Device nodes and symlinks in /dev for lvm containers

LXD
1

When I create a new container in a LVM pool a few things have to be created in /dev. Whether this is purely done by udev I don’t know. But I noticed something strange.

For volumes created by LXD there is one block node in /dev/<vg_name> and a symlink in /dev/mapper' pointing to the one in /dev/<vg_name>`. For example

$ sudo lxc init images:ubuntu/bionic/amd64 -p pool2 test04
Creating test04
$ ls -l /dev/rapper-vg2/*test04* /dev/mapper/*test04*
brw-rw---- 1 root disk 253, 6 mei 19 14:01 /dev/mapper/rapper--vg2-containers_test04
lrwxrwxrwx 1 root root     41 mei 19 14:01 /dev/rapper-vg2/containers_test04 -> /dev/mapper/rapper--vg2-containers_test04

And in this case there is also a /dev/dm6

$ ls -l /dev/dm-6
brw-rw---- 1 root disk 253, 6 mei 19 14:01 /dev/dm-6

If you create a volume with lvcreate then you get a symlink in /dev/<vg_name> and /dev/mapper

$ sudo lvcreate --name mytest01 --yes --wipesignatures y rapper-vg2/LXDThinPool --thin --virtualsize 10G
  Logical volume "mytest01" created.
$ ls -l /dev/rapper-vg2/*test01* /dev/mapper/*test01*
lrwxrwxrwx 1 root root 7 mei 19 14:01 /dev/mapper/rapper--vg2-mytest01 -> ../dm-5
lrwxrwxrwx 1 root root 7 mei 19 14:01 /dev/rapper-vg2/mytest01 -> ../dm-5

And of course a /dev/dm-5.

So far I see that this is consistent and I don’t understand why. Is it a timing issue with udev maybe?

And on one LXD system I see that it doesn’t even create the files for snapshots in /dev. Something’s fishy.

2

What version of LXD and is it from snap?

3

The latest version of LXD doesn’t create devices in /dev until the volumes are activated by LXD, so that may explain what you are seeing.

4

Yes, snap. I was going to say snap 4.0.1, but to my surprise it is snap 4.1 (and another one at snap 4.0.1)

5

If you’re running snap then you may also need to instruct your host-side lvm tools to rescan the volumes before your tooling will see the volumes, see https://github.com/lxc/lxd/issues/7376#issuecomment-629898371

6

Can you elaborate on that? What does this “activate” mean? The background to this question is that I want to create a snapshot and then I want to mount it to make a backup.

7

See setactivationskip in the LVM manpage for more info.

You can create a snapshot yourself so it will be at a known location and activated, and parse the output of lvs command so you know which volume name to take a snapshot of (taking into account the the github comment I linked to from @stgraber).