VMware bought Sunnyvale storage startup Virsto today for an undisclosed sum.

The acquisition of Virsto, which is 50-people strong, is significant because it helps VMware fend off Microsoft.

Virsto allows a company's storage to be managed like it's one big hard drive, even if the company is using different types of storage technologies or the storage is located in different areas.

Microsoft is breathing down VMware's neck with a competing technology known as Hyper-V. Hyper-V is included in Microsoft's flagship server operating system, Windows Server 2012.

For many  years, Hyper-V was considered the cheaper, less desirable alternative to VMware's flagship software. But Microsoft loaded WS2012 with features that, in some cases, leapfrog VMware's. One of these better features is called Storage Spaces, which works a lot like Virsto.

VMware owns 90% of the enterprise market, and Microsoft's Hyper-V isn't going to change that. But the issue is growth, which will come from small companies and large, cloud-provider data centers. Both can be very cost-conscious, which makes Microsoft more appealing to them.

Silicon Angle's John Furrier offers an excellent deep-dive analysis of the Virsto acquisition.