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The art of optimising VM performance

Physical and virtual hiccups

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Lab While it is unfair to say (as many vendors do), that server virtualisation will take over the world during the course of the next fifteen minutes, we know from the readers of The Register that ever-expanding numbers of virtual machines (VMs) are being spun up by organisations large and small.

A primary driver for early server virtualisation projects has been to consolidate footprint onto a smaller number of physical servers, each running multiple VMs. But are virtual systems making best use of the resources at their disposal?

You also told us that the routine management of virtualised systems can be problematic, to put it lightly. But considering the drive for server consolidation in particular, there has been nowhere near as much discussion around how to configure the physical resources allocated to VMs at run time. The same goes for sizing physical host servers in order to optimize service delivery while keeping control of costs.

So how does an administrator set about specifying the amount of RAM, disk space and I/O for each virtual machine? A good starting point is to monitor the physical resource consumption of the original servers hosting the applications over their typical work cycles. This information can then form the basis for working out just how much resources the corresponding VMs will consume, after adding in the requirements of the virtual server software itself.

So while some testing and experimentation can provide an idea of the needs of each individual VM the next question becomes one of working out which VMs to run on which physical servers. Getting this right can be tricky, especially in terms of working out how to satisfy the I/O requirements of the combined virtual servers and, more particularly, to ensure that all of the networking needs are adequately resourced.

Of course, real life is rarely so accommodating and we know that many of you report significant challenges in getting resource allocation right. There is a risk that virtual machines running on your systems have been allocated more physical resources than they actually require, especially if management tools cannot provide visibility into opportunities for optimising resource consumption.

If you have any examples of how to manage the physical resources allocated to each VM we would be very interested to hear what you do. Your comments in the past have indicated that VMs tend to be set up once and then left alone until something changes the status quo, thus leaving open the potential that physical resources may have been allocated but not used.

We also need to remember that today the majority of environments run their virtual servers in a relatively static mode. Virtualisation holds the promise of flexibility to cater for changing workloads and business demands, but so far few organisations have taken advantage of this capability. In order to do this properly, a way of dynamically allocating physical resources to virtual machines is required. We’d like to hear about how you’re making all this work, whether you’re doing it manually or if you’re one of those ‘dynamic’ types. ®

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