Why Storage DRS May Not Be Available for a Virtual Disk
Avoid problems by knowing these situations.
Storage DRS (distributed resource scheduler) allows for aggregate storage management within a datastore cluster. When enabled, Storage DRS can give you recommendations for VM disk placement in an effort to balance disk consumption and storage I/O across a datastore cluster. However, there are some situations in which Storage DRS might be disabled for a VM, even though it's enabled for the datastore cluster.
There are a number of different conditions that can cause this particular problem. For example, if a virtual machine's (VM) virtual disk is actually an ISO file, Storage DRS will be disabled for the virtual disk. Similarly, VMware disables Storage DRS for independent disks (except for special situations involving relocation or cloning).
Storage DRS can also be disabled as a result of a VM's use of legacy datastores. For example, distributing base/redo files across separate datastores can cause Storage DRS to be disabled if legacy datastores are used.
Similarly, Storage DRS will be disabled for a home disk if the VM has system files stored on a separate datastore from a legacy home datastore. In both cases, the solution is to use Storage vMotion to move data from the legacy datastore to the DRS-enabled datastore cluster.
In other cases, VMware's failure to enable DRS for a virtual disk may be caused by VMware's efforts to protect the VM. If for example, a VM's home disk has been made highly available, and moving that home disk would cause vSphere HA to lose the ability to protect the disk, VMware will disable DRS for the virtual disk rather than risking a loss of availability.
There can be any number of other conditions that can cause Storage DRS to be disabled for a virtual disk. For example, Storage DRS is not enabled for template VMs or for VMs that are vSphere Fault Tolerance-enabled, or for a VM that shares files between its disks. You can access VMware's full list of reasons why Storage DRS may be disabled for a virtual disk here.
Brien Posey is a 19-time Microsoft MVP with decades of IT experience. As a freelance writer, Posey has written thousands of articles and contributed to several dozen books on a wide variety of IT topics. Prior to going freelance, Posey was a CIO for a national chain of hospitals and health care facilities. He has also served as a network administrator for some of the country's largest insurance companies and for the Department of Defense at Fort Knox. In addition to his continued work in IT, Posey has spent the last several years actively training as a commercial scientist-astronaut candidate in preparation to fly on a mission to study polar mesospheric clouds from space. You can follow his spaceflight training on his Web site.