Yazou ( 12 years ago )
Hi BillThanks for the great question, and one I find myeslf explaining to customer’s allot.Historically Storage Networks have always been 2 separate networks, SAN A and SAN B, this as I'm sure you are aware is to provide two distinctly separate paths for the storage traffic in order to provide full resiliency.These two separate networks obviously provide multiple paths between the Host (Initiator / Server) and the Array containing the logical disk (Target), and all these paths can be intelligently used by multipathing aware drivers (MPIO, EMC PowerPath etc..)OK so back to your question about why don’t we dual attach the Fabric Interconnects to the SAN Switches like we do the Array Controllers.Well as you may know in the default N Port virtualization (NPV) mode the Fabric Interconnects act like an Initiator (N Port) as far as the upstream SAN Switches are concerned. So as with any initiator it is best practice to have one HBA to SAN A and a Separate HBA to SAN B, so you can think of the Fabric Interconnects as HBA’s. So that answers your first question.Your second question around, if the point is to have two completely separate SAN Networks why are the Array Controllers often dual attached to both fabrics.This actually provides several benefits the main one being around fail over.As you may know whenever you create a LUN on an array, that LUN is active on a single Storage Processor (SP), if the active SP ever failed then the secondary SP would take ownership of that LUN. However imagine the scenario in the above diagram if Fabric Interconnect A or SAN Switch A failed. If the Blade was using a LUN which was being owned by SP A and SP A was only attached to SAN Switch A, the server would now be isolated from its LUN, as all paths to SP A would be broken.Now depending what failed and what array you have this at best would cause the LUN to failover to the other SP which while works well is a “Moving Part” in the failover process that should be avoided as a best practice.Hence the best practice to dual attach the SP’s in the Array to both fabrics, as this enables the Host to have a path to its LUNs across both fabrics regardless to the SP that owns them.I mentioned this also has other benefits; the one I capitalise on most is when performing a migration of a host from one SAN to another or even a SAN replacement.For example a few weeks back I moved a client from a Brocade SAN to a Nexus SAN and had to move all their Servers and Arrays to the new Nexus Fabric with no downtime.This was a fairly easy task as all the Arrays (VNX and Clariion in this case) were dual attached to both fabrics. So it was a case of, install the Nexus Fabric, configure all the aliasing and zoning etc.. Then move each host and array off the Brocade SAN A and onto the Nexus SAN A. This obviously breaks SAN A but as long as all paths across both fabrics were active this will not cause the Host to lose connectivity to its LUNs (Regardless of which SP own s them)Then once done confirm all is well and all paths are back up and then do the same for SAN B.Appreciate we got a bit off topic, but thought it was a good opportunity provide some context and examples around the topic for other readers.RegardsColin

danaos commun club. ( 12 years ago )
Prepare now staged, only casual,or hidden to dont know to footage. Please....and -- FROM FRONT ---
Thanks.