HBAs: Why some are better than others


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The host bus adapter (HBA) is the Rodney Dangerfield of the storage area network (SAN). It gets no respect. An often unconsidered part in the deployment and adoption of enterprise SANs is the ubiquitous Fibre Channel (FC) HBA found in servers and storage arrays alike. But buyers beware: All HBAs aren't alike, and they are starting to incorporate some pretty sophisticated technology and features.

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SAN connectivity
While SAN deployments today are frequently associated with fiber optic, the Fibre Channel (FC) specifications don't require fiber optic cabling. All HBA vendors support and provide copper connector kits that generally cost up to $200 less than their fiber optic HBA counterparts.

While copper connectivity may seem like a small detail, it permits an organization that lacks the finances or personnel with fiber cabling expertise to enter the SAN space. While copper is a robust solution, a copper deployment does have its limitations. First, copper has a 100MB/s limitation and there doesn't appear to be a current upgrade path for this technology. Second, it can only cover a maximum distance of around 100 ft., so it won't provide the same flexibility in distance that comes with a Fibre solution.

With the increased emphasis in today's environment to do more with less, HBAs provide a logical, cost-effective place to start. Every device on a SAN--absent the switch--requires an HBA. With most HBAs now sporting a retail price between $1,000 and $3,000, the value one vendor's HBA may offer to your organization over another may well justify the additional investment or savings, depending on what functionality you may need or want. But before we get into how they differ, and what features HBA vendors plan for the future, it's important to list what all HBAs have in common.

Most HBA buyers know they offer either 1Gb or 2Gb speeds (see "Decoding conventions"). They know that the HBAs have to work with the existing bus architecture on their servers and have to support some common FC protocols. Additionally, they realize that HBAs use an unconventional naming scheme, have an onboard I/O processor and understand how HBAs physically connect to the SAN.

Bus speeds
Another commonly understood design fact relates to the bus architecture of the server the HBA fits into, which is a prerequisite to buying the HBA because it has to be either SBUS, PCI or PCI-X. But what are you truly getting--or not getting--when you buy one of these architectures and what are the advantages of each?

The SBUS architecture was designed by Sun Microsystems, Inc. in 1989 and was the standard I/O interconnect for Sun computers for nearly the next decade. Beginning in July 1997, Sun moved to the more commonly accepted PCI bus starting with its UltraSPARC computers. SBUS HBAs now primarily exist to support the many older Sun servers and their underlying SBUS architectures.

Currently, three vendors--Emulex, Costa Mesa, CA, JNI in San Diego, CA, and QLogic, Aliso Viejo, CA--support this technology, and none of the vendors contacted appear to have any plans for further development of this technology. In fact, the underlying SBUS architecture won't support the higher throughput inherently offered in the 2Gb FC protocol due to the architectural design limitations in SBUS, therefore additional development is pointless.

This contrasts with the current PCI architecture. While initially designed as a 32-bit 33MHz PCI bus, many cards now offer 64-bit 66MHz speed. The capabilities of this bus--even with the higher bit count and speeds--began to be stretched by the late 1990s by the increasingly high bandwidth needs of many of today's applications. While still prevalent, the PCI bus has rapidly been giving way to the newer PCI-X technology.

The PCI-X bus technology currently addresses many of the needs of today's high bandwidth, high performance environments. It's a 64-bit solution that can perform at up to 133MHz/s with effective throughput of over 1Gb/s on the server bus. As a result, it delivers nearly a 10-fold performance increase over the standard PCI bus. All HBA vendors included in this article offer a product that supports this architecture and should be backwards compatible with the older PCI interfaces.

Yet QLogic's vice president of marketing, Frank Berry, cautions potential buyers of PCI-X technology that this backwards compatibility with PCI isn't necessarily a given. He says there are two voltage levels which the PCI-X architectures operates at. If the voltage at which the PCI-X HBA differs from that of the PCI bus architecture on the server, the PCI-X HBA won't work.

A feature you'll find on every HBA and what gives an HBA its intrinsic value is its onboard I/O processor which offloads the block-level storage I/O from the server's CPU onto the HBA. This is the primary value of an HBA because block-level storage I/O activity is extremely CPU-intensive.

LSI Logic, Milpitas, CA, believes its adapters have an edge in this space, by providing interrupt coalescing, which can be tuned per system and application environment. By offloading the server I/O processing to the HBA, it permits almost any servers--regardless of its CPU capabilities--to join the SAN. This CPU offloading differs from network interface cards (NIC) which primarily rely on the server's CPU for processing the network traffic. While NIC manufacturers are currently working on what is called a TCP/IP Offload Engine (TOE) to simulate this functionality in HBAs, HBAs currently provide the most proven method for deploying this CPU offload technology.

This was first published in December 2002

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