Thursday, May 1, 2014

SIOC - VMware vSphere Storage I/O Control

What is SIOC ?

A cool feature in VMware vSphere.. Well all the features in VMware vSphere platform are way cooler than they look. Trust me other virtualization vendors are no where even close when it comes to useful features.

Enough blabbing.. back to SIOC or Storage I/O control.

"SIOC or Storage I/O control is a mechanism to prioritize I/O for virtual machines running on group of vSphere hosts and sharing a common pool of storage."

Storage problems are most easily identified by high device latency. When storage takes a long time to service IOs (over 20ms by my definition), application owners will soon start complaining. The goal of SIOC is to identify this trend at the VMFS volume level and take corrective action to protect high priority virtual machines

Lets see how it works and some prerequisites.
  • SIOC throttles (decreases in most cases) VM's throughput to limit access to host device queue.
  • SIOC is enabled per datastore.
  • SIOC only applies disk shares when a certain threshold(Device Latency, most likely 30ms but configurable) has been reached.
  • SIOC modifies array queue on host (based on average DAVG per host on a datastore). Please refer to Frank Denneman's excellent post
  • SIOC will enforce limits in terms of IOPS when specified on the VM level.
  • SIOC requires Enterprise Plus license.
  • SIOC also supports NFS datastore in vSphere 5.0 and above.
  • SIOC is NOT same as traditional VM disk share but a level up. 
  • SIOC is NOT compute cluster based.

You can see the working of SIOC in a youtube video SIOC in action


Storage I/O Control (SIOC) is an Enterprise Plus feature that is used to control the I/O usage of a virtual machine and to gradually enforce the predefined I/O share levels according to the business needs. Even if equal shares are desired, fairness cannot be guaranteed between VMS on different hosts without SIOC.

It is supported on Fibre Channel, iSCSI and NFS storages, and can automatically set the best latency threshold to achieve maximum throughput. This enables you to make the best out of your shared storage, and helps you easily manage a better virtual environment.

This feature is a must have for organizations that want to achieve higher consolidation ratios or host VMs for multiple tenants (like public clouds) as it helps reduce the effect of a noisy neighbor trying to hog storage resources on the rest of the well-behaving VMs.

I plan to write another blog on enhanced features in SIOC in vSphere 5.1 and implications on NFS datastores. 

Until next time :)

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