What's the point of rootwait/rootdelay?

Sometimes the OS can't distinguish a peripheral that's slow to respond from a peripheral that's not there or completely hosed. The most obvious example is a root filesystem coming from the network (TFTP, NFS) where a slow network link or an overloaded server are difficult to distinguish from a severed network link or a crashed server. A timeout tells the kernel when to give up.

This can also happen with disks that are slow to spin up, RAID arrays that need to be verified and so on. rootdelay instructs the kernel not to give up immediately if the device isn't available. The kernel can't know whether a SCSI drive is a local disk or some kind of RAID bay.

rootwait is provided to wait indefinitely. It's not always desirable, for example a system may want to fall back to a different root filesystem if the normal one takes too long to respond.