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Virtualizing Resources on the Mainframe

How to virtually get the most from your System z platform.

How to virtually get the most from your System z platform.

Memory Virtualization

Server memory demands can increase exponentially when servicing strategic programming models such as Java* and XML, creating increased virtualization demands on a system's most heavily virtualized resource: program-addressable RAM.

The System z platform provides two fundamental memory virtualization technologies:

  • Physical Memory Partitioning - This is an LPAR memory-virtualization technology used by the PR/SM LPAR hypervisor to segment the server's program addressable physical memory into multiple logical memory spaces. Each is then assigned to a single LPAR instance. The System z platform provides up to 60 high-performance and secure LPAR address spaces, all of which are controlled by hardware-enforced address translation controls. These controls operate with no measurable memory access overhead and have achieved the highest Evaluation Assurance Level 5 (EAL5) security certification.
  • Dynamic Address Translation (DAT) - In DAT memory virtualization, a given memory of size x is virtualized to appear to programs as having the greater total capacity of x+y. System z DAT-virtualization capabilities are provided by server hardware controls.

Within each LPAR-address space, the System z platform provides for the dynamic creation of many 64-bit virtual memory address spaces. System z DAT technology is used by z/OS*, z/VM and Linux* OSs to enable the activation and concurrent use of many (hundreds/thousands) of 64-bit virtual memory spaces by each OS instance. z/OS uses individual virtual-memory spaces to encapsulate and protect control program functions, subsystem functions (e.g., WebSphere* and DB2*) and to insulate multiple independent application instances. z/VM uses DAT to create unique virtual memory spaces for each VM encapsulation. It also exploits DAT to accommodate highly efficient sharing of both data and programs between participating virtual machines.

DAT hardware provides two concurrent levels of nested DAT for use by the z/VM hypervisor. Nested DAT is exploited to provision and manage large numbers of virtual machines with minimum memory virtualization overhead and to transparently share its LPAR memory space (down to the 4K memory page size) with each VM instance it controls. In turn, each VM OS may exploit multiple 64-bit virtual address spaces to accommodate their individual processing requirements as if each were deployed in a separate LPAR memory space.

System z DAT provides the software instance within each LPAR and VM an almost unlimited supply of virtual memory and virtual memory spaces - an increasing necessity for the emerging class of memory hungry service-oriented architecture (SOA) applications and associated software infrastructures.

System z LPARs and VMs can be configured with dedicated sets of processors or, more commonly, they're configured to dynamically and transparently share physical processors with fine-grained processor consumption levels.

Les Wyman has held numerous technical and technical-leadership positions in programming, channel engineering and mainframe architecture during his IBM career. Les can be reached at lwyman@us.ibm.com.


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Virtualizing Resources on the Mainframe

How to virtually get the most from your System z platform.

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