bcachefs/rootfs

Warning
bcachefs support is currently in its experimental stages in Gentoo and the Kernel so dragons beware! Please report Kernel issues upstream.

This article has some todo items:

• Encrypted root (using LUKS and native?)

• Test one or both Dracut hooks: first, second

This article is focused on using bcachefs as the root file system on Gentoo and is intended to be followed alongside the Handbook.

Disk setup

Important
Grub does not currently have a bcachefs driver, although a bug is opened, so having a unified /boot is required (or a /boot on a different partition)

The disk layout for bcachefs is similar to a normal disk layout using ext4 or XFS: a vfat boot partition, a swap partition, and the bcachefs partition.

root #mkfs.vfat -F 32 /dev/sda1

root #mkswap /dev/sda2

root #mkfs.bcachefs /dev/sda3

Or, using bcachefs's cli tools:

root #bcachefs format /dev/sda3

Before proceeding, follow the handbook and resume this guide after reaching Applying a filesystem to a partition.

Subvolumes

Subvolumes in bcachefs are similar to those in btrfs, but with one benefit: they don't need to be mounted! When the root file system is mounted, the subvolumes are mounted along with it.

To create a subvolume for /home, run the following:

root /mnt/gentoo #bcachefs subvolume create home

Important
Listing subvolumes is still a work in progress so for now, verifying the subvolumes exist will not be possible.

Continue following the Handbook and return upon reaching Configuring the Linux kernel

Kernel Config

For configuring the kernel, following the manual configuration guide in the Handbook should be sufficient, all that needs to be changed for bcachefs is:

KERNEL Adding bcachefs support

File Systems --->
   <*> bcachefs filesystem support (EXPERIMENTAL)
      [ ]   bcachefs quota support
      [ ]   bcachefs erasure coding (RAID5/6) support (EXPERIMENTAL)
      [*]   bcachefs POSIX ACL support
      [ ]   bcachefs runtime info
      [ ]   bcachefs debugging
      [ ]   bcachefs unit and performance tests
      [ ]   bcachefs lock time statistics
      [ ]   disable latency accounting and time stats

If a lscpu shows ssse3 and/or avx2 it is recommended to enable also:

KERNEL Adding bcachefs support - Accelerated Cryptographic Algorithms

-*- Cryptographic API  --->
    Accelerated Cryptographic Algorithms for CPU (x86)  --->
        [*] Ciphers: ChaCha20, XChaCha20, XChaCha12 (SSSE3/AVX2/AVX-512VL)
        [*] Hash functions: Poly1305 (SSE2/AVX2)
        [*] Hash functions: SHA-224 and SHA-256 (SSSE3/AVX/AVX2/SHA-NI)
        [*] CRC32c (SSE4.2/PCLMULQDQ)

After building the Kernel, if the Distribution Kernel config was used, install the initramfs with

Note
Using sys-kernel/gentoo-sources should automatically run dracut after make install if sys-kernel/installkernel is built with the dracut USE flag, but in case it does not, this instruction should help.

root #dracut --kver=6.12.16-gentoo

/etc/fstab

An example fstab for a bcachefs rootfs looks like:

FILE /etc/fstab

/dev/sda1 /boot vfat     defaults 1 2
/dev/sda2 none  swap     sw       0 0
/dev/sda3 /     bcachefs defaults 0 0

Important
Dracut will fail to boot if the option fs_passno in the fstab (the second number in 0 0 columns) is set to >0 due to it thinking the file system is ext4, set it to 0 in the meantime. (There is a dracut hook but this guide has not tested it yet.)

and finally finish the Handbook, resuming at configuring the system.

Userspace Tools

Important
Installing these tools for bcachefs on root are required!

To manage bcachefs in userspace, the package sys-fs/bcachefs-tools will need to be installed.

root #emerge --ask sys-fs/bcachefs-tools

Issues

Multi Device Root FS

Using the command to add a second partition to one bcachefs pool currently leaves the system unbootable when used on the rootfs, so should not be used until bcachefs support is added to sys-boot/grub and other bootloaders. This will work fine if you have the rootfs on one pool and create a second pool which uses multiple partitions.

To tracker this bug please follow this bug report.

Note
This could be worked around by creating your own custom initramfs however this will need a lot of management on the user's part over the lifetime of the system.