Recently, I’ve been working on a design for a virtual infrastructure, based on VMware Virtual Infrastructure 3 with HP ProLiant servers and a small SAN – an HP MSA1500cs with MSA30 (Ultra320 SCSI) and MSA20 (SATA) disk shelves.

The MSA is intended as a stopgap solution until we have an enterprise SAN in place but it’s an inexpensive workgroup solution which will allow us to get the virtual infrastructure up and running, providing a mixture of SATA LUNs (for VCB, disk images, templates, etc.) and SCSI LUNs (for production virtual machines). The MSA’s Achilles’ heel is the controller, which only provides a single 2Gbps fibre channel connection – a serious bottleneck. Whilst two MSA1500 controllers can be used, the default configuration is active-passive; however HP now has firmware for active-active configurations when used with certain operating systems – what was unclear to me was how VMware ESX Server would see this.

I asked the question in the VMTN community forums thread entitled Active-Active MSA controller config. with VI3 and MSA1500 and got some helpful responses indicating that an active-active configuration was possible; however as another users pointed out, the recommended most recently used (MRU) recommended path policy seemed to be at odds with VMware’s fixed path advice for active-active controller configurations.

Thanks to the instructor on my VMware training course this week, I learned that, although the MSA controllers are active-active (i.e. they are both up and running – rather than one of them remaining in standby mode), they are not active-active from a VMware perspective – i.e. each controller can present a different set of LUNs to the ESX server but there is only one path to a LUN at any one time. Therefore, to ESX Server they are still active-passive. I also found the following on another post which seems to have been removed from the VMTN site (at least, I couldn’t get the link from Google to work) but Google had a cached copy of it:

“The active/active description”… “seems to imply that they are active/active in the sense that both are doing work but perhaps driving different LUN’s? i.e. if you have 10 volumes defined you might have 5 driven by controller A and 5 driven by controller B. Should either A or B fail all ten are going to be driven by the surviving controller. This is active/active yes [but] this is also the definition of active/passive in ESX words (i.e. only one controller have access to one LUN at any given time).”

Based on the above quote, it seems that MSA1500 solutions can be used with VMware products in an active-active configuration (which should, theoretically, double the throughput) but the MRU recommended path policy must be used as only one controller can access as LUN at any given time.