When Linux 6.7 (already end of life) was released some time ago another file system made it into the kernel: bcachefs. This is another copy on write file system like ZFS or Btrfs. The goal of this post is not to compare those in regards to features and performance, but just to give you the necessary bits to get started with it. If you want to try this out for yourself, you obviously need at least version 6.7 of the Linux kernel. You can either build it yourself or you can use the distribution of your choice which ships at least with kernel 6.7 as an option. I’ll be using openSUSE Tumbleweed as this is a rolling release and new kernel versions make it into the distribution quite fast after they’ve been released.
When you install Tumbleweed as of today, you’ll get a 6.8 kernel which is fine if you want to play around with bcachefs:
tumbleweed:~ $ uname -a
Linux tumbleweed 6.8.5-1-default #1 SMP PREEMPT_DYNAMIC Thu Apr 11 04:31:19 UTC 2024 (542f698) x86_64 x86_64 x86_64 GNU/Linux
Let’s start very simple: Once device, on file system. Usually you create a new file system with the mkfs command, but you’ll quickly notice that there is nothing for bcachefs:
tumbleweed:~ $ mkfs.[TAB][TAB]
mkfs.bfs mkfs.btrfs mkfs.cramfs mkfs.ext2 mkfs.ext3 mkfs.ext4 mkfs.fat mkfs.minix mkfs.msdos mkfs.ntfs mkfs.vfat
By default there is also no command which starts with “bca”:
tumbleweed:~ # bca[TAB][TAB]
The utilities you need to get started need to be installed on Tumbleweed:
tumbleweed:~ $ zypper se bcachefs
Loading repository data...
Reading installed packages...
S | Name | Summary | Type
--+----------------+--------------------------------------+--------
| bcachefs-tools | Configuration utilities for bcachefs | package
tumbleweed:~ $ zypper in -y bcachefs-tools
Loading repository data...
Reading installed packages...
Resolving package dependencies...
The following 2 NEW packages are going to be installed:
bcachefs-tools libsodium23
2 new packages to install.
Overall download size: 1.4 MiB. Already cached: 0 B. After the operation, additional 3.6 MiB will be used.
Backend: classic_rpmtrans
Continue? [y/n/v/...? shows all options] (y): y
Retrieving: libsodium23-1.0.18-2.16.x86_64 (Main Repository (OSS)) (1/2), 169.7 KiB
Retrieving: libsodium23-1.0.18-2.16.x86_64.rpm ...........................................................................................................[done (173.6 KiB/s)]
Retrieving: bcachefs-tools-1.6.4-1.2.x86_64 (Main Repository (OSS)) (2/2), 1.2 MiB
Retrieving: bcachefs-tools-1.6.4-1.2.x86_64.rpm ............................................................................................................[done (5.4 MiB/s)]
Checking for file conflicts: ...........................................................................................................................................[done]
(1/2) Installing: libsodium23-1.0.18-2.16.x86_64 .......................................................................................................................[done]
(2/2) Installing: bcachefs-tools-1.6.4-1.2.x86_64 ......................................................................................................................[done]
Running post-transaction scripts .......................................................................................................................................[done]
This will give you “mkfs.bcachefs” and all the other utilities you’ll need to play with it.
I’ve prepared six small devices I can play with:
tumbleweed:~ $ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sr0 11:0 1 276M 0 rom
vda 254:0 0 20G 0 disk
├─vda1 254:1 0 8M 0 part
├─vda2 254:2 0 18.6G 0 part /var
│ /srv
│ /usr/local
│ /opt
│ /root
│ /home
│ /boot/grub2/x86_64-efi
│ /boot/grub2/i386-pc
│ /.snapshots
│ /
└─vda3 254:3 0 1.4G 0 part [SWAP]
vdb 254:16 0 4G 0 disk
vdc 254:32 0 4G 0 disk
vdd 254:48 0 4G 0 disk
vde 254:64 0 4G 0 disk
vdf 254:80 0 4G 0 disk
vdg 254:96 0 4G 0 disk
In the most simple form (one device, one file system) you might start like this:
tumbleweed:~ $ bcachefs format /dev/vdb
External UUID: 127933ff-575b-484f-9eab-d0bf5dbf52b2
Internal UUID: fbf59149-3dc4-4871-bfb5-8fb910d0529f
Magic number: c68573f6-66ce-90a9-d96a-60cf803df7ef
Device index: 0
Label:
Version: 1.6: btree_subvolume_children
Version upgrade complete: 0.0: (unknown version)
Oldest version on disk: 1.6: btree_subvolume_children
Created: Wed Apr 17 10:39:58 2024
Sequence number: 0
Time of last write: Thu Jan 1 01:00:00 1970
Superblock size: 960 B/1.00 MiB
Clean: 0
Devices: 1
Sections: members_v1,members_v2
Features: new_siphash,new_extent_overwrite,btree_ptr_v2,extents_above_btree_updates,btree_updates_journalled,new_varint,journal_no_flush,alloc_v2,extents_across_btree_nodes
Compat features:
Options:
block_size: 512 B
btree_node_size: 256 KiB
errors: continue [ro] panic
metadata_replicas: 1
data_replicas: 1
metadata_replicas_required: 1
data_replicas_required: 1
encoded_extent_max: 64.0 KiB
metadata_checksum: none [crc32c] crc64 xxhash
data_checksum: none [crc32c] crc64 xxhash
compression: none
background_compression: none
str_hash: crc32c crc64 [siphash]
metadata_target: none
foreground_target: none
background_target: none
promote_target: none
erasure_code: 0
inodes_32bit: 1
shard_inode_numbers: 1
inodes_use_key_cache: 1
gc_reserve_percent: 8
gc_reserve_bytes: 0 B
root_reserve_percent: 0
wide_macs: 0
acl: 1
usrquota: 0
grpquota: 0
prjquota: 0
journal_flush_delay: 1000
journal_flush_disabled: 0
journal_reclaim_delay: 100
journal_transaction_names: 1
version_upgrade: [compatible] incompatible none
nocow: 0
members_v2 (size 144):
Device: 0
Label: (none)
UUID: bb28c803-621a-4007-af13-a9218808de8f
Size: 4.00 GiB
read errors: 0
write errors: 0
checksum errors: 0
seqread iops: 0
seqwrite iops: 0
randread iops: 0
randwrite iops: 0
Bucket size: 256 KiB
First bucket: 0
Buckets: 16384
Last mount: (never)
Last superblock write: 0
State: rw
Data allowed: journal,btree,user
Has data: (none)
Durability: 1
Discard: 0
Freespace initialized: 0
mounting version 1.6: btree_subvolume_children
initializing new filesystem
going read-write
initializing freespace
This is already ready to mount and we have our first bcachfs file system:
tumbleweed:~ $ mkdir /mnt/dummy
tumbleweed:~ $ mount /dev/vdb /mnt/dummy
tumbleweed:~ $ df -h | grep dummy
/dev/vdb 3.7G 2.0M 3.6G 1% /mnt/dummy
If you need encryption, this is supported as well and obviously is asking you for a passphrase when you format the device:
tumbleweed:~ $ umount /mnt/dummy
tumbleweed:~ $ bcachefs format --encrypted --force /dev/vdb
Enter passphrase:
Enter same passphrase again:
/dev/vdb contains a bcachefs filesystem
External UUID: aa0a4742-46ed-4228-a590-62b8e2de7633
Internal UUID: 800b2306-3900-47fb-9a42-2f7e75baec99
Magic number: c68573f6-66ce-90a9-d96a-60cf803df7ef
Device index: 0
Label:
Version: 1.4: member_seq
Version upgrade complete: 0.0: (unknown version)
Oldest version on disk: 1.4: member_seq
Created: Wed Apr 17 10:46:06 2024
Sequence number: 0
Time of last write: Thu Jan 1 01:00:00 1970
Superblock size: 1.00 KiB/1.00 MiB
Clean: 0
Devices: 1
Sections: members_v1,crypt,members_v2
Features:
Compat features:
Options:
block_size: 512 B
btree_node_size: 256 KiB
errors: continue [ro] panic
metadata_replicas: 1
data_replicas: 1
metadata_replicas_required: 1
data_replicas_required: 1
encoded_extent_max: 64.0 KiB
metadata_checksum: none [crc32c] crc64 xxhash
data_checksum: none [crc32c] crc64 xxhash
compression: none
background_compression: none
str_hash: crc32c crc64 [siphash]
metadata_target: none
foreground_target: none
background_target: none
promote_target: none
erasure_code: 0
inodes_32bit: 1
shard_inode_numbers: 1
inodes_use_key_cache: 1
gc_reserve_percent: 8
gc_reserve_bytes: 0 B
root_reserve_percent: 0
wide_macs: 0
acl: 1
usrquota: 0
grpquota: 0
prjquota: 0
journal_flush_delay: 1000
journal_flush_disabled: 0
journal_reclaim_delay: 100
journal_transaction_names: 1
version_upgrade: [compatible] incompatible none
nocow: 0
members_v2 (size 144):
Device: 0
Label: (none)
UUID: 60de61d2-391b-4605-b0da-5f593b7c703f
Size: 4.00 GiB
read errors: 0
write errors: 0
checksum errors: 0
seqread iops: 0
seqwrite iops: 0
randread iops: 0
randwrite iops: 0
Bucket size: 256 KiB
First bucket: 0
Buckets: 16384
Last mount: (never)
Last superblock write: 0
State: rw
Data allowed: journal,btree,user
Has data: (none)
Durability: 1
Discard: 0
Freespace initialized: 0
To mount this you’ll need to specify the passphrase given above or it will fail:
tumbleweed:~ $ mount /dev/vdb /mnt/dummy/
Enter passphrase:
ERROR - bcachefs::commands::cmd_mount: Fatal error: failed to verify the password
tumbleweed:~ $ mount /dev/vdb /mnt/dummy/
Enter passphrase:
tumbleweed:~ $ df -h | grep dummy
/dev/vdb 3.7G 2.0M 3.6G 1% /mnt/dummy
tumbleweed:~ $ umount /mnt/dummy
Beside encryption you may also use compression (supported are gzip, lz4 and zstd), e.g.:
tumbleweed:~ $ bcachefs format --compression=lz4 --force /dev/vdb
/dev/vdb contains a bcachefs filesystem
External UUID: 1ebcfe14-7d6a-43b1-8d48-47bcef0e7021
Internal UUID: 5117240c-95f1-4c2a-bed4-afb4c4fbb83c
Magic number: c68573f6-66ce-90a9-d96a-60cf803df7ef
Device index: 0
Label:
Version: 1.4: member_seq
Version upgrade complete: 0.0: (unknown version)
Oldest version on disk: 1.4: member_seq
Created: Wed Apr 17 10:54:02 2024
Sequence number: 0
Time of last write: Thu Jan 1 01:00:00 1970
Superblock size: 960 B/1.00 MiB
Clean: 0
Devices: 1
Sections: members_v1,members_v2
Features:
Compat features:
Options:
block_size: 512 B
btree_node_size: 256 KiB
errors: continue [ro] panic
metadata_replicas: 1
data_replicas: 1
metadata_replicas_required: 1
data_replicas_required: 1
encoded_extent_max: 64.0 KiB
metadata_checksum: none [crc32c] crc64 xxhash
data_checksum: none [crc32c] crc64 xxhash
compression: lz4
background_compression: none
str_hash: crc32c crc64 [siphash]
metadata_target: none
foreground_target: none
background_target: none
promote_target: none
erasure_code: 0
inodes_32bit: 1
shard_inode_numbers: 1
inodes_use_key_cache: 1
gc_reserve_percent: 8
gc_reserve_bytes: 0 B
root_reserve_percent: 0
wide_macs: 0
acl: 1
usrquota: 0
grpquota: 0
prjquota: 0
journal_flush_delay: 1000
journal_flush_disabled: 0
journal_reclaim_delay: 100
journal_transaction_names: 1
version_upgrade: [compatible] incompatible none
nocow: 0
members_v2 (size 144):
Device: 0
Label: (none)
UUID: 3bae44f0-3cd4-4418-8556-4342e74c22d1
Size: 4.00 GiB
read errors: 0
write errors: 0
checksum errors: 0
seqread iops: 0
seqwrite iops: 0
randread iops: 0
randwrite iops: 0
Bucket size: 256 KiB
First bucket: 0
Buckets: 16384
Last mount: (never)
Last superblock write: 0
State: rw
Data allowed: journal,btree,user
Has data: (none)
Durability: 1
Discard: 0
Freespace initialized: 0
tumbleweed:~ $ mount /dev/vdb /mnt/dummy/
tumbleweed:~ $ df -h | grep dummy
/dev/vdb 3.7G 2.0M 3.6G 1% /mnt/dummy
Meta data and data check sums are enabled by default:
tumbleweed:~ $ bcachefs show-super -l /dev/vdb | grep -i check
metadata_checksum: none [crc32c] crc64 xxhash
data_checksum: none [crc32c] crc64 xxhash
checksum errors: 0
That’s it for the very basics. In the next post we’ll look at multi device file systems.
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