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Windows Server 2012 R2 included a Storage QoS feature that was designed to regulate storage IOPS consumption for Hyper-V virtual machines. Policies could be put into place on a per virtual machine basis that would either limit IOPS or reserve IOPS for the virtual machine in question. As well as this feature worked, though, it had one major shortcoming.
The most significant problem with the Windows Server 2012 R2 Hyper-V Storage QoS feature was that it worked on a per-host server basis. That means it is entirely possible (and likely) for multiple Hyper-V host servers to share a common storage device. The IOPS demands from one host server could potentially overwhelm the underlying storage, thereby impacting the performance of virtual machines that are running on other host servers.
So, how does the new Storage QoS feature improve upon the prior release? Rather than treating each Hyper-V host separately with regard to IOPS, the storage will be actively involved in the storage bandwidth reservation process. The bandwidth can be effectively managed across multiple Hyper-V hosts. The storage coordinates with the Hyper-V hosts to manage bandwidth rather than the Hyper-V hosts acting independently.
For storage pros, here is another chief consideration regarding this new feature: in order to take full advantage of the new Storage QoS feature, virtual machines will have to be stored on a compatible scale-out file server that is exposed to the hypervisor through the SMB 3.0 protocol. The Windows-based scale-out file server actively communicates with each Hyper-V host server. This means that Hyper-V storage IOPS policies can still be set on the host server, on a per-virtual-machine basis, but it will now be up to the file server to coordinate requests for storage bandwidth from the various Hyper-V hosts and to enforce the Storage QoS policies. The end result should be much more effective control over storage bandwidth than what is currently available.
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