Bcachefsのソースを表示

[[Category:ファイルシステム]]
[[en:Bcachefs]]
[https://bcachefs.org/ Bcachefs] は、 [[Btrfs]] と [[ZFS]] の機能をよりクリーンなコードベース、より安定性、より高速、 GPL 互換のライセンスを提供することを目的とした次世代の CoW ファイルシステムです。

主に Kent Overstreet によって開発されています。

== インストール ==

Bcachefs はまだアップストリーム [[カーネル]] にありませんが、 {{AUR|linux-bcachefs-git}} カーネルは [[AUR]]からインストールできます。

Bcachefs ユーザースペースツールは {{AUR|bcachefs-tools-git}} から入手できます。

== 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.

{{Note|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, {{ic|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 ==
{{Expansion|Missing details on which options should be used}}

Most options can be set at either during {{ic|bcachefs format}}, at mount time ({{ic|1=mount -o option=value}}), or through sysfs ({{ic|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.
{{Note|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 {{ic|1=bcachefs setattr ''file'' --option=value}}
* data_replicas
* data_checksum
* compression, background_compression
* foreground_target, background_target, promote_target

{{Note|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

{{Note|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

{{Note|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 ==

{{Expansion|Information on auto-mounting would be useful}}

=== Documentation ===

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

Check dmesg for more useful error messages.

== See also ==

* [https://www.patreon.com/bcachefs Kent Overstreet's Patreon page]
* [[Wikipedia:Bcachefs]]