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7.1.1. System V
System V is the classic boot process that has been used in Unix and Unix-like systems such as Linux since about 1983. It consists of a small program, init, that sets up basic programs such as login (via getty) and runs a script. This script, usually named rc, controls the execution of a set of additional scripts that perform the tasks required to initialize the system.
The init program is controlled by the /etc/inittab file and is organized into run levels that can be run by the user:
0 — halt
1 — Single user mode
2 — Multiuser, without networking 3 — Full multiuser mode
4 — User definable
5 — Full multiuser mode with display manager 6 — reboot
0 — halt
1 — Single user mode
2 — Multiuser, without networking 3 — Full multiuser mode
4 — User definable
5 — Full multiuser mode with display manager 6 — reboot
The usual default run level is 3 or 5.
Advantages
• Established, well understood system.
• Easy to customize.
Disadvantages
• Slower to boot. A medium speed base LFS system takes 8-12 seconds where the boot time is measured from the first kernel message to the login prompt. Network connectivity is typically established about 2 seconds after the login prompt.
• Serial processing of boot tasks. This is related to the previous point. A delay in any process such as a file system check, will delay the entire boot process.
• Does not directly support advanced features like control groups (cgroups), and per-user fair share scheduling.
• Adding scripts requires manual, static sequencing decisions.