The initrd is the "initial ramdisk". It is enough files stored in a ramdisk to store needed drivers . You need the drivers so that the kernel can mount / and kick off init.
You can avoid this file 'initrd.img' and eliminate the need of 'initrd.img', if you build your scsi drivers right into the kernel, instead of into modules. (Many persons recommend this).
The mkinitrd utility creates an initrd image in a single command. This is command is peculiar to RedHat. There may be equivalent command of mkinitrd in other distributions of Linux. This is very convenient utility.
You can read the mkinitrd man page.
/sbin/mkinitrd --help # Or simply type 'mkinitrd --help' usage: mkinitrd [--version] [-v] [-f] [--preload <module>] [--omit-scsi-modules] [--omit-raid-modules] [--omit-lvm-modules] [--with=<module>] [--image-version] [--fstab=<fstab>] [--nocompress] [--builtin=<module>] [--nopivot] <initrd-image> <kernel-version> (example: mkinitrd /boot/initrd-2.2.5-15.img 2.2.5-15) # Read the online manual page with ..... man mkinitrd su - root # The command below creates the initrd image file mkinitrd ./initrd-2.4.18-19.8.0custom.img 2.4.18-19.8.0custom ls -l initrd-2.4.18-19.8.0custom.img -rw-r--r-- 1 root root 127314 Mar 19 21:54 initrd-2.4.18-19.8.0custom.img cp ./initrd-2.4.18-19.8.0custom.img /boot
See the following sections for the manual method of creating an initrd image.
To create /boot/initrd.img see the documentation at /usr/src/linux/Documentation/initrd.txt and see also Loopback-Root-mini-HOWTO .
A cut from "http://www.linuxman.com.cy/rute/node1.html" chapter 31.7.
SCSI Installation Complications and initrd
Some of the following descriptions may be difficult to understand without knowledge of kernel modules explained in Chapter 42. You may want to come back to it later.
Consider a system with zero IDE disks and one SCSI disk containing a LINUX installation. There are BIOS interrupts to read the SCSI disk, just as there were for the IDE, so LILO can happily access a kernel image somewhere inside the SCSI partition. However, the kernel is going to be lost without a kernel module [lsqb ]See Chapter 42. The kernel doesn't support every possible kind of hardware out there all by itself. It is actually divided into a main part (the kernel image discussed in this chapter) and hundreds of modules (loadable parts that reside in /lib/modules/) that support the many type of SCSI, network, sound etc., peripheral devices.] that understands the particular SCSI driver. So although the kernel can load and execute, it won't be able to mount its root file system without loading a SCSI module first. But the module itself resides in the root file system in /lib/modules/. This is a tricky situation to solve and is done in one of two ways: either (a) using a kernel with preenabled SCSI support or (b) using what is known as an initrd preliminary root file system image.
The first method is what I recommend. It's a straightforward (though time-consuming) procedure to create a kernel with SCSI support for your SCSI card built-in (and not in a separate module). Built-in SCSI and network drivers will also autodetect cards most of the time, allowing immediate access to the device--they will work without being given any options [lsqb ]Discussed in Chapter 42.] and, most importantly, without your having to read up on how to configure them. This setup is known as compiled-in support for a hardware driver (as opposed to module support for the driver). The resulting kernel image will be larger by an amount equal to the size of module. Chapter 42 discusses such kernel compiles.
The second method is faster but trickier. LINUX supports what is known as an initrd image ( initial rAM disk image). This is a small, +1.5 megabyte file system that is loaded by LILO and mounted by the kernel instead of the real file system. The kernel mounts this file system as a RAM disk, executes the file /linuxrc, and then only mounts the real file system.
31.6 Creating an initrd Image
Start by creating a small file system. Make a directory [nbsp ]/initrd and copy the following files into it.
drwxr-xr-x 7 root root 1024 Sep 14 20:12 initrd/ drwxr-xr-x 2 root root 1024 Sep 14 20:12 initrd/bin/ -rwxr-xr-x 1 root root 436328 Sep 14 20:12 initrd/bin/insmod -rwxr-xr-x 1 root root 424680 Sep 14 20:12 initrd/bin/sash drwxr-xr-x 2 root root 1024 Sep 14 20:12 initrd/dev/ crw-r--r-- 1 root root 5, 1 Sep 14 20:12 initrd/dev/console crw-r--r-- 1 root root 1, 3 Sep 14 20:12 initrd/dev/null brw-r--r-- 1 root root 1, 1 Sep 14 20:12 initrd/dev/ram crw-r--r-- 1 root root 4, 0 Sep 14 20:12 initrd/dev/systty crw-r--r-- 1 root root 4, 1 Sep 14 20:12 initrd/dev/tty1 crw-r--r-- 1 root root 4, 1 Sep 14 20:12 initrd/dev/tty2 crw-r--r-- 1 root root 4, 1 Sep 14 20:12 initrd/dev/tty3 crw-r--r-- 1 root root 4, 1 Sep 14 20:12 initrd/dev/tty4 drwxr-xr-x 2 root root 1024 Sep 14 20:12 initrd/etc/ drwxr-xr-x 2 root root 1024 Sep 14 20:12 initrd/lib/ -rwxr-xr-x 1 root root 76 Sep 14 20:12 initrd/linuxrc drwxr-xr-x 2 root root 1024 Sep 14 20:12 initrd/loopfs/
On my system, the file initrd/bin/insmod is the statically linked [lsqb ]meaning it does not require shared libraries.] version copied from /sbin/insmod.static--a member of the modutils-2.3.13 package. initrd/bin/sash is a statically linked shell from the sash-3.4 package. You can recompile insmod from source if you don't have a statically linked version. Alternatively, copy the needed DLLs from /lib/ to initrd/lib/. (You can get the list of required DLLs by running ldd /sbin/insmod. Don't forget to also copy symlinks and run strip -s [lcub ]lib[rcub ] to reduce the size of the DLLs.)
Now copy into the initrd/lib/ directory the SCSI modules you require. For example, if we have an Adaptec AIC-7850 SCSI adapter, we would require the aic7xxx.o module from /lib/modules/[lcub ]version[rcub ]/scsi/aic7xxx.o. Then, place it in the initrd/lib/ directory.
-rw-r--r-- 1 root root 129448 Sep 27 1999 initrd/lib/aic7xxx.o
The file initrd/linuxrc should contain a script to load all the modules needed for the kernel to access the SCSI partition. In this case, just the aic7xxx module [lsqb ] insmod can take options such as the IRQ and IO-port for the device. See Chapter 42.]:
#!/bin/sash aliasall echo "Loading aic7xxx module" insmod /lib/aic7xxx.o
Now double-check all your permissions and then chroot to the file system for testing.
chroot ~/initrd /bin/sash /linuxrc
Now, create a file system image similar to that in Section 19.9:
dd if=/dev/zero of=~/file-inird count=2500 bs=1024 losetup /dev/loop0 ~/file-inird mke2fs /dev/loop0 mkdir ~/mnt mount /dev/loop0 ~/mnt cp -a initrd/* ~/mnt/ umount ~/mnt losetup -d /dev/loop0
Finally, gzip the file system to an appropriately named file:
gzip -c ~/file-inird > initrd-<kernel-version>
31.7 Modifying lilo.conf for initrd
Your lilo.conf file can be changed slightly to force use of an initrd file system. Simply add the initrd option. For example:
boot=/dev/sda prompt timeout = 50 compact vga = extended linear image = /boot/vmlinuz-2.2.17 initrd = /boot/initrd-2.2.17 label = linux root = /dev/sda1 read-only
Notice the use of the linear option. This is a BIOS trick that you can read about in lilo(5). It is often necessary but can make SCSI disks nonportable to different BIOSs (meaning that you will have to rerun lilo if you move the disk to a different computer).