If you are running a Dell EqualLogic SAN and are on an effected version then you need to lookout for this VMFS Heartbeat corruption issue with vSphere 5.5. The fix is to upgrade your SAN firmware.

It appears there is also an issue with EMC VNXe SANs and iSCSI. This one only effects dual SP systems. I don’t have a direct link for this one but the EMC KB is 169041, Cormac Hogan has a few more details on his blog. There is a hotfix for this issue.