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 shares used for memory and CPU resource allocation, which are described in Resource Allocation Shares. These shares represent the relative importance of a virtual machine regarding the distribution of storage I/O resources. Under resource contention, virtual machines with higher share values have greater access to the storage array. When you allocate storage I/O resources, you can limit the IOPS allowed for a virtual machine. By default, IOPS are unlimited.
The benefits and drawbacks of setting resource limits are described in Resource Allocation Limit. 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 back up application with 64 KB IOs to 10 MB per second, set a limit of 160 IOPS.