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The second step in the setup of Storage I/O Control is to allocate:
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 VM has been allocated.
Storage I/O shares are similar to the ones that are used for memory and CPU resource allocation. They represent the relative priority of a virtual machine regarding the distribution of storage I/O resources.
Under resource contention, VMs with higher share values have greater access to the storage array, which typically results in higher throughput and lower latency. There are three default values for shares: Low (500), Normal (1000), or High (2000) and a fourth option, Custom. Choosing Custom allows a user-defined amount to be set.
In addition to setting the shares, you can limit the IOPS that are permitted for a VM. By default, the IOPS are always unlimited as they have no direct correlation with the shares setting. If you prefer to limit based on MB per second, you must convert to IOPS using the typical I/O size for that VM. For example, to restrict a backup application with 64 KB I/Os to 10 MB per second, set a limit of 160 IOPS (10240000/64000).
You should allocate storage I/O resources to virtual machines based on priority by assigning a relative number of shares to the VM. Unless virtual machine workloads are similar, shares do not necessarily dictate allocation in terms of I/O operations or MBs per second.
Higher shares allow a virtual machine to keep more concurrent I/O operations pending at the storage device or datastore compared to a virtual machine with lower shares. Therefore, two virtual machines might experience different throughput according to their workloads.