PCI-based RAID controller advancements

Posted on December 01, 1999

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PCI-based RAID controller advancements

Bob Mohs

There`s been so much press and vendor news surrounding storage area network (SAN) architectures, products, and strategies, it`s easy to lose track of host-based RAID, still the most dominant method of storage attachment. SAN technology is exciting, but the publicity has overshadowed progress in PCI-based RAID.

Full-featured RAID controllers are available to meet the price/performance requirements of all users, from entry level to enterprise-class. In fact, cost reduction has made intelligent hardware RAID available in low-cost Intel architecture servers. This has fueled a move away from software RAID and non-RAID (JBOD) applications.

Dataquest, a market research firm in San Jose, estimates that by 2003, the cost of single-channel, entry-level PCI-based RAID controllers will have declined 41%. In contrast, the price of external RAID controllers is expected to remain about the same over that period.

Dataquest`s RAID forecast for Intel architecture servers shows a 357% increase in the number of installed host PCI-based RAID controllers from 1998 to 2003. In addition, the host PCI-based RAID market share will nearly double over the same period, with the number of servers with PCI RAID controllers expected to increase from 33% to 61%.

A key contributing factor is the growth of the server market itself over the next several years. Dataquest forecasts IA server unit sales will increase 143%, from about 2.5 million units in 1998 to more than 6 million units in 2003.

Because of the performance advantage, many low-cost server vendors have switched from software RAID or JBOD to host-based intelligent hardware RAID controllers. Intelligent controllers take advantage of embedded CPUs that are dedicated to off-loading interrupt requests and I/O processing functions from the host CPU, freeing the host to process user applications and requests.

Intelligent controllers can significantly increase data throughput and overall system responsiveness. Software RAID, like any other application, competes for host CPU cycles and system bus bandwidth, and can degrade overall server performance. But with PCI-based hardware RAID, all array functions— including resource-hungry XOR calculations—are processed by dedicated RAID hardware that`s embedded in the controller, off-loading the functions from the host.

The dominant host-based RAID controllers today include:

•Ultra2 SCSI RAID, available with one or multiple channels, which allows a maximum data transfer rate of 80MBps per channel.

•Fibre Channel RAID, available with one or two loops at 100MBps per loop.

These bandwidth ceilings are more than adequate, given the seek time and average rotational latency of today`s fastest Ultra2 SCSI disk drives. In 2000, host-based controllers supporting Ultra160 SCSI will become dominant, doubling the maximum channel data transfer rate to 160MBps. And the next generation of Fibre Channel controllers will allow up to 200MBps per loop.

Both SCSI and Fibre Channel host-based RAID controllers have the connectivity capabilities to configure large storage systems. Ultra2 and Ultra160 SCSI`s low-voltage differential (LVD) technology allows system designers to use a four-channel RAID controller to configure, for example, 60-drive arrays without reliability constraints, yielding both capacity and performance dividends.

Compatible Ultra2 and Ultra160 SCSI and Fibre Channel RAID controllers are also available to complement server vendors` introduction of 64-bit PCI motherboards. Backward compatible with 32-bit motherboards, 64-bit 66MHz RAID controllers can take advantage of the higher 528 MBps host-to-controller PCI burst rate.

Host PCI-based RAID controllers also offer the flexibility to customize servers to match user requirements. For example, you can simultaneously connect storage devices internal and external to the server, or group hard drives into separate logical arrays operating under different RAID levels. Many controllers also allow you to simultaneously connect both SCSI and Fibre Channel devices.

With e-commerce, transaction processing, video streaming, and multi-user applications requiring zero downtime, server clustering is growing in popularity. Many PCI-based RAID controllers now feature multi-initiator support, which is a prerequisite in clustered environments.

SCSI bus redundancy is achieved by striping RAID-5 sets across a controller`s multiple channels. Fibre Channel bus redundancy is achieved using dual loops and dual-ported disk drives with any RAID level or JBOD. Controller support for SAF-TE and SES cabinet status monitoring protocols give users notification of cabinet-related conditions critical to uninterrupted system operation.

Adding to the appeal of host-based RAID controllers are a variety of user interfaces, including Web and workstation-based graphical user interfaces that enable remote access to storage subsystems and storage management tools. BIOS-based utilities allow quick configuration and status checks exclusive of the host operating system; command line interface utilities give system administrators control with scripting capability.

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Bob Mohs is technical marketing manager at Distributed Processing Technology (DPT), in Maitland, FL (www.dpt.com).


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