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Perhaps a silly question this one, but I was wondering if it were possible to multipath a storage array across a FC connection and an iSCSI connection? Given that the main point of multipathing seems to be to increase resiliancy in the face of hardware failure, it seems logical that providing a storage pathway that depends on networking hardware would be a suitably resiliant alternative to a pathway that relies on FC.

I appreciate that the performance might not be the same across both, but might this not be an acceptable low-cost solution where there isn't enough budget for multiple FC connections (and fabric)?

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up vote 3 down vote accepted

theoretically, if device-mapper-multipath detected the same LUN over an iSCSI link it would add it to the available paths, and then it's up to you to make sure you set the failover and IO directions to the right priorities. I do however doubt you will see the same presentation of the LUN, because I haven't seen any arrays that allowed exporting LUNs and targets based on the same block device pool. If you know of such a device, please correct me, since I obviously haven't seen every possible SAN out there

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I was thinking along the lines of a Solaris ZFS based SAN, which I believe can export a LUN on both FC and iSCSI targets... – growse Oct 31 '11 at 21:55
if you're building your own SAN, anything is possible really – dyasny Nov 1 '11 at 9:01
Yes it's possible, you would have to write your own path checker (using linux multipath for reference) to score the paths properly. Solaris would also have to present identification for both luns that could be correlated by the multipath host as paths to the same lun e.g. INQUIRY, or you would roll your own and code your multipath agent to recognize this association. – ppetraki Jan 19 '12 at 16:12

I suppose that you could. It'll probably depend on your storage vendor. Personally I'd get and spend the budget for another fiber channel switch and more HBAs.

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I agree, I'm just thinking a little out of the box for situations where "more of the same hardware" might not be possible. I'm just not aware of any multipath software which will let you do it across both. – growse Oct 31 '11 at 21:56

As long as your SAN supports it (Compellent does, I don't know about others), you certainly can. However, be careful of the path selection policy you choose - that is, how MPIO does load balancing. Set up FC as your preferred path and use iSCSI for failover, or vice versa (whichever fabric is preferred based on speed, congestion, reliability, etc). Avoid doing load balancing (e.g. round robin) using paths from both protocols.

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