System information

Sharing and maintaining SLES 10 SP2 Linux under z/VM 3
򐂰 Staff productivity: fewer people are needed to manage a large-scale virtual server
environment running on z/VM
򐂰 Operational flexibility: companies can leverage and utilize their IT infrastructure to
enhance their business results
A word of caution and a disclaimer are necessary. The techniques described in this paper are
not simple to implement. Both z/VM and Linux on System z skills are needed. It is not
guaranteed that such a system would be supported. Check with your Linux distributor and
your support company to verify the changes described in this paper will be supported. This
being said, this paper is based on a system that has been implemented and is in production
at Nationwide Insurance and at Penn State University.
1.2.1 Conventions
The following font conventions are used in this paper:
Monospace and bold Commands entered by the user on the command line
<value> Value inside angle brackets is to be replaced with a value specific to
your environment
monospace File, directory and user ID names
The following command conventions are used in this book:
򐂰 z/VM commands are prefixed with ==>
򐂰 z/VM XEDIT subcommands are prefixed with ====>
򐂰 Linux commands running as root are prefixed with #
1.3 Background of read-only root Linux
The system is called read-only root because the root file system (/) is mounted with read
permission, not read-write. The common approach to sharing file systems is to ask “Which file
systems can be mounted read-only?”. The approach taken in this paper is the exact opposite.
The root file system is mounted read-only and the question that is asked is “Which file
systems need to be mounted read-write?”.
1.3.1 Why a read-only root?
By creating a read-only root file structure, the basic Linux system code can be shared among
many virtual Linux servers. This helps with Linux standardization. Most Linux servers can
share exactly the same version of Linux operating system. This environment makes
maintenance much simpler. It becomes possible to roll out a new version of Linux by updating
the master copy of the shared root and not each system that uses it.
1.3.2 Overview of the system
The root file system is read-only because it does not have to be read-write. Actually, four
directories are chosen to be read-write: /etc/, /var/, /srv/ and /root/. In addition, the /tmp/
directory is read-write, but is in-memory and built at boot time. The /proc/ and /sys/
pseudo-directories are abstractions of kernel control blocks and the permissions are not
under the control of the systems administrator.