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Linux on POWER5 architecture

The meshing of Linux with the POWER5 architecture is a big deal. If marketed properly, this may be the operating system's biggest success story to date, pushing Linux out into the enterprise in a big way.

The meshing of Linux with the POWER5 architecture is a big deal. If marketed properly, this may be the operating system's biggest success story to date, pushing Linux out into the enterprise in a big way.

While Linux has always been at the forefront in terms of lower total cost of ownership options on commodity-based Intel hardware, IBM is pushing another way. The company wants its Power5 architecture customers to be able to choose either proprietary Unix or Linux. (It actually even supports AS/400 partitions.) This in itself is a major philosophical breakthrough; no other vendor can claim that it natively supports Linux and Unix on its high-end systems (forget VMware).

We're not just talking about support here; we're talking major technological enhancements for the midrange that were formerly available only on mainframes. Linux has been running on IBM mainframes for years, but IBM's push on the p5 series targets the midrange and makes available on Linux nearly all of the Advanced Power Virtualization (APV) tools from its proprietary Unix (AIX).

To reiterate, when you create a logical partition (LPAR) from a hardware management console (HMC), you can actually choose whether to create an AIX, Linux or AS/400 partition. The HMC is the management station that the system administrator uses to interface with the POWER Hypervisor to manage your logical partitions and managed systems.

APV is not dynamic logical partitioning (DLPAR), which is a wonderful capability that was already available for Linux even on the older IBM p4 architecture. DLPAR offers the ability to react to workload requirements on the fly by moving CPUs or memory from partition to partition on a managed frame (Linux only supports moving CPUs at this time).

On the older Regatta systems like p690, the process could be either done manually or automated through tools such as the DLPAR toolset (only AIX), an IBM alphaWorks production. To automate the process of moving processes around on a Linux partition, you can create a script through an SSH session with the HMC.

The DLPAR toolset, workload manager and partition load manager (PLM) are unavailable on Linux partitions. This is not a major problem, because uncapped partitions can accomplish most of what you need more efficiently. This will be covered in a further section.

So what does the POWER5 do that previous releases of the POWER architecture do not? Let's start with APV.

APV reduces the total cost of ownership and improves IT responsiveness to business by providing flexibility and simplifying overall infrastructure management. Using the virtualization strategies of IBM, the business can significantly improve overall server utilization.

Ask a mainframe person if there is anything wrong with running at 90% CPU capacity and then ask the same question to a Unix or Linux administrator. The midrange administrator will say that 60% capacity means a problem. With shared partitions you can take advantage of unused clock cycles and not be concerned with 90% CPU utilization. After all, the CPUs were purchased in order to be used, not to accumulate dust. And the POWER5 puts them to use in a big way.

Other features of APV that are available on Linux partitions are:


This lets you slice CPUs into virtual partitions, giving you the flexibility to create a host machine with as little as a tenth of a CPU. This is a great feature if you need to deploy host systems (i.e. Web servers) that require minimal CPU resources. With micro-partitioning you can create up to 254 partitions on a 32-processor server, like the p590. The p595 has scales up to 64 processors. During server consolidation projects this feature provides enormous flexibility.

Virtual I/O

Do you need more flexibility with your adapters or disk? The virtual SCSI and shared Ethernet capabilities of APV handle I/O from virtual I/O servers. You can add I/O channels dynamically to a Linux application in response to heavy data loads, directing maximum I/O resources where they're needed and achieving the best sustainable system throughput.

This works because these VIO servers are actually partitions that have physical resources (adapters and/or disks). Your logical partitions, which are VIO clients, can then share these resources with other logical partitions. Virtual I/O is invaluable for environments that do not require a huge amount of network bandwidth, such as development or staging environments and environments that have run out of physical capacity in the managed frame.


Simultaneous multithreading (SMT) can give you an extra 30-40% improvement in horsepower, allowing two separate instruction streams (threads) to run concurrently on the same exact physical processor. To the operating system, each thread appears to run on an independent logical processor. SMT support is available only through the POWER5 architecture, whose core chipset consists of a dual processor core with each core supporting two hardware threads of execution.

Capped and uncapped partitions

Shared partitions offer the flexibility of setting uncapped partitions, which allow you to access untapped resources from your shared processor pool. Shared partitions can use as much of the shared pool as they need, based on weighting factors that you control.

What is extraordinary about the IBM virtualization engine and APV is that virtualization is not really implemented as operating system functionality, i.e. VMware. Hardware support for APV is actually built into the p5 architecture, which uses the hypervisor to integrate with the OS. The Power Hypervisor (PHYP) is the firmware layer that runs underneath AIX, Linux and i5/OS on pSeries machines.

From a Linux perspective, IBM has partnered with Red Hat and SuSE to integrate enhancements into their most recent distributions to maximize the benefits of Linux for this architecture. Because of this partnership, Linux can exploit the processor hardware in ways it could not do before. For more information, see You must be running either Red Hat Enterprise Linux 5 or SuSE Linux Enterprise Server 9 to take full advantage of POWER5.

For anyone not interested in creating partitions with Unix or AS/400, IBM also offers Linux-only systems in the form of OpenPower. OpenPower systems can cost up to 50% less than Power systems that can run Unix or AS/400 partitions because the hypervisor that powers the Virtualization Engine does not need to be as advanced. Because IBM also wants to promote Linux and sell more of these systems, it tends to offer substantial discounts with this server line.

More than any other hardware vendor, IBM promotes Linux in a big way. IBM hosts free APV across the country with labs that let people install Linux partitions. IBM's promotion of Linux is real, and perhaps more importantly, IBM's Linux technology is groundbreaking. My only criticism of IBM's Linux strategy is that too few people are aware of what they can accomplish with Linux on the p5. I hope that in the future IBM will work more with ISVs to ensure that more third-party applications are fully supported on this hardware platform.

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