Why virtualisation is struggling to keep up

• Tags:
• Scaling,
• Processors,
• Multicore,
• Virtualisation

Carl Claunch

Published: 25 Mar 2009 17:08 GMT

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...limits will matter in all layers of the software stack, not just the operating system.

The future Kilimanjaro version of SQL Server, scheduled for release at about the same time as Windows Server 2008 R2, is expected to support 256 processors.

z/OS
IBM's z/OS v.1.10 has a hard limit of 64 processors. As IBM continued to refine z/OS to improve scalability for all types of workloads, it increased the limits for the next generation, once it was satisfied that the scaling experience of all customers would be good.

This is a conservative approach, whereas many other vendors set hard limits that may not be achieved in real-world workloads.

Unix
Unix systems have been operating with larger processor counts for many years and have hard limits that reflect that. However, in some cases, they may be set by the largest server built rather than the intrinsic hard limits of the software. Solaris 10 has a hard limit of 512 processors. HP-UX 11i has a hard limit of 256 processors, but HP states it has a design limit of 2,048, allowing for future expansion. AIX 6 is limited to 128 processors, which is identical to the largest machine available today from IBM.

VMware
VMware ESX supports a maximum of 32 processors in the physical machine. It could handle up to, but not above, a four-socket system with the new generation of eight-core chips. The virtual machines have limits on the number of virtual processors each is given.

The hard limit for virtual machines is four virtual processors. The market has a fair number of people running on four-socket machines with quad-core chips, at 16 processors in total.

Hyper-V
The latest Hyper-V in Windows Server 2008 has a hard limit of 24 processors for the physical machine and up to four per virtual machine. It can support, at most, a two-socket server using the newest eight-core chips. In 2010, the Windows Server 2008 R2 version of Hyper-V will increase the hard limit to 32 processors.

One solution is to plan to run with a wider range of operating-system releases in production, because users installing bigger servers may be forced to run the latest software, despite the migration strategies for their other servers.

You should also carefully evaluate the hard and soft limits to scalability of important software you plan to deploy, to ensure it will perform as expected on the intended hardware platform. Finally, you need to look now to hard partitions as a way of overcoming the limitations of many virtualisation hypervisors.

Carl Claunch is a vice president and distinguished analyst at Gartner Research. He conducts primary research into grid computing, its markets and technologies, as well as cluster computing. One of Claunch's key areas of interest is technology trends for servers.

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