DRS Host Affinity

One new feature I would like to highlight in the vSphere 4.1 announcement is DRS Host Affinity. This feature allows you to set granular policies for VM movement. A great use case for this new functionality would be to limit a VM to certain hosts for licensing purposes. For example, Microsoft’s typically licensing policy for Cloud providers is on a per processor basis where the usage is computed monthly. The problem with this licensing model in a virtualized environment is that a VM could potentially run on every processor within every host contained in a cluster. Depending on the resource and availability requirements of the virtualized environment, features such as DRS and HA may be enabled. DRS may decide to move VMs because of increased load on the hosts and depending on the types of loads in the environment this movement could be quiet frequent. In the case of host failures or unplanned outages HA is used to restart VMs.
Prior to DRS Host Affinity the host a VM was relocated to could not be easily determined or controlled. Not being able to easily prove which processors a given VM touched in a month meant that companies may be charged for every processor within a cluster because a Windows VM resided in it. For Cloud providers this would mean increased costs, which would result in increased service fees, a model that does not scale well. DRS Host Affinity does not entirely solve the problem of knowing which host a VM will move to, but it does make it more predictable and as such easier to determine and reduce licensing costs. In the case where a VM is allowed to run only on two hosts the movement is always known. For large environments or ones with higher availability requirements, two hosts may not be sufficient and as such a larger subset of hosts may be selected.
This licensing example highlights one reason why DRS Host Affinity is important. While you may or may not have a use case for this feature, it is yet another tool in the VMware arsenal that you should be aware of. I am extremely interested in how the community will take to this new functionality.

© 2010, Steve Flanders. All rights reserved.

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