# btrfs in Arcline OS

Arcline OS is **btrfs-native**: the root filesystem layout is created at
install time and snapshots are first-class, not bolted on.

## Subvolume layout

| Subvolume    | Mount point   | Purpose                                        |
|--------------|---------------|------------------------------------------------|
| `@`          | `/`           | system root — rolled back on failed upgrade    |
| `@home`      | `/home`       | user data — not rolled back by default         |
| `@log`       | `/var/log`    | logs survive rollbacks                         |
| `@snapshots` | `/.snapshots` | read-only snapshots live here                  |

The layout is created by `btrfs/init.sh <device>` (this is what the installer
runs). `/etc/fstab` templates live in each edition manifest and reference these
subvolumes with `subvol=` mount options.

## Tooling

- `btrfs/init.sh <device>` — create the subvolume layout on a target device.
- `btrfs/snapshot.sh` — snapshot / list / prune. Installed in images as
  `/usr/local/sbin/arcline-snapshot` and triggered automatically by the
  `arcline-snapshot.timer` systemd unit (see `overlays/base/usr/lib/systemd/system/`).
- `btrfs/rollback.sh` — promote a snapshot back to `@`. Installed as
  `/usr/local/sbin/arcline-rollback`.

## How a rollback works

Rollbacks are done from a rescue/live environment (never against the mounted
`@`). The tool moves the current `@` aside to `@.rollback-<ts>` and promotes
the chosen snapshot to `@`. Because btrfs renames are atomic, a crash mid-way
leaves a recoverable system on disk.

## Compression

All subvolumes mount with `compress=zstd:3` (see the edition `fstab`
templates). Transparent compression is on by default.