snapshots should be disabled during distribution upgrade

[Kevin Squire]

During a distribution upgrade, multiple copies of apt are sometimes running simultaneously, and new apt-get processes are started frequently. This causes a couple of issues

1. First, many snapshots are created.
2. Second, the default granularity is such that, apt-btrfs-snapshot occasionally attempts to create two snapshots with the same name (in two different processes), which causes the upgrade to fail)

In reality, it would be useful if only one snapshot were taken, at the beginning of the upgrade.

(As an aside, and other than this issue, this package is a godsend!)

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: apt-btrfs-snapshot 0.3.4.2
Uname: Linux 3.16.3-031603-generic x86_64
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair wl
ApportVersion: 2.14.7-0ubuntu6
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Oct 15 11:40:36 2014
InstallationDate: Installed on 2014-09-29 (16 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: apt-btrfs-snapshot
UpgradeStatus: Upgraded to utopic on 2014-10-14 (1 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Id2ndR (id2ndr)]

In my case, my 15GB root partition is nearly full (btrfs filesystem df /) whereas it only contain 7.5GB of data (du -shx /).
In fact, I just discovered @apt-snapshot-release-upgrade-utopic-2014-10-23_23:24:46 volume (sudo btrfs subvolume list -p /), and I guess it used the other 7.5GB.

So just after dist-upgrading from trusty to utopic, my root volume just goes from 50% full to 99% full.