Many of you will have read various articles related to queue depths, especially in the area of LUN/device queue depths, and how these can be tuned to provide different performance for your I/O. However there are other queue settings internal to the VMkernel, which relate to how many I/Os a Virtual Machine can issue before it has to allow another virtual machine to send I/Os to the same LUN. What follows is some detail around these internal settings, and how they are used to achieve fairness and performance for virtual machine I/O.

Warning: These settings have already been pre-configured to allow virtual machines perform optimally. There should be no reason to change these unless guided to do so by VMware Support Staff. This is all about performance vs fairness. Failure to follow this advice can can give you some very fast virtual machines in your environment, but also some extremely slow ones.

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The resource management team is interested in your opinion about a feature in development; storage reservations.

The current version of Storage I/O Control provides tools to prioritize and provide fairness to I/O streams of virtual machines. However the current version does not provide a function to specify number of IOPS to guarantee a minimum level.
Because the storage subsystem can be shared, external workloads can impact the performance capacity (in terms of IOPS) of the datastores and therefore a guarantee cannot be met temporarily. This is one of the challenges that must be taken into account when developing storage reservations and we must understand how stringent you want the guarantee to be.

One of the questions we are dealing with is whether you would like a strict admission control or a relaxed admission control. With strict admission control, a virtual machine power-on operation is denied when vSphere cannot guarantee the storage reservation (similar to compute reservations). Relaxed admission control turns storage reservations into a share-like construct, defining relative priority at times where not enough IOPS are available at power-on. For example: Storage reservation on VM1 = 800 and VM2 = 200. At boot 600 IOPS are available; therefore VM1 gets 80% of 600 = 480, while VM2 gets 20%, i.e. 120 IOPS. When the array is able to provide more IOPS the correct number of IOPS are distributed to the virtual machines in order to to satisfy the storage reservation.

In order to decide which features to include and define the behavior of storage reservation we are very interested in your opinion. We have created a short list of questions and by answering you can help us define our priorities during the development process. I intentionally kept the question to a minimum so that it would not take more than 5 minutes of your time to complete the survey.

Disclaimer
As always, this article provides information about a feature that is currently under development. This means this feature is subject to change and nor VMware nor I in no way promises to deliver on any features mentioned in this article or survey.

Any other ideas about storage reservations? Please leave a comment below.

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