The Impact of Virtualization on Storage

By Drew Robb

The benefit of more powerful processors in virtualization hosts is clear: if you double the processing power, you should be able to run twice as many virtual machines as you did previously. That has important financial implications when it comes to per-processor virtualization software licensing costs.

Unfortunately, server virtualization is not as simple as that. A physical server running twice as many virtual machines produces twice as many I/O operations, and these become random operations rather than sequential ones thanks to the "I/O blender" effect. In fact, the amount of I/O operations is probably more than double as virtual machines are moved around.

The result? Storage systems that struggle to cope and applications that run slowly in their virtual machines.

When it comes to addressing the "write" side of the problem, one solution is to use a storage hypervisor such as VMware's Virsto. The storage hypervisor takes over the handling of I/O traffic from the standard virtualization hypervisor, sending writes to a high-performance staging area which immediately acknowledges them. The staging area then optimizes the writes—essentially making them sequential streams rather than random ones for each virtual machine—and sends them on to a storage pool for final storage

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This article was originally published on October 10, 2013