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May 23rd, 2014 14:00
Failover Mode 4
I understand that Failover Mode 4 is Async Active/Active... if optimal path failed, the IO did not route to non-optimal path. I am confused here. Since the LUN has an owner, IO is still trying to access through the another HBA connection and then through CMI if the backend fail. PowerPath does not issue to trespass the LUN. Statistic also collect Implicit and explicit trespass info. Can you please explain why we recommend to have failover mode 4 when customer request any given LUN without trespassing the LUN. May be I don't understand clearly with Async Active/Active. Thank you.
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AdamFox
254 Posts
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May 23rd, 2014 14:00
Failover mode 4 relies on ALUA, which is an industry standard used by array vendors as well as host multi-path drivers (such as PowerPath, but just about everyone supports this) to help with multi-pathing. The array will advertise optimal and non-optimal paths and, if properly configured, the host will use the optimal paths unless none are available. In the case of VNX, the optimal paths are FE ports that reside on the same SP as the LUN owner as that is the quickest path the LUN since there is no traversing the CMI. Just because using the other path may not cause a trespass doesn't mean that using the non-optimal path is a good idea unless it is necessary (e.g. the optimal path(s) is/are down, or the SP has failed over, etc.). That is what is meant by Asymmetic Active/Active. Both paths are active (unlike a truly active/passive config), but all paths are not equal to the LUN, thus they are asymmetric. Systems that have Symmetric Active/Active (such as VMAX as well as a limited configuration on VNX2) do not have to advertise optimal and non-optimal paths because all paths are equal regardless of which FE ports and which controller is used because all paths are equal. But all VNX1 and nearly all VNX2 use cases today are asymmetric, thus we recommend failover mode 4 for VNX systems.
AdamFox
254 Posts
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May 23rd, 2014 15:00
Here's an article that goes into more details than I did.
https://community.emc.com/message/792131#792131