How I mount my encrypted bcachefs array at boot.

In /etc/fstab/ include a x-systemd.requires=unlock-bcachefs-vault.service mount option. This service does the unlocking before the mount is triggered. Example line as in my /etc/fstab:

UUID=bf512699-6643-4d96-a793-daaf3f1d34f1 /mnt/vault bcachefs nofail,x-systemd.requires=unlock-bcachefs-vault.service 0 0

The content of the unlock service:

# cat /etc/systemd/system/unlock-bcachefs-vault.service
[Unit]
Description=Unlock bcachefs Vault
After=-.mount
Before=mnt-vault.mount

[Service]
Type=oneshot
ExecStart=/usr/sbin/bcachefs unlock -f /path/to/key/for/vault /dev/disk/by-uuid/bf512699-6643-4d96-a793-daaf3f1d34f1

[Install]
WantedBy=multi-user.target

Notes

• I'm not sure the Before=mnt-vault.mount is required. Probably more flexible to use x-systemd.after=unlock-bcachefs-vault.service.
• This requires 'newish' versions of bcachefs-tools. I believe newer than 1.6.0; assumingly since commit 5b21631, tested using 1.7.0.