New mclock I/O Scheduler in vSphere 5.5 – Some details

schedulerMy colleague Duncan wrote a post relatively recently around the new mclock I/O scheduler which VMware introduced in vSphere 5.5. He also mentioned some caveats with the new scheduler, especially around the I/O size (32K) used with the IOPS setting, which may lead to some unexpected behaviour. As Duncan mentioned, the reason for introducing the new scheduler is primarily to provide a better I/O scheduling mechanism that allows for limits, shares and reservations. Unfortunately, we didn’t do a very good job of announcing this change in I/O scheduling, or documenting the behaviour, and it has led to a number of additional questions from our customers. I hope to address some of these here.

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Does Storage DRS work with Adaptive Queuing?

Many of you will be aware that Storage DRS uses Storage I/O Control (SIOC) for load balancing based on I/O metrics. However a statement in one of our white papers has raised a few questions recently with both our customers and partners. The statement is as follows:

“Queue depth throttling is not compatible with Storage DRS”. (pg.34) from  http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.5.pdf.

This assertion led many to believe that Storage DRS would not work well with Adaptive Queuing (AQ), another of VMware’s queue depth throttling mechanisms. However internally, many felt that this wasn’t a true statement, but some work was needed to verify that it would not cause any issues. This led to a number of tests being run with Storage DRS and both of our queue throttling features, SIOC and Adaptive Queuing. I am using this post to share those results.

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Adaptive Queueing vs. Storage I/O Control

This post is to look at two different technologies available in vSphere to manage the queue depth on your ESXi host(s). A queue determines how many outstanding I/Os can be sent to a disk. In the case of vSphere environments, where many hosts can be doing I/O to the same shared disk device, it can be helpful to throttle the LUN queue depth from time to time when congestion arises. In this post, we will compare and contrast Adaptive Queues with Storage I/O Control (SIOC).

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Monitoring Storage I/O Control activity with new vSphere 5.1 counters

There was an interesting question posted recently around how you could monitor Storage I/O Control activity. Basically, how would one know if SIOC had kicked in and was actively throttling I/O queues? Well, in vSphere 5.1, there are some new performance counters that can help you with that.

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vSphere 5.1 Storage Enhancements – Part 8: Storage I/O Control

Storage I/O Control (SIOC) was initially introduced in vSphere 4.1 to provide I/O prioritization of virtual machines running on a cluster of ESXi hosts that had access to  shared storage.  It extended the familiar constructs of shares and limits, which existed for CPU and memory, to address storage utilization through a dynamic allocation of I/O queue slots across a cluster of ESXi servers. The purpose of SIOC is to address the ‘noisy neighbour’ problem, i.e. a low priority virtual machine impacting other higher priority virtual machines due to the nature of the application and its I/O running in that low priority VM.

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