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The Other Side of Thin Provisioning: Managing Oversubscription

In my post, "Religious Issue #6: Thin Provisioning," a number of people piped up with comments, as I expected. I'm a fan of thin provisioning and use it in almost every situation for my virtualization practice. But, there is one side of thin provisioning that is more of an art than a science: managing oversubscription.

The basic premise of oversubscription is that by the space savings of thin provisioning, the provisioned amount can exceed the threshold of raw capacity. In my personal lab, I have oversubscribed the primary VMFS datastore by over 18 TB (see Fig. 1). This isn't what I would do in the real world, but nonetheless it is tracked by vSphere.

Oversubscription is not necessarily a bad thing; in fact, SAN admins have been doing this for some time. For the typical Windows administrator who has graduated to virtualization administrator, managing oversubscription may be a new skill.

I'd like to point out one caveat to thin provisioning: performance. Thin provisioning by itself has minimal impact on a virtual machine's performance. In Tuesday's post, Scott Drummonds posts a few of his thoughts and a link to VMware thin provisioning white paper. (Thanks, Scott!) In that resource, Figure 4 shows that thin provision has effectively no impact on performance.

Thin provisioning and oversubscription

Figure 1. The VMFS volume has allocated virtual machines much more storage than is available. (Click image to view larger version.)

My observation on thin provisioning is that the same datastore may have been heavily consumed by free space of a virtual machine, is now populated with the real usage of a virtual machine. We are now able to achieve a greater datastore density, and based on the workloads on that type of disk, we may observe performance differences simply by consolidating more.

Managing oversubscription is best served by the built-in vSphere alerts. How do you hedge off oversubscription, and is the 15-percent threshold (default value) adequate for your needs? Share your comments here.

Posted by Rick Vanover on 09/23/2010 at 9:03 AM


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