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.

You can use squashfs to compress disk images and then mount them.

Create the disk image

If you haven't got a disk image yet use dd to create one:

dd if=/dev/sda1 of=/tmp/sda1.img bs=4M

Compress the image with squashfs

Install squashfs:

apt-get install squashfs-tools

Compress the image:

mksquashfs /tmp/sda1.img /tmp/sda1.squash.img

Or Stream the compression (don't need a temporary dd file)

mkdir empty-dir
mksquashfs empty-dir squash.img -p 'sda_backup.img f 444 root root dd if=/dev/sda bs=4M'

Compliments to terminator14 at UbuntuForums.org. Definitions/Explanations:

• empty-dir - "source" dir. Basically in our case, just an empty dir to satisfy mksquashfs' input arg format
• squash.img - the destination and filename of the output squashfs file
• sda_backup.img - the name of the dd backup INSIDE the squashfs file
• f - specifies that sda_backup.img is a regular file (as opposed to a directory, block device, or char device)
• 444 - permissions of the sda_backup.img file inside the squashfs image
• root root - UID and GID for the sda_backup.img file inside the squashfs image. Can be specified by decimal numbers, or by name
• dd if=/dev/sda bs=4M - the dd command used to read the device we want backed up

Mount the image

First mount the squashfs image:

mkdir /mnt/squash
mount /tmp/sda1.squash.img /mnt/squash

This will present the un-compressed disk image for you to mount:

mkdir /mnt/sda1
mount /mnt/squash/sda1.img /mnt/sda1

Or if it's a full drive image (partitioned) you could use losetup to attach the dd image to a loop device (possibly optional) and then kpartx -a or partprobe to find & separate the partitions to separate devices, or even vgscan / vgchange -ay if there's LVM.