IOPS Limit for Virtual SAN Objects

Virtual SAN version 6.2 introduced the ability to limit the amount of IOPS a virtual machine object could consume on a per-second basis. Virtual SAN normalizes the IO size, either read or write IO, in 32 KB blocks when it performs calculations. The throttling is specified as part of the storage policy and will be applied to each virtual machine object.

The following table demonstrates how actual IOPS are calculated on a Virtual SAN virtual machine object.

VM IO Size VSAN IO Size VSAN IOPS Limit Actual IOPS Limit
32 KB 32 KB 10,000 10,000
64 KB 32 KB 10,000 5,000
128 KB 32 KB 10,000 2,500

Once a virtual machine encounters the IOPS limit specified in the storage policy, any remaining IO is delayed until the next one-second window. The current implementation creates a spikey IO profile for the virtual machine objects, which is far from ideal.

SIOC or Virtual SAN Limits?

A key differentiator between the Virtual SAN IOPS limit and Storage I/O Control, is the IOPS limit specified in the Virtual SAN storage policy is applied regardless of any congestion that may or may not exist on the datastore. As a result, a virtual machine object can be throttled even though the Virtual SAN datastore has plenty of unused or free IOPS.

As mentioned in the previous post, SIOC provides the ability to assign resource shares to a virtual machine and/or a limit. The current Virtual SAN feature is only a limit — meaning it will be enforced regardless of the overall IO resource consumption on the Virtual SAN datastore. As such, it is not a feature I have seen being regularly implemented inside a hyper-converged infrastructure (HCI) architecture. It would be nice to see the SIOC functionality added entirely to Virtual SAN in the future.