2.2 Deciding where to run Linux on zSeries

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The first planning decision to make is where to run Linux on your zSeries system. There are three alternatives.

Run natively on the zSeries hardware (except on the latest models)
Run natively in a logical partition (LPAR)
Run as a guest under VM

The first method could be used when running in a small processor, such as a z800 or Multiprise 3000—but it is not viable in a larger processor, for several reasons: only one Linux system can be run at a time; the Linux server owns all the hardware attached to the server; and Linux does not support all the hardware which can be attached to a zSeries processor. It is not supported in the latest zSeries models. Because of its limited use, we did not consider this any further while writing this redbook.

We do consider the other two methods to be viable, depending on your circumstances; which one you choose depends on your needs. We now describe those two methods in more detail and discuss the advantages of each.

2.2.1 Logical partition

A zSeries processor can be partitioned into 15 different logical partitions (LPARs). With the z990, the number of partitions increases to a maximum of 30.

Logical partitions are an allocation of the available processor resource, either shared or dedicated. Processor sharing is possible on a system with any number of CPUs, even if there is only one. The processors can be either standard zSeries processors or Integrated Facility for Linux (IFL) processors.

Advantages

You can run up to 30 individual Linux systems in a zSeries processor. Perhaps more compelling, however, is the ability to run Linux LPARs on the same processor with a z/OS or z/VM LPAR. Systems in LPARs are well isolated from each other, from a security perspective.

Disadvantages

Linux was not written to maximize usage of resources on zSeries. You will probably be underutilizing the resources that are allocated to an LPAR controlled by Linux.

You must have access to a system console, such as the hardware maintenance console (HMC), to start and restart a Linux partition, or to define or change the definition of a partition. This makes managing your Linux servers more complex and requires the Linux system programmer to work closely with operations.

2.2.2 VM guest

Linux can run as a guest in one, or multiple, VM virtual machines. z/VM uses architectural hardware functions in zSeries to virtualize the zSeries instruction set. This means that each guest machine thinks it has its own dedicated zSeries system. A number of guest machines (Linux or others, including z/OS) can run concurrently. The number of concurrent guests is limited only by the resources available to virtualize them.

Advantages

The guest console can be accessed by any networking method supported by z/VM, including rlogin, telnet, 3270 through TCP/IP, and 3270 through SNA. This means you can start and restart remotely. Your virtual machine is protected by a user ID and password. You can automate, manage, and administer virtual machines using CMS.

There is also better resource management under VM, because memory and CPU are shared among the Linux guests, and resources are assigned to each Linux guest as it needs them. z/VM was written to optimize resource allocation and utilization among all guests on a zSeries system.

Disadvantages

If you want to run multiple Linux systems concurrently, z/VM skills are needed and a number of new concepts need to be learned. Also, VM guest systems are not quite as isolated from a security standpoint as those in separate LPARs.

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