• Request for enhancement (RFE)

background: I'm the bcachefs author. bcachefs supports multiple devices (raid, tiering).

Currently, we typically mount a multiple device filesystem by listing all the component devices together separated by colons, like so:

mount -t bcachefs /dev/sda:/dev/sdb /mnt

This works, and fstab entries using this syntax also work - i.e., if you have that entry in fstab mount /mnt will work. But booting does not work, because systemd doesn't understand the device field and will try to wait for a device that doesn't exist.

The btrfs approach of listing extra devices in the options field isn't a real solution if we ever want implement booting in degraded mode sanely.

I think for now, what would work is a way to hook into the fstab generator based on filesystem type - and we're probably going to need that anyways, i.e. for degraded mounts after a timeout we want to check if it's possible to mount without loss of data, inform the user of the situation and ask if they want to mount. For encrypted filesystems, it would be much better to unlock the filesystem from the mount unit rather than from an initramfs hook as it's down now (unlock also depends on the devices being ready).

I would also really like to see a standard agreed on method for listing multiple device filesystems in fstab - I think this is going to be necessary too, for mount by uuid/label to work with multi device filesystems. It doesn't have to be colon separated - comma separated might be better, so there isn't ambiguity about it being an nfs mount.