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playing around with a btrfs (4.4) machine (server 16.04) I ran into this

scenario

ubuntu server 16.04 installed on btrfs with standard subvolumes for / and home

what I want to achieve

put /var into it's own subvolume (on the same harddrive for now)

what I did

  1. sudo mkdir /subvol
  2. sudo btrfs subvolume create /subvol/var
  3. sudo rsync -azv /var/* /subvol/var/
  4. create a line in /etc/fstab for the subvolume (which I have copied from the existing line for the @home subvolume. It looks like this
    UUID=79a74d84-2f0e-4098-9c58-f721eab913ee /mnt/var btrfs \ defaults,compress=lzo,subvol=@var 0 2

  5. sudo mount -a

expected behavior

the new subvolume var (located in /subvol/var) get's mounted on /var

actual behavior

:~$ sudo mount -a mount: mount(2) failed: No such file or directory

Not sure whether the approach generally is good. It's the idea I could come up with. Any hints what my problem here is?

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As per the btrfs section on the ubuntu wiki (presuming that layout is still applicable), the default setup is a btrfs root containing the @ and @home submvolumes.

/subvol/var is here presumably created inside the mounted @ and hence its path relative to the btrfs root (subvolid=5) would be <FS_TREE>@/subvol/var, instructions below are based on this assumption.

So you probably either want to move the subvolume to <FS_TREE>/@var, via something like

mnt -o subvolid=5 /mnt
mv /mnt/@/subvol/var /mnt/@var
umount /mnt

and keep the fstab entry as it is.

Or, edit the fstab entry to match the current location

UUID=79a74d84-2f0e-4098-9c58-f721eab913ee  /mnt/var btrfs \
defaults,compress=lzo,subvol=@/subvol/var   0   2

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