Hands-On Review: Celerity FC-24XL


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Fibre Channel networking model

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This diagram illustrates the five layers of the Fibre Channel networking model as defined by the industry standard. The high-performance design of the Celerity FC-24XL adheres to the model standards.

Built for performance
The Celerity FC-24XL boosts the performance of these layers with its Intel XScale processor. The design approach is similar to the TCP offload engines available on iSCSI storage adapters. Although much of the FC protocol is implemented in hardware and is immune to the overhead associated with processing IP headers, the additional features (port-to-port streaming and hardware RAID) offered by the FC-24XL benefit by having an integrated processor.

The Celerity FC-24XL also comes standard with 128MB of SDRAM (upgradeable to 512MB) for direct memory transfers from the storage device without involving the application server's CPUs. This may appear to be overkill, but for applications that are sensitive to CPU interrupts, this isn't excessive.

When plugged into a PCI-X bus slot operating at 64 bits and 133MHz, Atto says the FC-24XL is capable of sustaining 1GB/sec transfer rates between its attached devices and the CPU. Overprovisioning the bus with four 2Gb/sec (4Gb/sec full-duplex) ports is similar to how you would provision initiators over ISLs. Considering the likelihood of concurrent full-duplex communication across all four ports, overprovisioning this resource makes sense.

The Celerity FC-24XL has eight 512-byte buffer credits for delivery into the fabric. This may not seem like much, but because the HBA operates over the Class 3 connectionless service and the maximum payload of a FC frame is 2048 bytes, the initiator needs only to receive an R_RDY frame from the communicating device to send the next data frame. In addition, the HBA is outfitted with multimode lasers. So the laser's light propagation distance is limited to 300 meters, and the R_RDY frame received by the HBA after the first frame is sent will arrive before the HBA is done sending the second frame.

Both initiator and target mode is supported on the FC-24XL. When configured for initiator mode, the HBA sends frames into the fabric. When configured for target mode, the HBA receives frames from the fabric and could, for example, support a storage virtualization server.

The FC-24XL also supports SNIA's HBA API, a common API developed by the standards organization. This is intended to improve the manageability of HBAs in a SAN without regard to proprietary-vendor APIs and is an essential part of the SMI-S. Product offerings from vendors complying with SMI-S can be managed using other vendors' software, thus avoiding vendor lock-in. Conformity to this specification should be considered mandatory when considering SAN hardware components.

The Celerity FC-24XL comes with two transceivers; two additional transceivers may be purchased separately. The retail price is approximately $2,600, which puts it out of reach for application servers that have simple access and throughput requirements. It's appropriate for data-hungry applications in film production, high-end engineering applications or even general-purpose servers running storage virtualization target software.

Although priced moderately higher than other four-port FC HBA offerings, such as LSI Logic Corp.'s LSI7402XP-LC and QLogic Corp.'s QLA2344, the Celerity FC-24XL sports upwards of 512MB of onboard SDRAM memory for media-rich applications that can take advantage of direct memory access.

The FC-24XL is a high-end product designed for the most data-driven applications. The density and speed of its ports provides unbridled access to storage content, especially large media-rich files. In addition, storage virtualization and disk-to-disk backups are also potential applications that can make use of the number and speed of the ports. Its adherence to FC standards at every level of the reference model should help allay compatibility concerns.

The performance characteristics of the four ports when combined with the Intel processor, multiple dual-channel SERDES circuits and up to 512MB of DMA memory makes the Celerity FC-24XL a must-have for applications plowing through terabytes of data daily, such as audio-visual content servers. You'll pay a premium for the Celerity, but if you're running demanding applications, you're likely to find that it's worth the price.

This was first published in July 2004

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