Changes/VarSubvol4SilverblueKinoite

Put /var on its own Btrfs subvolume by default, on Silverblue and Kinoite

Summary

Silverblue and Kinoite: For new clean automatic installs, create a "var" subvolume to be mounted at /var.

Owner

• Name: Chris Murphy
• Email: bugzilla@colorremedies.com

Current status

• Targeted release: Fedora Linux 36
• Last updated: 2022-01-18
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

Currently, Silverblue and Kinoite mimic other Fedora desktops. There is a "root" subvolume mounted at / and a "home" subvolume mounted at /home. This proposal will add a "var" subvolume to be mounted at /var. It will be located at the top-level of the Btrfs file system, along-side the "root" and "home" subvolumes, with an entry in /etc/fstab to mount it into the proper position during startup.

Benefit to Fedora

Users who opt into Btrfs features like snapshots and rollbacks.

• By moving /var into its own subvolume, it will be excluded from snapshots and rollbacks of the "root" subvolume, which contains /etc and /usr.
• Users will find it straightforward to rollback "root" while not rolling back "var": including logs, VMs, databases, and so on.
• The ability to snapshot only "var" and use Btrfs send/receive to replicate only "var" permits for an efficient way of backing up the variable system data.
  • A clean install can restore the "root", therefore it doesn't strictly need to be backed up. Meanwhile "var" and "home" can be restored using snapshot replication via send/receive.

Scope

• Proposal owners:
  • changes to lorax and anaconda as needed so that Silverblue and Kinoite variants have their own installation kickstart, such that automatic/guided installation automatically creates "var".
    • possible liability, determine whether the the addition of /var mount point for Btrfs scheme results in /var mount point for other schemes (and inhibit)

• Other developers:

• Release engineering: #Releng issue number

• Policies and guidelines: N/A (not needed for this Change)

• Trademark approval: N/A (not needed for this Change)

• Alignment with Objectives:

Upgrade/compatibility impact

Change will not be applied to upgrades. But we can document steps to apply the change to existing installations.

How To Test

• Do a clean installation and check df and /etc/fstab for an explicitly listed /var mount point.

User Experience

• The change won't generally be noticeable to users
• Users will see an additional /var mount point in /etc/fstab, and df
• Some utilities, notably backup programs like borg backup, and rsync with -x option, will treat Btrfs subvolumes as separate file systems and may not descend (recursively) into them.

Dependencies

• Anaconda/blivet, lorax, and possibly kickstarts

Contingency Plan

• Contingency mechanism: (What to do? Who will do it?) N/A (not a System Wide Change)
• Contingency deadline: beta freeze (not a System Wide Change)
• Blocks release? No

Documentation

No significant documentation is planned other than this change proposal.

N/A (not a System Wide Change)