Hands-On Review: Celerity FC-24XL


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Atto Technology Celerity FC-24XL

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The Celerity FC-24XL is a high-performance host bus adapter suitable for applications that use large files and require high throughput.

Usability and interface
To better understand Atto's Celerity FC-24XL, we'll map the functionality of the HBA to the five layers (FC-4 to FC-0) of the FC networking model to see how it stacks up against the FC standards. (See "Fibre Channel networking model")

FC-4. Device drivers operate at the FC-4 layer by packaging information units (e.g., SCSI command/data/status) to be transported down to the other layers and out over the glass wire. Because device driver code is often accessed in main memory, it's important that this code is small and efficient. The Mac OS X device driver for the Celerity FC-24XL is just over 200KB, due mainly to the functionality of Mac OS X's I/O developers' kit.

In contrast, the packaged driver for the Windows platform is more than 4,000KB. FC-3. The still-developing FC-3 layer is described as a set of common services that serve multiple ports on the same node. Encryption, the striping of outgoing data across multiple ports, and the definition of multicast and hunt groups can all be done at this layer.

The potential to exploit the functionality of the FC-3 layer with the Celerity FC-24XL is great. For example, a real-time application that needs the greatest bandwidth with the least latency could be configured to access a four-disk LUN through each of the HBA's four ports that are connected to four separate fabric ports that are ultimately connected to four separate storage ports. This is disk striping at its best because the probability of overrunning a fabric or storage port drops to almost zero with the proper buffering at each interconnect.

In addition, using the features of the Alias Server (a well-known service in the fabric), frames destined for the node can now be directed to all four ports (multicast) or the least busy port (hunt group). A storage virtualization product can take the Celerity FC-24XL's four ports and present them as one logical path from a storage server to its client application hosts in the form of a hunt group. When application hosts access their storage over this path, load balancing is automatically achieved by the native functionality of the hunt group.

FC-2. The framing and signaling portion of the standard, FC-2 receives information units from the upper layer protocols and packages them into frames to be sent to the destination port. Depending on the class of service in use, some level of signaling is used to either ensure that the frame arrived, or that there are sufficient buffers for the delivery of the next frame.

The Celerity FC-24XL only supports the Class 3 connectionless service, and thus doesn't guarantee the delivery of frames with receipt acknowledgment. Therefore, upper layer protocols that don't manage delivery errors (e.g., IP/UDP) aren't suitable applications.

FC-1. FC-2 frames are eventually serialized at the FC-1 layer and beamed across the physical interface for delivery to the destination port. Serialization is performed immediately before transmission, and is used to establish a clock frequency between the communicating ports by encoding timing bits into the data stream. It's also used to maintain DC balance (equal number of zeros and ones) on the glass wire. The quality (jitter) of the rise and fall times of the optical signal is a direct reflection of the quality of the serialization-deserialization (SERDES) function on the HBA.

When considering SERDES functionality, focus on the number of integrated circuits per independent channel, power dissipation and the number and speeds of protocols supported by the circuit. This helps define an upgrade path for the HBA, or even uncover the full range of protocols that can be transmitted using the HBA.

The FC-24XL is outfitted with two dual-channel FC controllers to perform serialization functions on behalf of the upper layer protocols. There's one SERDES channel for each port to support the independence of the data stream as it flows to and from its destination.

FC-0. This layer describes the supported variants of the physical interface and their associated speeds. Twisted pair, copper and fiber optic cabling are the supported media over which FC frames can be transported. The different connectors associated with the cabling options are also defined.

The FC-24XL uses four 2Gb/sec, small form-factor pluggable LC-type connectors to achieve maximum port density. Designed for high-throughput local applications, the HBA is outfitted with multimode lasers. Thus, multimode fiber optic cabling--preferably the 50 micron type--must be used to connect the Celerity FC-24XL to the fabric or end device.

This was first published in July 2004

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