Tuesday January 20, 2015

Why 64-bit Hypervisor Support Is a Big Deal

Citrix just released XenServer 6.5, their first 64-bit hypervisor آ– software that lets one machine create and run many "virtual" machines. So, what makes 64 the magic number? In the past, we’ve operated on 32-bit systems. But the proliferation of new devices that XenServer supports created the need for 64-bit support. For XenServer to use a device (say, a GPU), the device needs to be mapped into the input/output (I/O) space of XenServer’s management operating system, also known as "dom0." Up until now, XenServer’s dom0 has been a 32-bit Linux operating system. That means all devices need to be mapped into just 4GB of system address space. That’s because 4GB is the most memory a 32-bit operating system can support. So a dom0 can only support a limited number of devices.

That’s a problem. In today’s servers, there are a lot of devices that need to get loaded into this 4GB address space. Storage controllers, network adaptors, management interfaces آ… and GPUs. Most systems today support at least two of our GRID boards. Some support as many as eight. That eats up a lot of space and memory. So with the 32-bit XenServer, eight was the limit for many servers. With the release of the 64-bit XenServer 6.5, Citrix has removed those memory and software limitations. A 64-bit dom0 OS allows us to load more stuff into a much larger I/O address space. That lets you scale. And scalability آ– the ability to pile more users into a system آ– is one of the biggest questions in virtualization.