Cannot setup LXD to use existing containers get Requested btrfs subvolume exists but is not empty

#1

I had to re-install the server that my LXD installation was on. Before the re-install I had several LXD containers stored on the BTRFS parition /srv/lxd, which is a separate LVM LV formatted as BTRFS:

/dev/mapper/MainVG-lxd on /srv/lxd type btrfs (rw,relatime,space_cache,subvolid=5,subvol=/)

After the re-install, which involved wiping / and /boot but no other partitions, I’ve run LXD init:

Would you like to use LXD clustering? (yes/no) [default=no]: 
Do you want to configure a new storage pool? (yes/no) [default=yes]: no
Would you like to connect to a MAAS server? (yes/no) [default=no]: 
Would you like to create a new local network bridge? (yes/no) [default=yes]: no
Would you like to configure LXD to use an existing bridge or host interface? (yes/no) [default=no]: 
Would you like LXD to be available over the network? (yes/no) [default=no]: 
Would you like stale cached images to be updated automatically? (yes/no) [default=yes] 
Would you like a YAML "lxd init" preseed to be printed? (yes/no) [default=no]:

And now I’m attempting to ‘reconnect’ the existing LXD storage pool but get this error:

$ lxc storage create lxd-partition btrfs source=/srv/lxd
Error: Requested btrfs subvolume exists but is not empty

$ ls /srv/lxd/
containers            custom            images            virtual-machines-snapshots
containers-snapshots  custom-snapshots  virtual-machines
#2

You need to follow the disaster recovery instructions in the backup part of the documentation.

You’ll have to bind-mount your /srv/lxd over /var/lib/lxd/storage-pools/NAME or /var/snap/lxd/common/lxd/storage-pools/NAME (depending on whether using deb or snap), then use lxd import NAME for each of your containers. This will automatically re-add the storage pool for you and re-create as much of the database as possible from the serialized backup data that’s part of the instance volumes.

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#3

No luck, I just get “The instance “xx” does not seem to exist on any storage pool”:

sudo mkdir /var/snap/lxd/common/lxd/storage-pools/lxd-partition
sudo mount -o bind /srv/lxd /var/snap/lxd/common/lxd/storage-pools/lxd-partition

# should see containers, custom, images etc.
ls /var/snap/lxd/common/lxd/storage-pools/lxd-partition
containers            custom            images            virtual-machines-snapshots
containers-snapshots  custom-snapshots  virtual-machines

# list containers to import
sudo ls /srv/lxd/containers/
cntrdtop  emby-bionic  focaltest  sncedge  ttrss  vpncntr


sudo lxd import emby-bionic
Error: The instance "emby-bionic" does not seem to exist on any storage pool
#4

Try:

sudo nsenter --mount=/run/snapd/ns/lxd.mnt -- mount -o bind /var/lib/snapd/hostfs/srv/lxd /var/snap/lxd/common/lxd/storage-pools/lxd-partition

And then attempt that lxd import again.

The reason for this is that LXD when run as a snap is operating in a mount namespace different than the host’s so the mount you did manually would only have worked had LXD not been started yet.

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#5

Thanks! That worked.

The full procedure was (where my storage pool is named lxd-partition and it’s located at /srv/lxd):

sudo mkdir /var/snap/lxd/common/lxd/storage-pools/lxd-partition
sudo nsenter --mount=/run/snapd/ns/lxd.mnt -- mount -o bind /var/lib/snapd/hostfs/srv/lxd /var/snap/lxd/common/lxd/storage-pools/lxd-partition
# list containers to import
sudo ls /srv/lxd/containers/
# for each container
sudo lxd import containername

As a suggestion, could you add this as a working example to the documentation at https://lxd.readthedocs.io/en/latest/backup/? I can see it’s partly documented, but there are important parts missing like using nsenter for Snap-installed LXD. And TBH it’s very non-intuitive without an example :smiley:

#6