What is /dev/ram0?

steveglimache · n00b Joined: 21 Dec 2004 Posts: 12

Greetings,
I successfully finished my first Gentoo install ever. Everything during the installation seemed to have gone smoothly (except for figuring out the zillion options when configuring the kernel using genkernel --menuconfig all, but that is a subject for another thread...). At the end of the kernel build, there was a message telling me that the following boot options are required:

Moloch · Posted: Wed Dec 22, 2004 6:02 pm Post subject:

Fully reread: http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?part=1&chap=10#doc_chap2

What is is saying is:
First option is if you configured your kernel by hand "make menuconfig". You are simply telling it your root[ / ] is on /dev/hda3. Most people don't enable the initrd stuff when they do it by hand.

Second configuration is if you compiled your kernel with genkernel. Which is an automatic script. It adds some more complex options (which is why a lot of us don't like it). There options require a "mini virtual root"(initrd) before loading the real root. Note the option "real_root=/dev/hda3". /dev/ram0 is simply a ramdisk that the initrd is loaded into.
_________________
Understanding is a three-edged sword: your side, their side, and the truth. --Kosh
1010011010

NeddySeagoon · Posted: Wed Dec 22, 2004 6:33 pm Post subject:

steveglimache,

/dev/ram0 is the mountpoint for mouting an initrd file if you have one.
Getting a computer started (booted) is a problem whatever OS you use. On linux you get to see it. Simply stated, its the need to be able to load the operating system from disc without an operating system in memory to read the disc.
The term Bootstrap (shortened to boot over the years) comes from Greek mythology, from the God who pulled himself up to heaven by tugging on his own bootstraps.
Clearly this infinite recursion needs to be broken.
It starts in the BIOS. The BIOS loads the very first block off the boot disk (the Master Boot Record) into memory and jumps to the first instruction. Thats the BIOS done.
The MBR loads some more sectors from the disk. For Grub its about 23, using calls to the BIOS. These sectors are loacated after the MBR and before the start of the first partition. This is Grubs, stage 1.5.
Stage 1.5 knows enough to read the /boot dorectory structure to find and load stage 2, which loads the kernel and initrd, if one was specified. (/boot is not mounted for this)
The kernel is now in memory but not yet started and the problems are not over yet.

When grub passes control to the kernel, it initalises itself and mounts the root partition. There is a problem here if the modules needed to mount the root partition have been made as modules and are on disk because the kernel needs the modules to read the disk where the modules are.
One way out of this is to ensure that everything needed for booting is built in. Its always needed anyway. Another way is to have a fake root partition that holds everything needed to get started,
This is what an initrd (INITial Ram Drive) is.
The kernel knows how to read a ram drive, so the bootloader attaches it to /dev/ram0.
At startup the kernel mounts this ae the root filesystem, loads its modules to read the real root filesystem and off it goes.

The advantages of an initrd are that it allows a lean generic kernel to be made, that can be booted across a whole range of systems without leading to bloat because the initrd is unloaded when startup is complete.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

steveglimache · n00b Joined: 21 Dec 2004 Posts: 12

Thank you NeddySeagoon and Moloch. You clearly helped me understand what inirtd is and how it relates to /dev/ram0.

I still don't understand however, why using the statement "root=/dev/ram0" (in my LILO) as instructed/recommended rendered the system unbootable, while changing it to "root=/dev/hda2" made it bootable.

If initrd is required (because I am using genkernel), why did its ommision help?

I now reall that I didn't put exactly the following line:

Moloch · Posted: Wed Dec 22, 2004 7:24 pm Post subject:

That could be it. I don't use initrd. Not 100%. Actually I don't think using initrd is required by genkernel (I know before I said otherwise). It can make use of it. Like displaying a pretty startup screen similar to the LiveCD.
_________________
Understanding is a three-edged sword: your side, their side, and the truth. --Kosh
1010011010

NeddySeagoon · Posted: Wed Dec 22, 2004 7:37 pm Post subject:

steveglimache,

Failing to specifiy the ramdisk size was almost certainly your problem. By default, the kerenl expects to deal with 4Mb ramdisks (compile time option). The initrd produced by genkerenl is bigger than this so you get errors when you try to read beyond 4Mb

You posted that you used genkerenl with menuconfig. If you changed the default genkerenel options, its quite possible to have it produce a kernel that dosen't need an initrd to get started.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

steveglimache · n00b Joined: 21 Dec 2004 Posts: 12