I recently blogged about the oddity of EMC CX arrays not being supported with VAAI on vSphere 5.0 anymore and - like many others - I wondered what are the real reasons behind this. Today Chad Sakac (EMC) explained the riddle in a web cast covering a VNX engineering update and - well - exactly this issue.
If you missed it: he just published the presentation that he showed on his blog. Here is a summary:
Starting with vSphere 5.0 VMware changed the qualification tests for VAAI to include a performance component. The goal of this is to ensure that the different VAAI primitives not only work functionally but also run with the performance VMware expects. The CX arrays are rather old now (they were made generally available in 2008) and their storage processors do have far less power than the ones of the current VNX series. This is why they failed to complete the XCOPY primitive in the expected time. As a consequence the CX failed the VAAI test as a whole and is no longer officially supported for VAAI in vSphere 5.0.
Nevertheless Chad recommends to keep VAAI enabled for CX arrays when running vSphere 5.0, and he assured that both EMC and VMware will support anyone with this setup, but may ask you to disable VAAI to rule it out as a cause of problems during troubleshooting.
Chad raised a discussion on whether VMware is doing right with its change to the qualification process and whether EMC should try to back port some of the performance enhancements they made for the VNX to the CX series (which could in turn hurt the stability of the CX because of its limited resources). His blog post contains a link to a survey that you can take to join this discussion. If you feel affected go and make yourself heard.