Bcachefs

Bcachefs is a next-generation CoW filesystem that aims to provide features from Btrfs and ZFS with a cleaner codebase, more stability, greater speed and a GPL-compatible license.

It is built upon Bcache and is mainly developed by Kent Overstreet.

Installation

Bcachefs is not in the upstream Kernel yet but the linux-bcachefs-git^AUR kernel can be installed from the AUR.

The Bcachefs userspace tools are available from bcachefs-tools-git^AUR.

Setup

Single drive

# bcachefs format /dev/sda
# mount -t bcachefs /dev/sda /mnt

Multiple drives in RAID0/1

Bcachefs defines a replica as any instance of data, so 1 replica with 2 drives is equivalent to RAID0, 2 replicas with 2 drives is equivalent to RAID1, etc.

# bcachefs format /dev/sda /dev/sdb --replicas=n
# mount -t bcachefs /dev/sda1:/dev/sdb1 /mnt

RAID0/1 with SSD caching

Bcachefs has 3 categories of storage: background, foreground, and promote. Writes to the filesystem prioritize the foreground drives, which are then moved to the background over time. Reads are cached on the promote drives.

A recommended configuration is to use an ssd group for the foreground and promote, and an hdd group for the background, as in the following example.

ノート: These are not separated "tiers" of storage. They are just guidelines for a single large pool. Writes will go directly to the background if the foreground is full, or to promote if they both are. Metadata can be written to any of them. In this configuration, metadata_replicas should be at least 2, so that a cache drive may be able to fail without causing data loss.

# bcachefs format \
    --group=ssd /dev/sda /dev/sdb 
    --group=hdd /dev/sdc /dev/sdd /dev/sde /dev/sdf \
    --data_replicas=1 --metadata_replicas=2 \
    --foreground_target=ssd \
    --background_target=hdd \
    --promote_target=ssd
# mount -t bcachefs /dev/sda:/dev/sdb:/dev/sdc:/dev/sdd/dev/sde:/dev/sdf /mnt

Configuration

Most options can be set at either during bcachefs format, at mount time (mount -o option=value), or through sysfs (echo X > /sys/fs/bcachefs/UUID/options/option). Setting the option during format or changing it through sysfs saves it in the filesystem's superblock, making it the default for those drives. Mount options override those defaults.

ノート: The filesystem must be mounted for sysfs to be available. All operations except fsck are possible on a live filesystem

• data_checksum, metadata_checksum (none, crc32c, crc64)
• (foreground) compression, background_compression (none, lz4, gzip, zstd)
• foreground_target, background_target, promote_target

The following can also be set on a per directory or per file basis with bcachefs setattr file --option=value

• data_replicas
• data_checksum
• compression, background_compression
• foreground_target, background_target, promote_target

ノート: Disk usage reporting currently shows uncompressed size. Compression is otherwise complete.

Changing a device's group

# echo group > /sys/fs/bcachefs/filesystem_uuid/dev-X/label

ノート: This requires a remount to take effect.

Adding a device

# bcachefs device add --group=group /mnt /dev/device

If this is the first drive in a group, you'll need to change the target settings to make use of it. This example is for adding a cache drive.

# echo new_group > /sys/fs/bcachefs/filesystem_uuid/options/promote_target
# echo new_group > /sys/fs/bcachefs/filesystem_uuid/options/foreground_target
# echo old_group > /sys/fs/bcachefs/filesystem_uuid/options/background_target

ノート: Only new writes will be striped across added devices. Existing ones will be unchanged until disk usage reaches a certain threshold, when the disk rebalance is triggered. It is not currently possible to manually trigger a rebalance/restripe.

Removing a device

First make sure there are at least 2 metadata replicas (Evacuate does not appear to work for metadata). If your data and metadata are already replicated, you may skip this step.

# echo 2 > /sys/fs/bcachefs/UUID/options/metadata_replicas
# bcachefs data rereplicate /mnt
# bcachefs device set-state device readonly
# bcachefs device evacuate device

To remove the device.

# bcachefs device remove device
# bcachefs data rereplicate /mnt

Tips and tricks

Documentation

Up-to-date documentation is only available via bcachefs --help. The man page, for instance, includes the now-useless --tier option.

Check dmesg for more useful error messages.

See also

• Kent Overstreet's Patreon page
• Wikipedia:Bcachefs