I spent about half an hour the other day trying to figure out why a new Usermode Linux 2.6 kernel wouldn’t boot. Most of the time I focus on the 2.4 kernels, since adding “TTY Logging” to that kernel is really easy. (Yes, I’m watching your every keystroke)

For some reason, every time I’d boot the kernel, I’d get:

“Kernel Panic – VFS: Cannot open root device “98:0” or unknown-block“

The obvious problem would have been misspelling the flag “ubd” “udb”… but that wasn’t it. After scouring the Internet for any help I could find (and coming up empty), it turns out I missed a really simple but necessary config option when compiling the kernel:

“CONFIG_BLK_DEV_UBD” – Don’t leave home without it.