Wednesday, December 8, 2010

RHEL 6 ditches System V init for Upstart: What Linux admins need to know

With the release of RHEL 6.  There is a mantra of new ways of doing things; those that generally change the way a system administrator does his/her job.  On this post I will give you a brief backgrounder on how things sounds.

The release of Red Hat Enterprise Linux (RHEL) 6, Red Hat will use the new Upstart boot service, as a replacement for the old init. In this article you'll learn about the changes to this essential Linux process, and what it means for your work as an administrator.

The disadvantage of the old System V init boot procedure is that it was based on runlevel directories that contained massive amounts of scripts that all had to be started. Upstart is event driven, so it contains scripts that are only activated when they are needed, making the boot procedure a lot faster. A well-tuned Linux server that uses Upstart boots significantly faster than an old system using System V init.

In an attempt to make it easier to understand, the Upstart service still works with an init process. So you'll still have /sbin/init, which is the mother of all services. But if you have a look at the /etc/inittab file, you'll see that everything has changed.

Understanding the changes from init to Upstart
The good news, the changes to the boot procedure on RHEL 6 are minimal. You still work with services that have service scripts in /etc/init.d, and there is still a concept of runlevels. So after adding a service with yum, you can still enable it like you were used to by using the chkconfig command. Also, you can still start it with the service command.
But if you are looking for settings that you used to apply from /etc/inittab, you'll see that many things have changed. The only thing that didn't change, is the line that tells your server what runlevel to use by default:
All other items that were previously handled by /etc/inittab are now in individual files in the /etc/init directory (not to be confused with /etc/init.d, which contains the service scripts). Below you can find a short list of the scripts that are used:

/etc/init/rcS.conf handles system initialization by starting the most fundamental services
/etc/init/rc.conf handles starting the individual runlevels
/etc/init/control-alt-delete.conf defines what should happen when “control-alt-delete” is pressed
specify how terminals are to be handled

Apart from these generic files, some additional configuration is in the /etc/sysconfig/init file. Here, some parameters are defined that determine the way that startup messages are formatted. Apart from these not so important settings, there are three lines that are of more interest:


Of these, you can give the first line the value yes, to have your system detect swap devices automatically. Using this option means that you don't have to mount swap devices from /etc/fstab anymore. The ACTIVE_CONSOLES line determines which virtual consoles are created. In most situations, tty[1-6] works fine, but this options allows you to allocate more or less virtual consoles. But be aware that you should never use tty[1-8], because tty7 is reserved for the graphical interface.
Last but not least, there is the SINGLE=/sbin/sushell line. This line can have two parameters: /sbin/sushell (the default) which drops you in a root shell after starting single-user mode, or /sbin/sulogin, which launches a login prompt where you have to enter the root password before single user mode can be started.
With Upstart, RHEL 6 has adopted a new and much faster alternative for the old System V boot procedure. With the adoption of this new service, Red Hat has still managed to keep the old management routines in place, meaning that as an administrator you can still manage services the way you are used to – well almost that way - with some changes to the settings in the /etc/inittab file.

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