Traditionally, booting an Ubuntu installation with the root filesystem on a degraded RAID drops the system into a busybox prompt in the initramfs.
This is a very conservative approach, allowing the system administrator to consciously recognize that the system has lost a RAID disk, and preventing the system from booting into an unprotected situation.
SwitchFS is a lightweight replacement for UnionFS, RAID and LVM.
SwitchFS (or SFS) is a lightweight file system layer designed to combine file systems located on multiple drives and/or multiple servers into a single logical file repository. SFS is built on top of FUSE (the "filesystem in userspace" layer).
DRBD is a block device which is designed to build high availability clusters. This is done by mirroring a whole block device via (a dedicated) network. You could see it as a network raid-1.