ESX 3.5 U3 and ESXi U3 are the first releases of ESX 3.x to provide official VMware support for the internal SAS networked storage that exists within the Intel® Modular Server. VMware has identified an interoperability issue between ESX 3.5 U3 and the dual SAS Storage Controller Modules (SCMs) that provide access to, and management of virtual disk storage, together with RAID functionality for the underlying SAS Hard Disk Drives (HDDs) that plug into the Intel® Modular Server chassis backplane. This interoperability issue precludes the ability of both ESX and the SCMs to provide failover between the SCMs in the event of an internal SAS link, SAS controller port, or SAS expander failure. Figure 1 below provides an illustration of the internal organization of these SAS components.
Upon the occurrence of any of the above SAS link or port related failures, the remaining SCM expects the host OS (ESX) to initiate explicit fail-over of the virtual disks the SCM presents from the underlying HDDs belonging to the now inaccessible SCM. ESX expects the remaining SCM to automatically execute implicit fail-over of these HDDs. Consequently, the HDDs belonging to the SCM that has experienced the SAS link or port related failure remain with that SCM, and thus their corresponding virtual disks remain inaccessible to ESX.