Among the many improvements in VMware vSphere 5.0 is a new means to load balance virtual machine datastores: the Storage Dynamic Resource Scheduler, or Storage DRS. Like its pre-existing compute load-balancing counterpart (plain old DRS), Storage DRS promises to make managing large virtualization environments easier by taking some of the guesswork out of virtualization storage provisioning. But proceed with caution: Storage DRS can result in unintended consequences that may prevent customers who could most benefit from it from being able to use it at all.
Storage DRS allows you to group together multiple datastores into datastore clusters. Those clusters can consist of either block-level VMFS volumes or NFS mount points -- providing welcome support for NFS, which has been neglected in previous VMware storage tech releases. Once configured, virtual machines can be load balanced across all datastores in a cluster based on available capacity, datastore performance, or both. The load balancing can operate autonomously, or it can simply recommend when it thinks you should make a change and allow you to approve it -- providing an easy way to determine how it will work in your environment before you let it off its leash.
Just as standard DRS makes use of vMotion to move virtual machines from one host to another, Storage DRS utilizes Storage vMotion to move virtual machines from one datastore to another. As with many other components of vSphere, Storage vMotion has also seen significant changes in vSphere 5.0.