॥ श्री ॥

Persistence Strategy

Concepts 2026-04-01

Shanios selectively persists data across immutable system updates through bind mounts and dedicated Btrfs subvolumes.

Three Categories

❌ Replaced on Update

The active slot (@blue or @green) is overwritten:

  • / (root — @blue or @green)
  • /usr /bin /sbin /lib
  • /opt /srv (pre-installed)
  • /boot/efi (ESP — UKI updated)

✅ Persistent (Survives All Updates)

Dedicated Btrfs subvolumes that are never touched by updates:

| Location | Subvolume | |----------|-----------| | /home | @home | | /root | @root | | /data | @data — overlay + service state | | /var/log | @log | | /var/cache | @cache | | /var/lib/flatpak | @flatpak | | /var/lib/containers | @containers | | /nix | @nix — Nix package store | | /var/lib/libvirt | @libvirt (nodatacow) | | /var/lib/waydroid | @waydroid | | /var/lib/snapd | @snapd |

@nix and @flatpak are shared by both slots.

🔄 Volatile (Cleared on Reboot)

systemd.volatile=state kernel parameter mounts a tmpfs over /var:

  • /var (tmpfs)
  • /tmp (tmpfs — fstab entry)
  • /run (tmpfs)
  • /dev /proc /sys (virtual filesystems)

Critical service state is bind-mounted back from @data on every boot.

The /etc Overlay

/etc is an OverlayFS — a three-layer combination:

  • Lower layer (read-only): /etc from the active slot (@blue or @green)
  • Upper layer (writable): /data/overlay/etc/upper/ — only your changes live here
  • Work dir: /data/overlay/etc/work/ — required by the kernel for atomic copy-up
/etc appears fully writable to all processes.
Upper overrides lower — your changes take precedence.
Unchanged files are served from the read-only lower layer.
Changes in upper (@data) survive OS updates — only the lower is replaced.

fstab entry:

overlay /etc overlay rw,lowerdir=/etc,upperdir=/data/overlay/etc/upper,workdir=/data/overlay/etc/work,index=off,metacopy=off,x-systemd.requires-mounts-for=/data 0 0

View overlay modifications:

ls -la /data/overlay/etc/upper

Bind-Mounted Service State

Because /var is volatile, all service state that must survive reboots is stored in @data and bind-mounted back at boot.

Key bind mounts from @data/varlib/ → /var/lib/:

| Category | Examples | |----------|---------| | System Core | dbus, systemd, fontconfig | | Networking | NetworkManager, bluetooth, firewalld | | File Sharing | samba, nfs | | Remote Access & VPN | caddy, tailscale, cloudflared, geoclue | | Display Manager | gdm, sddm, colord | | Audio & Peripherals | pipewire, cups, sane, upower | | Auth & Security | fprint, AccountsService, boltd, sshd, tpm2-tss | | Spool | cron, at, cups print queue, postfix |

All bind mounts use:

bind,nofail,x-systemd.after=var.mount,x-systemd.requires-mounts-for=/data