Initial RAM disk image
An initial RAM disk holds files, programs, or modules that are not included in the kernel image but are required for booting.
For example, booting from DASD requires the DASD device driver. If you want to boot from DASD but the DASD device driver has not been compiled into your kernel, you must provide the DASD device driver module on an initial RAM disk. If your image contains all files, programs, and modules that are needed for booting, you do not need an initial RAM disk.
Ubuntu Server provides a RAM disk in /boot, named initrd.img-<version>, where <version> is the same as the output of the uname -r command. There is often more than one RAM disk installed, the currently active RAM disk is indicated by a symbolic link named initrd.img, without version suffix.