[Bug 1773564] [NEW] Disc full during upgrade with / on btrfs fs and using docker (subvolumes)

[Jan Katins]

Public bug reported:

I experienced a disc full event on a `do-release-upgrade` when upgrading
my ubuntu 17.10 to 18.04 with my root partition on a btrfs fs. `df`
reported 50% free (= a lie), but apparently docker created so man
subvolumes (which apparently df doesn't know about) that the disc was
almost full.

The upgrade crashed while installing packages taking some of my running
GUI apps with them and not allowing me to start any new ones. I had to
boot into a rescue console and in the end deleted all docker subvolumes
in a for loop:

```
$ btrfs subvolume list /
ID 338 gen 94967 top level 5 path var/lib/docker/btrfs/subvolumes/0d2bd1457a1a6ed78b1b3be1c1ff57cc1c709b1f4b25edc27c6d31ce1bbee8c8
ID 339 gen 94969 top level 5 path 
[...]
$ # remove the echo...
$ for subvol in $(ls /var/lib/docker/btrfs/subvolumes); do echo btrfs subvolume delete $subvol ; done
$ btrfs balance start /
```

Afterwards running `apt install -f` and whatever it asked me to run
(some dpkg stuff to finish the installs) got me back into a running
system.

-> Please check that a btrfs root partition has enough left space to
support a upgrade with e.g. `btrfs fi show /`  and warn if a out-of-
space situation is likely.

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1773564

Title:
  Disc full during upgrade with / on btrfs fs and using docker
  (subvolumes)

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  I experienced a disc full event on a `do-release-upgrade` when
  upgrading my ubuntu 17.10 to 18.04 with my root partition on a btrfs
  fs. `df` reported 50% free (= a lie), but apparently docker created so
  man subvolumes (which apparently df doesn't know about) that the disc
  was almost full.

  The upgrade crashed while installing packages taking some of my
  running GUI apps with them and not allowing me to start any new ones.
  I had to boot into a rescue console and in the end deleted all docker
  subvolumes in a for loop:

  ```
  $ btrfs subvolume list /
  ID 338 gen 94967 top level 5 path var/lib/docker/btrfs/subvolumes/0d2bd1457a1a6ed78b1b3be1c1ff57cc1c709b1f4b25edc27c6d31ce1bbee8c8
  ID 339 gen 94969 top level 5 path 
  [...]
  $ # remove the echo...
  $ for subvol in $(ls /var/lib/docker/btrfs/subvolumes); do echo btrfs subvolume delete $subvol ; done
  $ btrfs balance start /
  ```

  Afterwards running `apt install -f` and whatever it asked me to run
  (some dpkg stuff to finish the installs) got me back into a running
  system.

  -> Please check that a btrfs root partition has enough left space to
  support a upgrade with e.g. `btrfs fi show /`  and warn if a out-of-
  space situation is likely.

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