Support the creation of an empty snapshot

[davidcassany]

In snapper from an image builder perspective we are missing the capability of creating a new empty snapshot. By empty I mean creating a snapshot that is not based on a previous one.

The reasoning is the following, in Elemental, we aim to install/update images as snapshots with following sequence:

1. Start transaction (new snapshot playground is created)
2. include data into the new snapshot as desired
3. Close transaction (make the new snapshot the new default root)

Pretty similar to what tukit from transactional updates does. The problem we have is that we implement this procedure for the very first snapshot too (our first root is considered to be a snapshot too like in MicroOS). We are basically missing a way to call snapper to create a new empty subvolume as an snapshot, that way, snapper could be really our wrapper around btrfs utility.

Let me expose how we create the first root to be seen as a snapshot by snapper (roughly translated into shell steps):

# /mnt is a btrfs partition
# Enable quota management and create subvolumes
btrfs quota enable /mnt
btrfs subvolume create /mnt/@
btrfs subvolume create /mnt/@/.snapshots

mkdir -p /mnt/@/.snapshots/1

btrfs subvolume snapshot /mnt/@ /mnt/@/.snapshots/1/snapshot

volid=$(btrfs subvolume list --sort path /mnt | head -n 1 | cut -d" " -f2)
btrfs subvolume set-default ${volid} /mnt

# Create snapshot 1 info
date=$(date +'%Y-%m-%d %H:%M:%S')
cat << EOF > /mnt/@/.snapshots/1/info.xml
<?xml version="1.0"?>
<snapshot>
  <type>single</type>
  <num>1</num>
  <date>${date}</date>
  <description>first root filesystem</description>
</snapshot>
EOF

# Dump content into the first snapshot subvolume

# set first snapshot as readonly
btrfs property set /mnt/@/.snapshots/1/snapshot ro true

The so the problem we have is that we need to wrap btrfs to create subvolumes, create snapshot metadata, handle subvolumes IDs vs snapshot IDs, etc. It feels a bit like we are re-implementing core functionalities of snapper itself.

What we'd love to see in snapper is something similar to:

# Create required /mnt/@/.snapshots subvolumes structure
snapper --configdir custom-cfg --root /mnt --no-dbus init

# Create a new empty snapshot, use --empty or something similar instead of '--from ID'
snapshotID=$(snapper --configdir custom-cfg --root /mnt --no-dbus create --empty --read-write --print-number --description "First root install" --userdata "first snapshot")

# Dump content into the first snapshot subvolume

# Set just created snapshot as read only
snapper --configdir custom-cfg --root /mnt --no-dbus modify --read-only $snapshotID

So to my eyes is essentially bringing to snapshot the capability to create empty snapshots (no parent) and the capability to provide custom configuration beyond /etc. This is good for imaging and creating systems like Micro OS in which the very first / is considered a snapshot already. This would also be valuable for KIWI to simplify some of the logic, as there in order to support Micro OS builds there is also btrfs custom logic to mimic snapper.

[aschnell]

Creation of an empty snapshot is very easy since it is partly already implemented. YaST
also needs that functionality but it is so far only used in the "installation-helper".

https://github.com/openSUSE/snapper/blob/master/client/installation-helper/installation-helper.cc#L106

Since you use-case uses --no-dbus anyway it is not even required to extend the DBus
interface.