The Software RAID Revolution Has Begun! - Technology Information

Computer Technology Review, April, 2000 by Bret Cox

In the minds of many people today, the term "software RAID" immediately conjures up images of a low-end, inferior solution to a problem that's already been solved. This is unfortunate, though not terribly surprising. Built some twenty years ago, the very first RAIDs employed a software or "host-based" approach (wherein RAID tasks are handled by the host CPU), but this scheme was quickly abandoned due to insufficient host memory, slow bus, and communication architectures and the fact that host CPUs simply weren't fast enough to simultaneously process RAID tasks and application tasks. Hardware-based RAID (wherein RAID tasks are offloaded to firmware residing on a dedicated external controller) was the obvious solution to this dilemma and it has remained entrenched as the dominant RAID paradigm ever since.

The irony is that in today's world of super-fast CPUs, bus architectures, and ever-falling memory prices, this dominance makes virtually no sense. Today's CPUs are literally thousands of times faster than those of 15-20 years ago; memory is cheap and abundant; new, fast communication protocols like Fibre Channel are maturing and by today's standards, the processing overhead incurred by RAID tasks is negligible. Examined in this light, hardware-based RAID becomes redundant and altogether unnecessary. Not only is host-based RAID cheaper, faster, more flexible, and easier to implement/maintain, it is the only paradigm able to fully leverage newer technologies like Fibre Channel and Storage Area Networks (SANs)--technologies which imply an openness and connectivity that even the best hardware RAID solutions cannot provide. Once the facts and figures are considered, it becomes obvious that high-performance, host-based RAID is the best and most logical technology to usher in the new age of enterprise-level storage systems.

Host-Based RAID: Not What It Used To Be

Due to its initial shortcomings and the ensuing dominance of hardware RAID in the marketplace, host-based RAID has long been viewed as an inferior, low-end solution, unsuitable for enterprise-level storage demands. While this stereo type may have held some truth in the past, it is simply not valid today. Unlike the feeble SCSI-based RAID software of yesteryear, the new generation of software RAID applications are being specifically designed and developed to take advantage of Fibre Channel and today's multi-processor 64-bit computing architectures. Accordingly, they offer a level of performance unrivaled by even the most elaborate hardware RAID implementations. Key advantages include:

* Cost and Longevity - On average, hardware-based RAID solutions cost four to seven times as much as host-based systems of similar capability. The reason for this huge difference is simple: RAID controllers are expensive pieces of hardware. They are just as vulnerable to depreciation, obsolescence and failure as all other computing devices because they are pieces of hardware. As the march of technology gradually renders them obsolete, old RAID controllers (those purchased more than, say, three years ago) are either kept stubbornly in place or they are thrown away and replaced. Neither choice is cost-effective. In stark contrast software RAID systems can never be rendered obsolete by advances in hardware technology; in fact, the performance of software systems only increases as the underlying hardware improves.

* Speed - In a typical hardware RAID configuration, each controller is assigned to a finite array of disk drives and is responsible for handling all I/O traffic between those drives and the host machine. These controllers use intermediate memory for caching purposes, which means that data going to or from the disks has to be copied twice--once from the source device to the controller's memory and again from the controller's memory to the target device. Host based RAID applications interface directly with the disks and, thus, enjoy a significant built-in speed advantage (See Fig).

If the RAID software is optimized for Fibre Channel, this advantage becomes even more pronounced. While a single fibre-connected, hardware-controlled RAID subsystem is limited to a maximum transfer rate of 100Mbps (200Mbps if using the latest 2Gbps fibre), host-based RAID software is not handicapped by such limitations. The number of disk drives and the number of Host Bus Adapters (HBAs) that are plugged into the host limit bandwidth. For a single high-performance server like a Silicon Graphics Origin2000 (which can accommodate dozens of HBAs), this can equate to sustained transfer rates of gigabytes per second and tens of thousands of I/Os per second. If one continues adding hosts, HBAs, and disk drives into the mix, these performance figures could be increased to a theoretically infinite value. Traditional hardware RAID systems cannot--and will not--ever approach the speed or throughput of an optimized host-based RAID system.

* Dynamic Growth/Shrinkage of the File System - Individual disks can't easily be added/removed as storage needs change because each, controller in a hardware RAID system is "assigned" to a finite array of disks. Expanding most hardware RAID systems today usually involves taking the system offline, backing the RAID's data onto tape, adding the new drives to the volume's configuration files, then copying the RAID's data back again. With the cutting-edge software RAID applications that are now coming to market, that same process can now be accomplished on the fly, without ever shutting down the system or interrupting a single byte of traffic. The steps would literally consist of: 1) Put additional drives in place, 2) Click the Add button on the software's interface. If available drives already existed some where else on the Fibre Channel network (and the host can "see" and target those drives through a switch), the process would be even easier.