z/VM and z/Linux is like biscuits and gravy

Mmmm.... Of course that’s a complete dramatization, but you get the point.   

As I indicated in my last post, virtualization is a requirement for many reasons. Today's System z mainframe has it built in. Each machine has a type 1 Hypervisor built in to transform physical resources into virtual ones. Infact, System P and I models are equipped with Hypervisor technology as well.  Also virtualizations of memory, etc has been around in operating systems for years. Yes, z/VM, a virtual machine OS, facilitates z/Linux installations. But, how? Why wouldn't I just want want to use a PR/SM setup instead? The answer is related to how many Linux machines do you need as well as other considerations we will be discussing in this article.   

PR/SM (Processor Resource/Systems Manager) of course is derived from VM technology and is included with your z series processor. By creating Logical partitions, it also allows these LPARS to share processor and I/O resources by using MIF (Multiple Image Facility) and MCSS (multiple channel subsystem). These components allow I/O channel paths and subsets of devices attached to those paths to be shared between LPARS. PR/SM has improved over the years and allows for dynamic LPAR; the ability to add and delete processors, memory, and I/O between LPARS while they are still active.  

So why not just use this instead of paying of z/VM? This answer can be related to the trade off of managing a variety of LPAR (logical partition) configurations. You need to get the right mix of LPARs and massage them so that they will not have underutilized resources allocated to them. There is also a current limit in PR/SM of 60 LPARS.   

One of the reasons for virtualization I indicated earlier was if you want to drive up resource utilizations. Of course you could make OS images more application complex by consolidating. This had been done before on Mainframes. Yet, there were many reasons for moving applications to other machines. In the old days of MVS, you might have had a problem with IMS and DB2 co-existing well in shared memory. The distributed environment was also moving applications to their own servers with regard to change control or machine stability. Ultimately, this drove up the number of underutilized machines. With larger number of servers being used, there begins a boundary where one type 1 Hypervisor VM (PR/SM) or another one like z/VM may be more appropriate to a given shop.  

Here is but a few of Z/VM's advantages: 

• More VM: Hundreds of VMs can be run under one z/VM. The number of VMs that you can run is limited only by the amount of system resources available. 
• Starts everything: Either in System Config file or using the System userid for startup "autolog1", the VM administrator can define resources and start the Linuxes. 
• Dynamic reallocation: Using execs, PROP, or Performance Toolkit, the VM administrator can have a wide range of on-demand allocation of resources as needed including setting the share priorities of the Linuxes as well as monitoring and reporting. 
•  Over-allocation: Because of virtualization of memory for example, the over-allocation of resources is handled by z/VM as paging. 
• Upgrade in place: With shared resources like disks, Linux upgrades can be easily handled by add/replace with larger or more disks. 
• Vswitch controller: z/VM's TCP/IP stack initializes the OSA-Express device and continues communication as guest host couples to the virtual switch. In the event of OSA failures, the Linuxes can be isolated from the recovery that z/VM provides with the TCP/IP stack as this controller switches to alternate OSA. Recoveries can be even be more robust with controller failures being replaced by another controller. Starting with z/VM 5.3 there is support for IEEE 802.3ad Link Aggregation, Vswitch management (SNMP), native VLAN, improved diagnostics with Diagnose x'26c', etc.  

At the IBM Infocenter, they write, "virtualization can improve IT resource utilization by allowing system administrators to access and manage resources across a homogenous and heterogeneous environment."  In short, depending on your installation, virtualization is a given and PR/SM may be just fine. However, z/VM may be just the ticket for those shops that would like to take advantage of many of its features. As a Linux administrator and z/VM offering the ability to have any volume available to any Linux, mmmm tasty in deed.  I will talk more about that in latter posts.  Stay in Touch....