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12 z/VM and Linux on IBM System z: The Virtualization Cookbook for Red Hat Enterprise Linux 6.0

2.4 Memory planning

Planning memory may be the most difficult issue for z/VM and Linux on System z, yet the

most important to ensure adequate performance. The simplest solution may appear to

involve having enough central memory (storage) in the LPAR so that z/VM never pages and

Linux never swaps. However, such resources are often not realistically available. A best

practice is to allocate memory on a just enough basis for each Linux server. A good starting

point is to set a virtual machine size by changing the memory allocation value at just over the

value at which the guest starts to swap at the Linux system level when under normal loading.

If some level of sustained swapping is inevitable due to the nature of the workloads, then

ensure virtual disks are used for the swap media.

To better understand of memory planning, refer to the following resources:

򐂰 Linux on IBM System z: Performance Measurement and Tuning, SG24-69266

򐂰 The IBM z/VM Performance Resource pages in general, which can be found on the web

at:

http://www.vm.ibm.com/perf/

򐂰 The IBM z/VM page specifically discussing memory allocation, which can be found on the

web at:

http://www.vm.ibm.com/perf/tips/storconf.html

One rule is to have as few virtual machines logged on (or disconnected) as possible to handle

the workload being presented. Every virtual machine that is not required should be logged off

where appropriate, as this will mean more memory for the other virtual servers that are still

running.

2.5 Password planning

Good passwords are critical to good security. However, requiring many different passwords

generally leads to people writing them down, which clearly detracts from good security.

Sometimes it is difficult to balance these two extremes.

This book considers different system administration roles:

򐂰 The z/VM system administrator

򐂰 The Linux system administrator

򐂰 The Linux virtual server end users

The z/VM and Linux system administrator may be the same person.

The method of backing up z/VM data onto the Linux cloner means that the Linux

administrator will have access to all z/VM passwords. Therefore, the examples in this book

set all z/VM and Linux system administration passwords to the same value, lnx4vm. If the

z/VM and Linux system administrator roles must be kept separate and the Linux administrator

is not to have access to the z/VM passwords, then a different method of backing up z/VM

data must be chosen.

Important: However you choose to layout the minidisks, it is important that the golden

image and all target Linux user IDs have two minidisks of the same size at virtual

addresses 100 and 101. These assumptions are coded into the clone.sh script.

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Red Hat LINUX VIRTUAL SERVER 4.6 - ADMINISTRATION Manuale di Installazione Pagina 32