IBM has announced its z13 mainframe, which is the 13th system in its Complementary Metal-Oxide Semiconductor (CMOS)-based platform that debuted in 1994. Some IT pros may question the relevance of a mainframe in the modern data center, but there are segments of the market in which this is an important product. For data center managers struggling to obtain a higher density for virtualized workloads, the z13 is an intriguing option. (Read ZDNet analysis of IBM’s Q4, during which every core unit saw revenues decline.)

IBM’s new mainframe has impressive stats, such as it can support an eyebrow-raising 10 TB of RAM. The number that really caught my attention is that the system can run up to 8,000 virtual machines (VMs). According to IBM, the z13 requires two floor-tiles of space. The physical specifications of the z13 indicate the mainframe takes up the same amount of floor space as a standard 42U rack.

RISC vs. x86 virtualization

Comparing the VM density to x86 server virtualization highlights the challenges faced by data center managers looking to optimize space. It’s common to get 15 virtual machines per core on an x86-based blade. A 16-core blade system can host 240 VMs. With 16 half-height blades in an HP c7000 Chassis, for example, a single chassis could host approximately 3,840 VMs. Without accounting for heating/cooling and power, at 10U per chassis, up to four fully populated c7000s can fit inside a standard 42U cabinet. This puts the total capacity of HP’s x86 solution at 15,360 VMs per cabinet.

IBM’s 8,000 VMs pure two floor-tile capacity seems unimpressive when compared to x86 capacity; however, these numbers can’t be compared directly when considering Unix servers. Take the same c7000 and populate them with Itanium blades running HP-UX, and the math changes quickly. An HP-UX engineer with several years of experience told me his typical rule of thumb is to limit one VM per CPU core. A full-height Itanium blade could host 16 VMs. With a fully populated cabinet, the Unix configuration of the c7000 maxes out at approximately 512 VM, which I’m told is the typical density for Unix virtualization based on non-86x hardware.

Even if the engineer I spoke with was overly conservative by a few hundred percent and a data center manager chooses to double the floor space needed to accommodate access to the z13 (in my experience, up to four is required for proper access for servicing), the numbers are compelling. IBM’s claims of 8,000 VMs per z13 are very impressive for a segment of the market that hasn’t seen the same advancements in server virtualization as the x86.


Organizations running big data and financial applications on RISC Unix systems could benefit greatly from the higher density of the z13 with the ability to slowly re-platform to Linux. And yet, some challenges remain.

The 8,000 VM count is a marketing number, and real-world capacity is yet to be measured. The z13 ultimately still runs z/OS, and thus requires the expertise of operations teams well versed in z/OS. Also, organizations are getting out of the business of hosting data centers; it’s difficult to find a hosted data center willing to take on the power requirements of a mainframe, as x86 racks are typically more power friendly.

Are the z13’s specs impressive enough to make you consider Big Iron? Let us know in the discussion.

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