If rootfs placed on btrfs filesystem, place root on @ subvolume

[iav]

It makes possible to work with snapshots of a root filesystem on btrfs

Using snapshots of a rootfs on btrfs needs to place root on a btrfs subvolume, not on root of btrfs volume itself.
Sometimes some of directories have to be placed on separate subvolumes.

After this change snapper can be installed, and do auto-snapshots on timer and apt operations.

This PR:

1. places root filesystem to "@" btrfs subvolume instead of a root of btrfs itself
2. adds 2 hook calls in fs creation process to make possible to create additional btrfs subvolumes
3. adds inactive example of usage — how to place /var, /var/log, /home directories on separate btrfs subvolumes

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

[coderabbitai]

Walkthrough

The changes update the prepare_partitions function within the partitioning.sh script to add support for the Btrfs filesystem. When the ROOTFS_TYPE is set to btrfs, the function now creates a set of Btrfs subvolumes including those for @home, @var, @var_log, @var_cache, and @srv. A default subvolume @ is also created for the root filesystem and is set as the default subvolume for mounting. Additionally, the script appends the necessary entries to the fstab file with appropriate mount options specific to Btrfs, ensuring the subvolumes are correctly mounted during boot. No changes were made to the declarations of exported or public entities.

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (2)

lib/functions/image/partitioning.sh (2)

317-342: Btrfs Subvolume Creation and Root Remounting

This new block introduces btrfs‐specific logic by:
• Setting the mount option for btrfs to "commit=120" (note that this now omits the leading comma compared to its earlier default, so please ensure consistency across mount option concatenations).
• Creating the root subvolume “@” with btrfs subvolume create $MOUNT/@.
• Retrieving the subvolume ID using a pipeline that includes grep 'path @' and cut before setting it as the default with btrfs subvolume set-default.

Please verify that the filtering with grep 'path @' is robust across different btrfs versions and that only the intended subvolume is matched. In addition, consider whether additional error handling or logging should be added in case the subvolume creation or default setting fails.

---

345-363: Btrfs fstab Entries for Additional Subvolumes

This section creates and mounts additional btrfs subvolumes (namely for /home, /var, /var/log, /var/cache, and /srv) and appends corresponding entries to fstab. The use of the call_extension_method hook to encapsulate these operations is a good approach for modularity. Ensure that:
• Directory creation is idempotent so that repeated runs do not result in errors.
• The fstab entries are unique and nonconflicting if the image is rebuilt or reconfigured.
• Appropriate logging or error checking is in place for these mount operations.

A brief review in various environments may help verify that these extra mounts integrate well with the overall mounting scheme.

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• lib/functions/image/partitioning.sh (2 hunks)

🔇 Additional comments (1)

lib/functions/image/partitioning.sh (1)

247-247: Minor Formatting/Whitespace Change

A new line appears to be added right after the partitioning command output. This seems intended to improve readability or visually separate stages in the script. Verify that this extra newline was intentional and does not affect downstream processing.

[The-going]

Using snapshots of a rootfs on btrfs needs to place root on a btrfs subvolume

Good health!

Maybe it's better to implement this feature in armbian-install?

[igorpecovnik]

LGTM.

I also agree that it would be nice to add this also to armbian-install.

But there is ongoing RFC of it. I did some front-end changed armbian/configng#384 and @adeepn was also looking into integrating (fresh modular design) armbian-install into armbian-config.

[iav]

I'll try to add this to armbian-install.