Q: I need to add switches to an existing multi-switch Fibre Channel fabric.
Should I stick with 1Gbps switches or mix in 2Gbps switches?
The theoretical answer to the question is yes: You should move forward with 2Gbps. Multi-switch fabrics are engineered with the notion that users will able to grow their storage area networks (SANs) over time and that they will be able to do so without ripping out entire fabrics as new switch models are introduced and old ones discontinued. In other words, new switch models are designed to co- exist with older models.
However, there are other things that you should consider before integrating any new switch technology into your existing fabric. Will the new switches interoperate flawlessly with your old switches? Will adding new switches unnecessarily complicate an otherwise simple fabric? Also, what are the cost benefits of integrating such technology? These are some of the questions you should ask.
Interoperability is a real concern with Fibre Channel switches. While the major fabric switch vendors have done a good job of ensuring compatibility among switches of different vintages, new models often mean new compatibility problems. Even with extensive quality assurance measures, unanticipated compatibility problems can arise.
My advice is to take compatibility guides seriously. First, make sure that your existing products are certified with 2Gbps, down to the firmware level. Second, be sure that your supplier supports hybrids of the switch models you wish to mix. If you purchased your switches from an OEM that private labels another manufacturer's switch, you need to make sure that your intended configuration is supported. Private labelers are often on different firmware revisions than the actual manufacturer and they may not have the support resources in place for anything other than simple fabrics.
As for complexity, newer switches should not add significant management challenges. The fabric should continue to operate as it always has. In fact, features unique to the 2Gbps switch may not even express themselves when intermixed with 1Gbps technologies. This is an important point: You may not be able to take advantage of new features or additional bandwidth when you mix 1Gbps and 2Gbps switch technologies.
Clearly, whether you see the benefits of 2Gbps technology will depend on the design and the position of the new switches within your fabric. When 2Gbps ports are connected to 1Gbps ports, they drop down to 1Gbps to ensure compatibility. Therefore, if you connect your 1Gbps host bus adapters (HBAs) and 1Gbps storage devices to a 2Gbps switch, you will see no performance benefit. Similarly, if you up-link a 2Gbps switch to a 1Gbps switch, the throughput of the inter-switch link (ISL) will only be 1Gbps.
A common mistake is to connect 2Gbps HBAs to a 2Gbps switch at the edge of the SAN. Unless you have 2Gbps connections all the way to the ports on the storage devices, you will see no performance improvement. Any 1Gbps port in the data path will drop the entire path to 1Gbps.
However, you can boost SAN performance with 2Gbps switches if:
1. Your fabric is designed to fan out connectivity to storage devices equipped with 2Gbps ports. By installing a 2Gbps switch at the edge of the fabric where the storage device connects, you will be able to take advantage of the storage device's bandwidth.
2. Your fabric is designed around a set of core switches. By moving 1Gbps switches from the core to the edge and inserting 2Gbps switches in the core, you could greatly improve total fabric throughput. This configuration allows you to realize tremendous benefits from 2Gbps switching, even if you only have 1Gbps hosts and storage devices. (Note: Most 2Gbps switches also feature ISL trunking, which allows even greater throughput between the switches in the core.)
3. You are able to connect 2Gbps hosts and 2Gbps storage devices into the new switches. If you have 2Gbps storage devices and 2Gbps HBAs, you can achieve 2Gbps performance by connecting them all to the same 2Gbps switch or to multiple 2Gbps switches that are up-linked together.
In short, there is no right or wrong answer to the question of whether you should or shouldn't mix 1Gbps and 2Gbps switches. My advice is that if you do not anticipate significant SAN growth, play it safe and stick with 1Gbps. If you do anticipate growth, move to 2Gbps switches, but make sure the technology is certified with your existing hardware and software. You can probably benefit from 2Gbps today-but at the very least you will be investing in higher bandwidth for the future.
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Jacob Farmer is the CTO of Cambridge Computer Services, a storage technology integrator and training provider based in Boston, MA. His team is currently writing a book on SAN and NAS technologies to be published in the spring/summer of 2002.