Suse Installation in Large Enterprise Environment

1. BIOS After the computer has been turned on, the BIOS initializes the screen and keyboard and tests the main memory. Up to this stage, the machine does not access any mass storage media. Subsequently, the information about the current date, time, and the most important peripherals are loaded from the CMOS values. When the first hard disk and its geometry are recognized, the system control passes from the BIOS to the boot loader.
    

2. MBR boot Loader The first physical 512-byte data sector of the first device defined in BIOS load chain is loaded into the main memory and the program that resides at the beginning of this sector takes over. It usually loads another loader but it can do so only if the latter is located at fixed disk addresses as no file system is present. That's why it is often referred to as Master Boot Record (MBR). The boot loader then passes control to the actual operating system, in this case, the Linux kernel at /boot partition. 
    

3. Kernel and initramfs MBR loads both the kernel and an initial RAM–based file system (initramfs) into memory. initramfs is a small cpio archive It contains a small executable that uses BIOS calls called init that handles the mounting of the real root file system.  It provides a minimal Linux environment that enables the execution of programs before the actual root file system is created. This minimal Linux environment is loaded into memory by BIOS routines and does not have specific hardware requirements other than sufficient memory.

   • init on initramfs This program performs all actions needed to mount the proper root file system on the installation media, like providing kernel functionality for the needed file system and device drivers for typical mass storage controllers. After the root file system has been found, it is checked for errors and mounted. If this has been successful, the initramfs is cleaned and the init program on the root file system is executed.
      
   • Before the root file system can be mounted and the operating system can be started, the kernel needs the corresponding drivers to access the device on which the root file system is located. These drivers may include special drivers for certain kinds of hard drives or even network drivers to access a network file system. The needed modules for the root file system may be loaded by init on initramfs. After the modules are loaded, udev provides the initramfs with the needed devices. Later in the boot process, after changing the root file system, it is necessary to regenerate the devices. This is done by boot.udev with the command udevtrigger.

The main purpose of init on initramfs is to prepare the mounting of and access to the real root file system. Depending on your system configuration, init is responsible for the following tasks.

• Loading Kernel Modules. Depending on your hardware configuration, special drivers may be needed to access the hardware components of your computer (the most important component being your hard drive). To access the final root file system, the kernel needs to load the proper file system drivers.
• Providing Device Special Files. For each loaded module, the kernel generates device events. udev handles these events and generates the required device special files on a RAM file system in /dev. Without those special files, the file system and other devices would not be accessible.
• Managing RAID and LVM Setups. If you configured your system to hold the root file system under RAID or LVM, init sets up LVM or RAID to enable access to the root file system later.
• Managing Network Configuration. If you configured your system to use a network-mounted root file system (mounted via NFS), init must make sure that the proper network drivers are loaded and that they are set up to allow access to the root file system.

When init is called during the initial boot as part of the installation process, its tasks differ from those mentioned earlier:

• Finding the Installation Medium. As you start the installation process, your machine loads an installation kernel and a special initrd with the YaST installer from the installation medium. The YaST installer, which is run in a RAM file system, needs to have information about the location of the installation medium to access it and install the operating system.
   
• Initiating Hardware Recognition and Loading Appropriate Kernel Modules. The boot process starts with a minimum set of drivers that can be used with most hardware configurations. init starts an initial hardware scanning process that determines the set of drivers suitable for your hardware configuration. The names of the modules needed for the boot process are written to INITRD_MODULES in /etc/sysconfig/kernel. These names are used to generate a custom initramfs that is needed to boot the system. If the modules are not needed for boot but for coldplug, the modules are written to /etc/sysconfig/hardware/hwconfig-*. All devices that are described with configuration files in this directory are initialized in the boot process.
   
• Loading the Installation System or Rescue System. As soon as the hardware has been properly recognized, the appropriate drivers have been loaded, and udev has created the device special files, init starts the installation system, which contains the actual YaST installer, or the rescue system.
   

• Starting YaST. Finally, init starts YaST, which handles package installation and system configuration.