The fact that there are several distributions gives you options. The distributions include Open Source packages; it is possible to modify, exchange one distribution for another, and upgrade at your own pace.
How do you choose a distribution? You may already have criteria for selection. In this section, we present some criteria that may be of use. We have found that companies tend to consider the amount of modification they need to make in order for a distribution to fit their needs. The amount of modification needed depends on how well the base distribution fills your needs, what device drivers it comes with, and so forth.
The Linux distributions for the mainframe all come with the unique extra functions for mainframe device drivers and communications features such as HiperSockets. You can choose between 31-bit distributions (Linux for S/390) and 64-bit distributions (Linux for zSeries). Table 9-1 lists some commercial mainframe distributions.
While other mainframe distributions may be available, those listed in Table 9-1 have been tested, not only by the suppliers but by IBM as well.
Any revision of a distribution may at any given time exploit more or less of the full Linux for S/390 or zSeries capabilities. IBM first released its new S/390 or zSeries technology patches to IBM's DeveloperWorks Web site. From there, distributors can download the code.
The distributors each have their own schedule for building new versions of Linux on the mainframe. Distributors "leap frog" each other with the software version they include. For example, Red Hat might have a newer version of glibc, but SuSE might have a newer kernel.
9.1.1 Choosing the right distribution
The price of the distribution will be part of your TCO equation. Contributing factors to the cost of a commercial Linux distribution are the charge for the support and maintenance as well as the rights to future upgrades
A goal of the Free Standards Group workgroup Linux Standard Base (LSB) is to promote compatibility among Linux distributions and thereby enable software applications to run on different Linux systems. The LSB defines a standard against which both the distribution and the middleware/application products can be certified. Ensuring compliance of any distribution (and application) you use with the LSB helps you avoid problems when you change distributions or need to move an application to run on another distribution.
Documentation can be another important aspect of your choice of distribution. Inspect it for coverage in those areas where you have the greatest need for clear information.
9.1.2 Modifying a Linux distribution
As explained in Chapter 1, "Introducing Linux," Linux consists of numerous Open Source packages. Different distributions bundle different versions of various packages. Together with specific installation and administration tools, configuration files, documentation, and so on, this bundle of packages makes up your Linux system. At some point, you may need to make a change to your Linux distribution. You probably already have a change management policy in place for most of your other operating system images, and maybe even for Linux. This section presents a few things unique to Linux that you might want to ensure are addressed.
You might want to change your chosen distributor's basic installation in order to:
Some changes, such as installing kernel patches, may invalidate the support contract. It is important that any changes made (for example, to the kernel, to the distribution, to the middleware, to third party applications, and so forth) are acceptable under the terms of the respective maintenance and support contract. Methods of change that usually are acceptable include:
Starting with a distribution that is closest to your desired system helps you minimize changes. If you do need to make changes, the preferred method is to use the specific configuration tools of the distribution.
Change management as a discipline is discussed in 16.2, "Change management."