Small snap loop devices visible in gnome-disk-utility OR what is the function of snap ubuntu-core

Question

I just realized using gnome-disk-utility (3.18.3.1) that my 16.04 installation is featuring several small loop devices.

In detail:

• 78 MB Loop Device (/var/lib/snapd/sn_ntu-core_352.snap -> /dev/loop0)
• 76 MB Loop Device (/var/lib/snapd/sn_ntu-core_216.snap -> /dev/loop1)
• 79 MB Loop Device (/var/lib/snapd/sn_ntu-core_423.snap -> /dev/loop2)
• 705 KB Loop Device (/var/lib/snapd/snaps/tpad_12.snap -> /dev/loop3)
• 684 KB Loop Device (/var/lib/snapd/snaps/tpad_11.snap -> /dev/loop5)
• 705 KB Loop Device (/var/lib/snapd/snaps/tpad_13.snap -> /dev/loop6)

Image showing gnome-disk-utility

My questions are as follows:

• What are those loop devices used for?
• Is that an expected behaviour in 16.04?
• Can I remove them? And if so - how (just by unmounting)?
• Is there any risk in removing the 2 installed snaps mentioned below (tpad and ubuntu-core)

UPDATE

Installed snaps

According to snap list I do have

• tpad (1.8.1)
• ubuntu-core (16.04.1)

installed. I remember I did tinker with snaps for a short time when I realized this option - but am pretty unsure if I installed those 2 snaps - or if they are pre-/auto-installed.

tpad is a Terminal text editor with GUI-like user interface I did install manually, as I don't use it I removed it now via sudo snap remove tpad. As a result the 2 loop devices pointing to tpad are gone in gnome-disk-utility.

So - at the current point I am still having 1 snap installed called ubuntu-core and I don't know what it is used for. The summary of this particular snap is

The ubuntu-core OS snap

Answer 1

Each individual snap is a read-only squashfs filesystem image. In order to access files within these images, snapd mounts the images, one for each installed snap version, inside /snap.

This list of mounted loop devices thus effectively includes the snaps you have installed, and is part of the normal operation of snapd. You should not attempt remove them. If you really don't want them there at all, then you can remove the corresponding snaps.

Various utilities are designed to show you all mounted filesystems, including loopback ones, which is why you see them. snapd is the first common daemon I've seen to maintain a large and variable set of loopback mounts in this way, which admittedly does hamper the UI of these utilities. Perhaps eventually we'll see these utilities hide loopback mounts by default.

Answer 2

You can remove old core by execute

sudo snap remove core --revision <number>

manually.

Answer 3

If you don't use snaps it than just uninstall them as you did. Ubuntu core is the snap that the others snaps need to run. Visiblity of snaps in gnome-disk-utility is probably not wanted but it's a side effect of how snaps are working.

Answer 4

Two ways using a script:

The best and simple:

sudo fdisk -l | sed -e '/ \/dev\/loop/,+5d'

And the funny geek way (for study of sed, IFS, etc) :

# Copies output to var x: 
x=`sudo fdisk -l`

# Tranform new lines in @@@
y=`sed ':a;N;s/\n/@@@/g' <<< "$x"`

# Inform linux the new separator
IFS='@@@'
         
fo=0  # A beautiful flag to "found" dev/sda

for a in $y
  do
  if [[  "$a" == *"/dev/sda"* || $fo == 1  ]]; then
    fo=1  # Yes, we found
    echo -e "$a"
  fi
  done
# Just do add a line separator above:
echo ''