Hi,
If you are using vSphere 5.X, then you can use SDRS , storage DRS, which automatically trigger storage migration of VM based on I/O load and storage space availability on datastore.
"vSphere Storage DRS continuously monitors storage space and I/O utilization across a pre-assigned pool of datastores and intelligently aligns storage resources to meet your business growth objectives."
Here is blog from VMware on SDRS
http://www.vmware.com/products/datacenter-virtualization/vsphere/storage-drs.html
http://blogs.vmware.com/vsphere/2011/07/vsphere-50-storage-features-part-5-storage-drs-initial-place...
Regards
Mohammed
Mohammed | Mark it as helpful or correct if my suggestion is useful.