Hi, * Hannes Reinecke > In moving the assignment to another controller you will have a more > efficient I/O throughput, as in general the internal link between those > two controller isn't the fastest. So you really want to move the > LUN to that controller which should handle the I/O. Yes. But the EMC will take care of moving the volume where it gets the most I/O on its own (they call it "implicit trespass"), no host-initiated volume movement is necessary. I'll elaborate on why I believe this to be better (especially in clustered environments): Consider a simple dual-fabric topology, which controller A connected to fabric A and controller B connected to fabric B. Ten nodes share one volume, which by default are owned by controller A. They all generate about the same amount of I/O. +-----------[ Switch - Fabric A ] | | | | | Ctrl A | | | | N1 N2 [..] N10 Ctrl B | | | | | | | | | +-----------[ Switch - Fabric B ] Normally all traffic to the volume are passed through fabric A to controller A, while fabric B and controller B are completely idle. Now, say that the patch cord between N10 and Fabric A breaks. In the situation when there's no host-initiated volume, N10 will start using the path through fabric B through the passive controller with a slight performance impact, while the CLARiiON will leave the volume on controller A since it can tell that that's where 90% of the I/O is coming in anyway. If the hosts all will move the volume ("explicit trespass") whenever they see fit, in the above scenario N10 would move the volume to controller B, making 90% of all I/O come in the wrong way. Depending on the failback settings one of N1-9 would move it back to controller A later, and until the broken patch cord was fixed the volume would keep moving back and forth between controllers - not exactly optimal. At least that's how I _think_ it would work, and that's why I don't use the ALUA bits. I'd appreciate your comments on whether or not this makes sense or not... Note that in the case of an error that redirects all I/O to the passive controller (for example if N10 was the only node using the volume, or the switch in fabric A failed), the volume would still get moved to controller B (even though the hosts aren't able to do this themselves), because of the "implicit trespass"-functionality. The only drawback that I can see of relying on this is that the I/O will pass through the passive controller for some minutes instead of being transferred immediately, which isn't really a problem as the performance degradation is very slight, at least not on the CX3; I'm having problems measuring it at all. > (Remember, the original EMC failover couldn't even handle I/O on > the non-active controller ...) Yes, PNR mode sucked. I'm really glad the new CLARiiONs support ALUA, now I don't need a Symmetrix anymore. :-) > Well, the big advantage of the ALUA support in the EMC is that even the > secondary path will function, albeit slower. Hence you wouldn't get > the I/O errors during boot anymore. Ah, yes, I am aware of this. That is how the HDS AMS that was discussed earlier in the thread works too, though I'm not sure if that is actually ALUA or some HDS-specific implementation that does basically the same thing. I understood that Domenico asked if this is functionally equivalent to the new ALUA mode on the CLARiiONs, not if the old PNR mode was equivalent to ALUA, which is what I think is maybe how you understood it? If so I agree with you - those two are indeed completely different. Regards, -- Tore Anderson -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel