Comment # 15 on bug 1222429 from Stefan Hundhammer

The way we do that in the installer is we are probing all attached disks for
existing filesystems, for installation as well as for a system upgrade.

We need to do this in a non-destructive way (because we might detect a
filesystem that belongs to another system, for example for parallel boot ), and
also in a way that does not get the whole installation process stuck for a long
time. In particular Btrfs was notorious for starting a long-overdue rebalancing
at that time if a user was unlucky, and you know how long that can take.

For a new installation, we use that information to calculate a proposed storage
layout (what filesystems to reuse, which ones to delete, which ones not to
touch at all).

For an upgrade, we need to check which filesystems might be eligible for an
upgrade; basically, which ones contain an existing older SUSE distro that we
can offer as candidates for an upgrade.

In both cases, we need to mount the filesystem to check for files such as
etc/fstab, etc/crypttab, etc/os-release.