You allocate the number of storage I/O shares and upper limit of I/O operations per second (IOPS) allowed for each virtual machine. When storage I/O congestion is detected for a datastore, the I/O workloads of the virtual machines accessing that datastore are adjusted according to the proportion of virtual machine shares each virtual machine has.
Storage I/O shares are similar to those used for memory and CPU resource allocation, which are described in the vSphere Resource Management publication. These shares represent the relative importance of a virtual machine with regard to the distribution of storage I/O resources. Under resource contention, virtual machines with higher share values have greater access to the storage array, which typically results in higher throughput and lower latency.
When you allocate storage I/O resources, you can limit the IOPS that are allowed for a virtual machine. By default, these are unlimited. If a virtual machine has more than one virtual disk, you must set the limit on all of its virtual disks. Otherwise, the limit will not be enforced for the virtual machine. In this case, the limit on the virtual machine is the aggregation of the limits for all virtual disks.
The benefits and drawbacks of setting resource limits are described in the vSphere Resource Management publication. If the limit you want to set for a virtual machine is in terms of MB per second instead of IOPS, you can convert MB per second into IOPS based on the typical I/O size for that virtual machine. For example, to restrict a backup application with 64KB IOs to 10MB per second, set a limit of 160 IOPS.