Dynamic Memory will highlight storage performance problems
Posted Friday, February 11, 2011 in Technology 0 comments
Service packs are coming out Feb 22 for Windows Server 2008 R2, and one of the key new features is dynamic memory (see Jon Brodkin's article). By boosting how efficiently Hyper-V makes use of memory, this should boost VM density on a given host for a given set of resources. This claim tracks pretty closely with one we've been making for storage resources - that with a simple hypervisor plug in we'll let you support more VMs on a given storage configuration by significantly increasing the performance you get on a per spindle basis.
Memory capacity is probably the main limiter of VM density, but storage is right behind it in terms of importance. How many of you haven't noticed that you need to add spindles to your existing storage configurations once you start hosting workloads in virtual environments? Anyone...? This issue doesn't really have anything to do with main memory, but has everything to do with how hypervisors in general (not just Hyper-V) handle I/O. You might have heard the term "VM I/O blender", and its an apt description of what happens to I/O in a host running multiple VMs. The extremely random I/O pattern pretty much means you get the worst performance your underlying disk spindles can support because rotational latencies and seek times start to dominate disk access times. With such a random I/O workload the hypervisor doesn't really have the opportunity to performance optimize how data is written to disk. What's needed is a storage architecture that is built to handle the I/O patterns common in virtualized environments, not one designed for the good old days of "one application/one server" that have dominated the client/server computing model. This problem exists in virtual server and virtual desktop environments, and the problem tends to be worse the more write-intensive the environment is.
How does this relate back to Microsoft's release of Windows Server 2008 R2 SP1? First, if you're having performance and/or density problems on Hyper-V, memory might be your problem, and it might not. If it isn't, then storage performance probably is, and you might want to check Virsto out. If main memory was your problem, and dynamic memory helps, that will probably pretty much just move the challenge downstream to storage.... and you might want to check Virsto out.
So I guess the bottom line here is that if you want the best performance you can get out of your existing storage configuration, you'll need to optimize along the entire I/O path, not just the main memory. The gains you'll get on Hyper-V from implementing both dynamic memory and Virsto will be pretty impressive, so don't just do one or the other... do both. You can see from case studies on our site that a lot of our customers get 3x or more the storage performance they were getting out of Hyper-V before they installed us while actually using fewer disk spindles. If you want to see what this might mean in your environment, you can download a trial version of our software for free at www.virsto.com.
Leave a Comment