Fibre Channel Directors: Myths, Realities, and Evaluations Page 3 -

Fibre Channel Directors: Myths, Realities, and Evaluations Page 3


All I can say about vendors is that there's often a lack of full disclosure of performance information when it comes to their switches. A Google search on a specific vendor, model, and performance will likely provide some interesting results for some vendors.

In the environment I work in, we require 2 Gb full duplex (200 MB/sec read and 200 MB/sec write) from any port to any port. As all director-class switches are composed of blades, we might connect from the host to a blade within the switch and then connect that blade to one RAID for write and to a different blade and another RAID for read. Each blade connection and the blade-to-backplane communication should support full duplex and consistent I/O performance, and this connectivity must work along with the other case of running the blade at full duplex rates.

Our working environment precludes us from affording the requirement of specifically connecting certain RAIDs and hosts to certain blades. This presents a few problems for my work, as we require specific data rates for streams of I/O. These problems include:

  1. Over time you have failures of ports and need to be able to keep the same performance regardless of the port configuration

  2. Requiring specific port configuration adds complexity to operational systems, especially at 3 AM when a port inevitably decides to fail

So what is the performance requirement for your site?

Defining the amount of performance required means you need to understand your application load, the application I/O request sizes, and the hardware involved in your environment. For example, even if you have a 2 Gb mid-range RAID and your applications are writing large block streaming I/O, if you are using the RAID write cache mirror feature, your I/O performance is limited in most cases to about ~140 MB/sec or so depending on the vendor. To really understand your performance requirements you need to look at:

  1. The application I/O requirements and how the application performs I/O. A database, for example, is not likely move data anywhere near the 2 Gb full duplex rates, given that the I/O request sizes are generally small and random for indexes,, and table space access is still not large block streaming I/O

  2. The HBAs need to be high performance 2 Gb and tuned for high performance I/O (see

  3. The RAIDs need to support and be tuned for high performance streaming I/O

So basically, from the performance point of view, unless you have an HPC application and environment, performance is not likely to be an issue for most director-class switches, even though some of them do not run at full rate.

Page 4: Is a Director-class Switch Needed?

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