Changes/ImproveBtrfsPreset

Improve the Anaconda Btrfs preset

Summary

A few small changes will make it easier to do snapshots and rollbacks on btrfs, in the near future.

Owner

• Name: Chris Murphy
• Email: chrismurphy@fedoraproject.org
• Product: Anaconda
• Responsible WG: Workstation, KDE SIG

Current status

• Targeted release: Fedora 33
• Last updated: 2020-06-15
• 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

/boot will be on btrfs. This is supported by GRUB for ~10 years, and recently in the installer. It will make snapshotting of /boot possible, to ensure parity with /usr on rollback.

Use subvolumes for the following mountpoints: /home /var/logs /var/lib/libvirt/images. This will make it possible to avoid snapshot and rollback of logs, VM images, and user home. NOTE: the design is WIP, but also is not user facing.

Prior work in this area: Revisiting How We Put Together Linux System containers/bubblewrap

Feedback

One big file system? What about clean installs and using /home?
Anaconda custom partitioning will let you reuse an existing 'home' subvolume to mount at '/home', without a reformat. A new 'root' subvolume must be created for '/' however. NOTE: Subvolumes are a bit special, but act mostly like directories, including having no size.

Benefit to Fedora

Optimize the 'btrfs preset' installation for today's use cases, and prepare for additional features. A significant benefit of btrfs is it can be a nearly drop-in replacement, and not burden users with learning esoteric things.

Default features:

• One big file system, no longer run out of space on / or /home.
• Full data integrity checking.ƒ
• (Rootless) Podman will use snapshots.ƒ

Available optionally now, integration in the near future:

• cgroupsv2 IO isolation, completes the resource control picture for better system responsiveness.ƒ
• Transparent compression, file system/directory/file level granularity.ƒ
• Snapshots and rollbacks, restore files using reflink copy.ƒ

ƒ These are btrfs only features.

Scope

• Proposal owners:
  • Implement /boot on dedicated ~1G btrfs volume with 'boot' subvolume
  • Design a subvolume layout focusing on snapshot and rollback functionality, and modify the btrfs preset kickstart accordingly.

• Other developers:
  • Bootloader team to review and ack changes related to /boot.
  • Anaconda to review and merge changes.
  • Might affect dnf snapper plugin behavior.

• Policies and guidelines: N/A

• Trademark approval: N/A

Upgrade/compatibility impact

This change will not apply to upgrades.

How To Test

Install and use the system however you usually do, and it should behave the same.

User Experience

• Transparent in normal use.
• Always-on integrity verifies metadata and data. Corrupt data results in EIO, it never reaches user space. User is notified by kernel message about affected files by path.
• Podman can use snapshots (default) or reflinks+overlayfs, at user discretion. Both are faster and more space efficient operations, than copy up on ext4.

• Users can opt-in to other btrfs features: snapshots, compression, cp --reflink efficient copies, online scrub, btrfs send/receive, at their own pace.

Dependencies

None

Contingency Plan

• Contingency deadline: Revert all changes.
• Blocks release? Yes.
• Blocks product? Workstation/KDE are release blocking desktops.

Documentation

The user is not expected to know btrfs specific things for day to day use.

man 5 btrfs - for information about general btrfs information, mount options, and features.
man btrfs - for information about the btrfs command and sub commands. NOTE: Subcommands can be arbitrarily shortened, as long as it isn't ambiguous. 'btrfs fi us' is the same as 'btrfs filesystem usage'.

• Users and developers alike, may be interested in:
  • Btrfs wiki: Full list of btrfs features
  • Btrfs wiki: Project ideas
  • btrfs-progs: libbtrfsutil
  • knorrie/python-btrfs