The initial RAM disk

The initial RAM disk is loaded by the boot loader. There are two uses for the initial RAM disk:

• The primary purpose of the initial RAM disk is mostly of interest to maintainers of Linux distributions. The initial RAM disk makes it possible to install a minimal kernel image on the hard disk without the file system or hard disk device drivers compiled in. At boot time these drivers are loaded as modules, not from the hard disk, but from an initial RAM disk. After doing that, the kernel would switch to the real root device (the hard disk). Without an initial RAM disk you would need the hard disk drivers (there are many possible SCSI host adapters) and the file system drivers compiled into the kernel, so you either have a huge kernel or many different kernels to choose from.

  Such an initial RAM disk needs a file system with the following:

  • A /dev directory with some essential devices.
  • Some directories for mount points.
  • A shell with some binaries. The busybox program would be all that is needed.
  • The required kernel modules.
  • A startup script that loads the required kernel modules.
• The initial RAM disk can also be used for rescue, install or special-purpose diskettes, much as you would use the other type of RAM disk. You simply do not use the feature that you can switch back to the real root device. This is the type of diskette described in my ``Getting Linux into Small Machines'' article.

There is no reason why you should not be able to install an initial RAM disk from a second diskette, but most boot loaders do not support this. This is a limitation of the boot loader, not of Linux. Using the pause command, GRUB can do this, as the following part of a configuration file shows:

title Linux with RAM disk on separate diskette
kernel (fd0)/boot/vmlinuz 
pause Insert the second diskette.
initrd (fd0)/boot/initrd.gz