How to boot your Linux box on USB media without initrd or initramfs

It is widely known you need initrd or initramfs to boot your Linux box over USB media, namely USB HDD or Flash memory drives, but you can do it without it.

Basically, USB media relies on SCSI driver for its recognition. The problem is that you have to wait for some time to make it recognized, namely one or two seconds or so. And during the time your kernel doesn’t wait for the media to be recognized - yes. it was the “common sense.”

However, you can make your Linux kernel to wait for root devices to be recognized. Simply add “rootwait” option on your kernel boot parameter and everything is done. Or, you can use “rootdelay=NN” option to make some timeout. These options are introduced in kernel 2.6, and I heard that there’s a patch for 2.4 kernels to do the same.

The only problem here is that you can’t make use of UUID or disk labels to recognize the boot media. Dynamic devfs like mdev or udev need to be run to assign UUIDs or read disk labels from each partition. So, chances are that you must rely on traditional static devfs - boot to /dev/sda or so.

Of course there’s a issue that you do may not know which device file is assigned to your boot media on boot. Your /dev/sda can be either SATA or SAS HDD, and there can be more than one - chances are, most of the time your boot media can be assigned as /dev/sdb or sdc or sdd or….. Whatever. In this case, you can compile your kernel to include only SCSI disk driver and USB media only and compile others(SATA, SAS, IDE…..) as modules so that during the initial boot kernel can recognize only USB media and other media can be recognized later on init stage when udev or mdev is populated(=daemon is started).

Hope this helps you simplifying your boot process of Linux from USB media. Good luck!