Why are there multiple loop devices for the same snap?

Question

I used lsblk to check connected devices, when I found that there are a total of 3 loop devices for the same snap "core", with revision numbers.

loop1    7:1    0  81.6M  1 loop /snap/core/4110
loop2    7:2    0  81.7M  1 loop /snap/core/4017
loop0    7:0    0  81.3M  1 loop /snap/core/3887

On checking the mount directory of snaps, this is what I get:

drwxr-xr-x 24 root root 321 Jan 22 13:17 3887
drwxr-xr-x 24 root root 321 Feb  6 20:48 4017
drwxr-xr-x 24 root root 321 Feb 20 22:37 4110
lrwxrwxrwx  1 root root   4 Feb 23 19:31 current -> 4110

and snap list shows this:

core  16-2.31.1  4110  canonical  core

Why is snapd not removing the old versions of snap core? Is it a normal behavior, or is there any way to remove this if this is not supposed to happen? I'm using Ubuntu 16.04.

Answer 1

In speaking with Ubuntu developers, the current default is to keep three prior versions of a snap so that you can roll-back to a prior version if needed. This default setting is not configurable.

You can use the command snap remove --revision=<an old one> snapname to remove an older revision of a snap.

They are considering whether to have only the current snap mounted as a loop filesystem, but I don't know the status of that.