Bcachefs Concepts

Terminology

Term	Description
Filesystem	A bcachefs instance spanning one or more devices
Device	A block device (disk, partition, or loop device) in the filesystem
Subvolume	Independent namespace within a filesystem
Snapshot	Read-only point-in-time copy of a subvolume
Superblock	Metadata block describing the filesystem

Copy-on-Write Architecture

Like ZFS and Btrfs, bcachefs uses copy-on-write (COW):

1. New data is written to free space
2. Metadata pointers are updated atomically
3. Old blocks are freed (unless referenced by snapshots)

Benefits:

• Atomic updates prevent corruption
• Efficient snapshots
• Data integrity through checksums

Comparison with ZFS

Aspect	Bcachefs	ZFS
License	GPL	CDDL
Kernel integration	External (DKMS)	External (DKMS/module)
Maturity	Experimental	Production-ready
Codebase size	~117K lines	~500K+ lines
Encryption	Built-in	Dataset-level
Compression	LZ4, gzip, zstd	LZ4, gzip, zstd, zle
RAID	Basic (RAID5/6 unstable)	Mature (RAIDZ1/2/3)
SSD caching	Native	L2ARC/SLOG
Community	Small	Large, established

Comparison with Btrfs

Aspect	Bcachefs	Btrfs
Kernel integration	External (DKMS)	In-kernel
Maturity	Experimental	Production (mostly)
RAID5/6	Unstable	Unstable
Encryption	Built-in	None (use LUKS)
Scrubbing	Supported	Supported
Quotas	Planned	Supported

Device Roles

Bcachefs supports multiple device types in a single filesystem:

Data Devices

Standard storage devices for filesystem data.

# Single device
bcachefs format /dev/sda

# Multiple devices (striped)
bcachefs format /dev/sda /dev/sdb

Cache Devices

Fast devices (SSDs, NVMe) for caching hot data:

bcachefs format \
    --label=hdd.hdd1 /dev/sda \
    --label=ssd.ssd1 /dev/nvme0n1 \
    --foreground_target=ssd \
    --promote_target=ssd \
    --background_target=hdd

Replication and Durability

Data Replication

Control how many copies of data are stored:

# RAID1-style mirroring
bcachefs format --replicas=2 /dev/sda /dev/sdb

Metadata Replication

Metadata can have different replication than data:

bcachefs format \
    --replicas=1 \
    --metadata_replicas=2 \
    /dev/sda /dev/sdb

Checksumming

All data and metadata is checksummed by default:

Algorithm	Description
crc32c	Fast, hardware-accelerated on modern CPUs
crc64	Stronger than crc32c
xxhash	Very fast hash
none	Disable checksums (not recommended)

Checksums enable:

• Detection of silent data corruption
• Automatic repair with replicated data
• Scrubbing to proactively find errors

Allocator

Bcachefs uses a sophisticated allocator:

• Buckets - Fixed-size allocation units
• Copygc - Background garbage collection
• Tiering - Automatic data movement between device classes