How to make initrd that contains root file system?

Caldus · 05-19-2010, 01:10 PM

Hello all,

I am attempting to PXE boot Redhat 5.4 and load the OS into RAM. Here is how far I have gotten so far:

I have successfully configured my DHCP, TFTP, and PXE servers. The PC that is booting up Redhat 5.4 is able to get a DHCP IP address from the server, grab the kernel and initrd from the TFTP server, and boot up from there. I have this setup working if I store the root file system on a NFS share on the server and use the kernel's NFSROOT parameter. Long story short, I now need to get this same setup working without using NFS if possible. I have spent several hours google searching how to create a RAM disk (initrd) that also contains the root file system. I must be searching the wrong keywords. Anyway, based on what I have read so far, I need to do the following:

1. I need to pass different kernel parameters. It needs to look something like:

kernel initrd=/initrd_with_root_file_system.gz rw root=/dev/ram0

2. It seems like newer versions of initrd are created using the cpio tool. So would this simply be a matter of booting to my hard drive that has Redhat 5.4 loaded on it and running:

% find . -depth -print | cpio -ov > initrd.img

to create the RAM disk with a root file system attached to it? I have a feeling it has to be more complicated than that but I just cannot find any sites that specifically explain what would be involved with creating an initrd file that can also be used as the root file system.

3. Would I need to modify the init script (many sites call it "linuxrc") at all in order to accomplish what I want to do? For example, I found this site:

http://www.jacobstoner.com/index.php...oot-on-ramdisk

My understanding of this page is that you have to create a linuxrc script that basically does the work of decompressing the file system into /dev/ram0 and mounting it to /root. Is something like this actually needed?

4. The kernel must have certain parameters set such as enabling RAM disk support.

I have most of this setup working. I just need to figure out how to basically store the contents of the OS on the TFTP server and then tell the kernel to load that OS into memory. Based on what I have read, this should be possible (otherwise, how do Live CD's work?).

Thanks for any help!

brucehinrichs · 05-19-2010, 01:40 PM

What did Redhat customer service say? I would pick their brains first, that's what you're paying them for.

Caldus · 05-19-2010, 01:48 PM

Quote:

Originally Posted by brucehinrichs

What did Redhat customer service say? I would pick their brains first, that's what you're paying them for.

I probably could, but it would take a looong while to finally get to someone who could actually help (big company politics and bureaucracy you know :P). I'm assuming that creating root file system RAM disks should be independent of what distribution I am running anyway.

brucehinrichs · 05-19-2010, 01:57 PM

Then why pay them? Use CentOS for free if you can't get the help you're paying for.

Anyway, not knowing what you tried already, a google search for 'root file system RAM disks' yields some promising results...

Caldus · 05-19-2010, 07:21 PM

Quote:

Originally Posted by brucehinrichs

Then why pay them? Use CentOS for free if you can't get the help you're paying for.

Anyway, not knowing what you tried already, a google search for 'root file system RAM disks' yields some promising results...

Wasn't my decision to go to Redhat. :P

Thanks for the keywords. It seems like based on what I have read, all I need to do is just put what I want into a new initrd.img file and change the kernel boot parameters to:

kernel initrd=/initrd.img rw ramdisk_size=2097152 init=/linuxrc root=/dev/ram0

(Allocate 2 GB for the ram disk)

The diskless PC has 4 GB of RAM, so I should be fine I would think.

Caldus · 05-19-2010, 10:33 PM

OK so here's what I tried out. I wanted to try and accomplish this in small steps. So what I tried first is to simply boot any PC up using a custom initrd.img. I figured out that I needed to use the following command to extract the contents of the existing initrd.img so that I can edit the image as needed. I ran the following commands:

Code:

mkdir /tmp/ramdisk
cp /boot/initrd.img /tmp/ramdisk
cd /tmp/ramdisk
mv initrd.img initrd.img.gz
gzip -d initrd.img.gz
cpio --extract < initrd.img

That will now allow me to see the files that are on the ram disk under /tmp/ramdisk. Once I am finished adding files, etc., I run these commands to create the new custom initrd.img:

Code:

find . | cpio -o -H newrc | gzip > /boot/initrd.img

After that, this is the part where I am still just totally confused about. I know I have to modify the kernel parameters somehow to tell it that the RAM disk is going to be the root file system. I tried all kinds of different combinations of kernel parameters with no success. Does anyone know what the correct kernel parameters are for loading a RAM disk image as the root file system?