NixOS with Btrfs

snapshot.sh

# I have 3 btrfs subvolume /root, /home, /nix-store and these are used as the name suggest
# Now, when i create new nixos configuration i can make a snapshot of root and home, before i switch to a new config.

CURRENT_GENERATION=`readlink /nix/var/nix/profiles/system | cut -d'-' -f2`; echo Determined current generation: $CURRENT_GENERATION
sudo mkdir -p /btrfs
# the / is not mounted in my btrfs filesystem by default, because my filesystem root is the /root subvolume
sudo mount /dev/mapper/NixOS /btrfs
sudo btrfs subvolume snapshot /btrfs/root /btrfs/root-$CURRENT_GENERATION-snapshot
# I also make a snapshot of home, because i use gnome or KDE, which contains a lots of config and cache, which can broke by a rollback.
sudo btrfs subvolume snapshot /btrfs/home /btrfs/home-$CURRENT_GENERATION-snapshot


# We can see how much space we use for the snapshots:
sudo btrfs filesystem du -s --mbytes --si /btrfs/*
#     Total   Exclusive  Set shared  Filename
# 2834.48MB      9.58MB   2504.99MB  /btrfs/home
# 2833.87MB      8.65MB   2505.27MB  /btrfs/home-7-snapshot
#12234.15MB  12234.15MB      0.00MB  /btrfs/nix-store
#  119.43MB      2.10MB    115.00MB  /btrfs/root
#  119.44MB      0.82MB    116.28MB  /btrfs/root-7-snapshot
#
# So there is 12 GB that we do not have to snapshot at all and also the root snapshot itself only takes around 12 MB