Debian 8.0 Jessie Gets A Release Date

[interested]

You have a 2-disk system with btrfs in RAID1 and grub on both disks so you can boot from both, you remove one disk. systemd would just keep looking for that disk.

Normally without systemd: with btrfs has a bunch of options which you can specify on the kernel commandline and then the system can boot in a degraded mode.

The problem with "degraded" mode is that neither the kernel, udev nor systemd knows anything about such stuff. They need some kind of "helper" program to tell them about what is acceptable or not. Without such a helper program it requires manual intervention to boot into degraded mode.

Not sure if "nofail" will help here, but in any case it should be used with care since allowing a raid system to go online in degraded mode can mean instant wipe out of the entire array; eg. the raid array is activated without any spares because a disk was late, then the disk shows up and the raid system starts to rebuild it. A single unrecoverable error while doing so may then wipe out the entire array.

I think mdadm raid on top of btrfs may work with allowing unconditionally booting into degraded mode.

Regarding dracut, then I believe every other initramfs solution in wide use offers similar functionality as to stop the boot sequence and drop into a shell at a specified point.