Performance can become an issue when sharing hardware between multiple Linux instances. Even the largest S/390's capacity is not limitless, and speculation abounds as to just how many Linux instances could do "real work" on an S/390. According to Boas Betzler, just how effective a multi-instance Linux deployment can be depends mostly on what kind of applications are being run. Betzler says the mainframe's strengths are extremely high I/O bandwidth and parallel access to the same I/O channel by more than one CPU at a time. Context switches and SMP management are also extremely fast, yielding a very high figure of merit as CPUs are added to a machine. Betzler says multiple Linux instances under VM make sense if the workload lends itself well to aggregation of CPU use, as might be the case if an ISP had hundreds of moderately-sized web hosting customers. Betzler says instance counts "into the hundreds" are feasible under VM/ESA, but adds that rigorous testing in the thousands range hasn't yet been done. This is all on a single SMP machine, incidentally. IBM offers a cluster of SMP nodes called a Sysplex, which can extend performance almost without limit.

The biggest weakness cited by users of the beta version has been poor documentation, especially for the installation process. Because there are numerous ways to install and to run Linux for S/390, new users have found the instructions quite confusing. This seems especially true when the system administrator has no UNIX background and is trying to learn a new operating system at the same time he or she is performing a complex system management procedure. IBM acknowledges that there is room for improvement with the documentation, and in fact there is a Redbook Residency program underway to address this need. (IBM Redbooks are detailed technical "cookbooks" similar to Linux HOWTO documents, only larger in scope. Redbooks are available online at http://www.redbooks.ibm.com.)

Another problem encountered by prerelease users has been the difficulty in managing direct access storage devices, or DASD. Especially in the VM/ESA environment, IBM allows disk drives and partitions thereof to be managed and defined in a variety of ways, and this adds to the confusion of anyone installing Linux for S/390. The latest version of the IBM DASD driver supports all common DASD configurations with much simpler semantics. Betzler says that SuSE and TurboLinux will be working with IBM to help improve the installation process for future releases.

Greg Burke, vice president of IBM's Linux for S/390 division, says Linux complements IBM's other operating systems more than it competes with them. He views the ability to rapidly develop and deploy cutting-edge e-commerce applications as a key strength of Linux, and says this agility will allow S/390 to penetrate new markets. OS/390, IBM's bread-and-butter mainframe operating system, is noted for scalability and stability, but not for rapid application development. Burke says the ability to run Linux and OS/390 side-by-side on the same hardware allows enterprises to capitalize on the strengths of both environments.

Burke sees Linux as an important part of IBM's e-commerce strategy. "We plan to take this to another era," he says, referring to this introduction of Linux as "another step" in the evolution of e-commerce. Burke noted that Linux is now the only operating system that can run on every IBM hardware platform from a PC laptop to the largest mainframe.

Further information about Linux for S/390 is available from IBM's S/390 web site, from IBM's DeveloperWorks site, or from the Marist College Web site.

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