We ran into something at work lately that caused some serious issues around PSODs and storage disconnects due to the way that VMware ESXi 5.5U2 changed the way VMFS heartbeats are used. Apparently VAAI now is used to manage the heartbeats in 5.5U2. If you need to disable this feature without turning off VAAI or reverting back to ESXi 5.5U1 you can used the advanced settings from esxcli below to accomplish this. You can then run the list command to validate that the setting for Default Int Value: 1 is now set so that heartbeating is disabled for VMFS. This is an issue specifically for ESXi 5.5U2 connecting to IBM SVC on microcode 126.96.36.199 and other versions but more importantly and issue around the change in VAAI controlling the heartbeating for VMFS datastores on IBM SVC storage.