VSP: After the server is powered on, Virtual System Platform (VSP) boots the system and performs diagnostic tests on the hardware such as the processor, memory, disks. After VSP completes the hardware tests, control is then passed to the Linux loader (GRUB).

GRUB: Grand Unified Bootloader (GRUB) is a Linux loader that reads the Linux kernel from the boot disk and transfers control to it. Basic input/output system (BIOS) reads phase 1 of GRUB in to memory. When Phase 1 begins executing, it reads in the rest of the GRUB program including the location of the Linux kernel. GRUB reads the Linux kernel, uncompresses it, and transfers control to the kernel.

Linux Kernel: The Linux kernel initializes the Pentium processor registers and its own data structures, determines the amount of available memory, and initializes the various compiled-in device drivers. When finished, the Linux kernel creates the first process, known as init.

Init: The init process creates the remaining processes for the system using the /etc/inittab file, which specifies runlevels, and a set of processes to run at each runlevel.

The rc script runs the service startup scripts in /etc/rc.d/rc4.d in numeric order (S00* through S99*). Each of these startup scripts starts a particular Linux service, for example, inetd. In addition to starting up the various services, the disk partitions are checked for sanity, and loadable modules are loaded.

These applications come up and start heartbeats to the Watchdog.

The Watchdog also starts up a script to monitor Linux services. It starts up threads to communicate with a Hardware-Sanity device.

Hardware-Sanity: The Watchdog periodically tells the hardware-sanity device how long to wait for before rebooting the system. If the Hardware-Sanity driver does not receive an update within that interval, the HW Watchdog’s timer resets the processor.

Arbiter: The Arbiter decides whether the server goes active or standby.