VMware is updating the VMware Virtual SAN Compatibility Guide (VCG) as part of our ongoing testing and certification efforts on Virtual SAN compatible hardware.
Specifically, we’re removing low-end IO controllers from the list, due to the impact these controllers have on Virtual SAN. The choice of IO controller in Virtual SAN really matters when it comes to sustained IO performance. Even with a design like Virtual SAN where a flash device caches IOs, in case the flash device is behind a controller, all IOs go through the controller in each server. The outstanding IOs are managed using a queue on the controller, and the IOs are de-staged from the queue to various storage devices. IO controllers with low queue depths are not well suited for the type of workloads that Virtual SAN is designed to support. These controllers offer very low IO throughput, and hence the probability of the controller queue getting full is high. When the controller IO queue gets full, IO operations time out, and the VMs become unresponsive.
The situation is exacerbated in the case of re-build operations. Although Virtual SAN has a built-in throttling mechanism for rebuild operations, it is designed to make minimal progress in order to avoid Virtual SAN objects from being exposed to double component failures for a long time. In configurations with low queue-depth controllers, even this minimal progress can cause the controllers to get saturated, leading to high latency and IO time outs.
Given the above scenario, VMware has decided to remove controllers with queue depth of less than 256 from the Virtual SAN compatibility list. While fully functional, these controllers offer too low IO throughput to sustain the performance requirements of most VMware environments.
For a complete list of controllers that will be removed from the compatibility list, please refer to this Knowledge Base article.
If you have purchased Virtual SAN for use with these controllers, please contact VMware customer care for next steps.
Going forward, in order to make it easy for our customers and partners to put together the appropriate Virtual SAN solution for their specific scenario, we are working with our partners to list the queue depth of all controllers in the VCG in the coming weeks. For additional hardware guidance on Virtual SAN, please refer to Virtual SAN Hardware Guidance.
