need to mount the root of the /dev/sd{b,c}1 filesystem:

sudo mkdir /mnt/media_root/
sudo mount -o subvol=/ /dev/sdb1 /mnt/media_root/
sudo btrfs subvol snap -r /mnt/media_root/@media/ /mnt/media_root/<snapshot_name>
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