Steps For Estimating Memory And Cpu Requirements - IBM ZVM - FOR LINUX V6 RELEASE 1 Getting Started

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Lower the memory size until you see Linux begin to swap, then increase the

virtual machine memory to the next bigger size.

Another way to reduce the memory requirements is through discontiguous saved

segments (DCSS). A DCSS is an area of virtual storage outside the address range of

a virtual machine. The area can contain read-only data or reentrant code. A DCSS

connects discontiguous segments to a virtual machine's address space so programs

can be fetched. DCSSs can be shared by many virtual machines, so total virtual

memory required might be reduced. This reduced requirement for virtual memory

can:

v Reduce CP paging requirements.

v Allow a given z/VM instance to support more Linux virtual machines.

v Reduce the amount of Linux disk I/O by having file systems, block devices, and

shared objects in DCSSs.

A Linux guest machine uses DCSSs through the DCSS block device driver.

Related information

v This manual shows you how to set up a swap disk on a z/VM minidisk. Other

options are available, such as using a virtual disk in storage. For more

information on swap disk options, see "Linux Performance when running under

VM" (http://www.vm.ibm.com/perf/tips/linuxper.html).

v For more information about DCSS block device drivers, see Linux on System z:

Device Drivers, Features, and Commands on the IBM developerWorks

System z Web site entitled "Documentation for Development stream" at:

http://www.ibm.com/developerworks/linux/linux390/documentation_dev.html

Real CPU requirements

The real CPU requirement is not simply a function of a single server: rather, the

requirement is a function of all your virtual servers combined (see "Overview of

z/VM capacity planning" on page 22). You must consider what each of your

applications require and then estimate the overall CPU requirement. Your IBM

representative, business partner, or consultant offer services to help you perform

this task.

When defining z/VM LPARs, it is recommended that you assign a minimum of

two logical processors. You have the option of dedicating the processors to the

LPAR or sharing them with other LPARs. It is also possible to set different

processing weights to LPARs, which gives more processor resources to one over

another. For instance, you might give your production LPAR 60% weight and your

test LPAR 40%.

Virtual CPU requirements

In general, follow this guideline: define as many virtual CPUs as needed

(maximum CPU resources required), but do not exceed the number of real

processors assigned to this LPAR. Extra virtual CPUs just add to the overhead and

potentially increase the software multiprocessing factors.