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A tape area network and speed

We created a TAN (Tape Area Network). The TAN consists of a Brocade 3800 (2Gb) FC switch, FC_AL only LTO tape units and several hosts (utilizing a third HBA). I have read that the Brocade supports Fibre Channel translative mode meaning that it will do the translation from FC_AL to FC_SW and vice versa. The HOSTs are able to backup data on the LTO Tape Units, but the speed is not what we expected.

The only question I have is, what is the overhead that is involved in the translation process from FC_AL to FC_SW? Will it be faster if I had all the hosts using FC_AL? FC_SW is a faster method since we are using a switch, but does the translation slow it down.

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There is some overhead within the switch when bridging between protocols, but I have no statistics on what that is. The feature you are referring to is called QuickLoop on Brocade. A QuickLoop enables connectivity into a switched fabric of legacy FC-AL devices. Address translation is performed within the switch so that all devices within the QuickLoop are accessible to fabric-based nodes.

Each QuickLoop on a switch can provide the full bandwidth of the switch port for devices connecting into the loop. Loop devices share the bandwidth of the port. Each port can be configured as a looplet, running at the port speed. This means you can load balance access to your tape drives by spreading the load on different looplets.

Try setting up multiple switch ports within the QuickLoop and spread the tape load across those ports. This should dramatically increase performance. Other things to consider when doing SAN based backup is the "feed speed" of the disks to the tape. The whole idea is to provide enough of a data stream to keep the tape moving across the tape heads without the "shoe shine" effect. When a data stream is interrupted, the tape needs to reposition itself on the heads to continue. This causes a "stop start" or "shoeshine" motion on the tape device that slows down performance. The feed speed is the maximum throughput of reading data off the disks, and sent to the tapes. Using "serverless" backup can help here because data moves directly from disk to tape via the "e-copy" extended copy command.

Check with your tape vendor for best practices in performance tuning with their solution.

For more information on QuickLoop capabilities, check out Brocade's Web site.

Your practice of using a separate TAN with a separate adapter in the host is a good one. This will provide non-impact backup during the day.

Chris

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This was first published in June 2002

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