Disk free space left check

I discovered I'd run out of disk space when installing a snap "froze". Annoying, because it's happening in a privileged space, it actually used 100% of the disk, instead of respecting the root-reserved section, which mean I didn't have enough free disk space to resize my LVM volume to give myself more disk space.

In this case I could resolve it by deleting the partially-downloaded snap and doing the resize, but I feel it's a significant bug that it managed to eat up all of the disk space and hang at the same time.

I ran into this in recover mode in uc20 with a small tmpfs mounted for ubuntu-data and got the following snapd log/panic https://pastebin.ubuntu.com/p/6BkjjssQpt/

I think that at a minimum, before the full fix of having snapd estimate how much disk space is required for an operation and cancel before filling up the disk, we should figure out how to at least make snapd service failover eventually fail such that snap-failure gets started up because as it is, snapd takes so long to start up (and thus times out) that we don't hit the StartLimitBurst and StartLimitIntervalUsec default settings (and snapd.service doesn't configure these for itself) and thus snap-failure never gets started.

If we at least hit the restart limit, then that would give snap-failure an opportunity to cleanup partial downloads so that snapd could start itself back up. But note that snap-failure currently would just revert snapd which may or may not be sufficient to unbreak snapd here. Ideally also snap-failure would detect that the disk is full and instead of reverting snapd revision it would just clean up the disk for snapd and leave some kind of flag for snapd to know that it should give up on it's current set of changes/tasks in flight due to disk space issues.

This is being worked on, it will involve a series of PRs, e.g. https://github.com/snapcore/snapd/pull/9002

Snapd is now much more aware of disk space constraints, especially around critical operations that rely on additional space, such as refreshes. The feature is still behind several experimental feature flags, and is not enabled by default.

Since Snap I have much problem with diskspace. I permanently get diskspace warnings It seems that Snap-apps I have, using around 50% of my Root disk space. It would be much easier If I could permanently deactivate or limit the backup of old SNAP versions to certain Apps.
e.g. 'sudo snap set system refresh.retain=0'

For the record cleaning old snap is only a temporary solution.

#!/bin/bash
# Removes old revisions of snaps
set -eu

LANG=en_US.UTF-8 snap list --all | awk '/disabled/{print $1, $3}' |
    while read snapname revision; do
        snap remove "$snapname" --revision="$revision"
    done

I would vote for a solution which autoremove oldest SNAP if free Diskspace is lower than double the disk-space warning.