Q

The real world difference between F_port and FL_port protocols, Part 2

F_port vs. FL_port protocol: What's the real-world difference?

This is a two-part question.

Part 1:

Please assume that a prospect is researching a modular vs. monolithic storage solution for his FC SAN.

Please also assume that -- except for one key difference -- the prospect found a modular system and a monolithic system that, for all intents and purposes, appeared to address all his business and application needs.

Question 1:

If one of the storage solutions communicated out to hosts through a FC switch using true switch/fabric/mesh F_port protocol and the other storage solution communicated out to hosts using classic arbitrated loop FL_port protocol, what would be the real-world differences and impacts?

(No need to talk about the oft-heard theoretical metrics of FC-AL loop vs. fabric, like 127-node limit in loop vs. thousands of nodes in fabric, etc. I'm looking for practical, in-the-trenches differences.)

Part 2:

What if the data sheet for the monolithic storage array in Question 1 above states that it provides support for only FC-AL to hosts (i.e., it doesn't indicate support for fabric; put another way, it doesn't indicate support for F_port protocol).

And, what if the same data sheet for the same monolithic system states that it is internally architected with switching technology that supports a Fibre Channel-based fabric of, say, 64 internal point-to-point paths between the disk, cache and server interface controllers.

Question 2:

Will this monolithic storage device support true fabric/F_port protocol in an FC SAN environment? Does the "internal, fabric-based switching technology" take the place of an external fabric switch? Or, does the FC SAN communications protocol remain FL_port as opposed to true F_port protocol?


Click here forPart 1.

Part 2:

The only storage arrays that use an internal switch are the HDS 9000 series, the Sun SE9000 series and the HP XP series. These storage arrays support both FC-AL and FC-SW connections.

Question 2:

Question: Will this monolithic storage device support true Fabric/F_port protocol in an FC SAN environment?

Answer: Yes. The ports can be set to "point to point, fabric on", which supports the true Fibre Channel switched protocol.

Question: Does the "internal, fabric based switching technology" take the place of an external fabric switch? Or, does the FC SAN communications protocol remain FL_port as opposed to true F_port protocol?

Answer: In the storage arrays mentioned above, the internal switched architecture is not a fabric switch, it's a crossbar switch. There is a big difference here. A fabric switch uses internal firmware to route messages through the switch to the ports of the switch. A crossbar switch uses hardware based arbitration logic to route data between the paths of the crossbar.

As the Fibre Channel frames enter the front end processor ports, the data and commands are stripped from the FC frame and written to both sides of the cache using a duplexed write. The control information is sent along a separate internal switched path from the data to the dual control cache. The data is sent to the data cache. All writes into the array are duplexed to both sides of cache. Parity is calculated in cache and the data is then sent through the crossbar down to the fibre channel disks.

Chris

Editor's note: Do you agree with this expert's response? If you have more to share, post it in one of our .bphAaR2qhqA^0@/searchstorage>discussion forums.


This was first published in November 2002

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