RAID cost/performance trade-offs

Posted on July 01, 2000


Buyers face a plethora of choices in interfaces, with Fibre Channel making inroads on the front end of high-performance RAID arrays and SCSI continuing to dominate at the drive level.

By John Haystead

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Selecting the best approach to RAID-based storage has always required a fair amount of application and requirements analysis, but today's end users and systems integrators are faced with a steadily increasing number of technology choices. Fibre Channel is slowly taking a larger share of the high-end market from SCSI, while less complex and less expensive host-based RAID systems are becoming increasingly attractive to low-end and mid-range users. And, while bus technologies continue to improve in speed, distance, capacity, and capabilities, drive prices continue to drop steadily.

FC, SCSI trade-offs

Not long ago, many industry analysts were predicting a future where any discussion of RAID would have implied Fibre Channel. This is certainly not the case today, however, and few observers expect such a scenario to ever develop.

Nevertheless, Fibre Channel is steadily seeding the RAID marketplace, settling into the predictable position of competing with SCSI and other interfaces on a case-by-case basis into the foreseeable future.

Currently, Ultra160 SCSI has a leg up in theoretical maximum transfer rate at 160MBps, but Fibre Channel will hit 200MBps later this year. However, at the disk drive level, transfer rate specs are largely a non-issue for most users.

Andreas Koepf, director of support at ICP vortex, points out that these specifications refer strictly to theoretical maximum throughput. "You'll never see 160MBps sustained data throughput on SCSI because of the command overhead," Koepf explains. He notes that to maintain compatibility with older devices, Ultra160 SCSI commands are still sent in asynchronous mode. "Command sets take a lot of transmission time compared to the data, so doubling the speed of data transmission doesn't double the total throughput speed."

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Sustained transfer rates depend on a number of factors, not the least of which is the application mix. So, what can you expect in actual transfer rates? "Realistically, you can get about 140MBps with Ultra160 SCSI drives," says Bob Morris, vice president of marketing at Chaparral Network Storage. "With Fibre Channel drives, it typically ranges between 80MBps and 90MBps today." Chaparral's controllers use Fibre Channel on the front end and Ultra160 SCSI drives on the back end.

At the disk drive level, Fibre Channel pricing is approaching parity with SCSI drive pricing. "In large OEM deals, there's no price premium for Fibre Channel drives," according to Morris, "but further up the channel they're paying about a 10% price premium for Fibre Channel drives." Morris says that, at the controller level, Fibre Channel has about a 20% price premium, but he expects that price differential to disappear over the next few months.

Aside from speed specs and cost issues, Fibre Channel does offer significant RAID advantages in areas other than drive-level interfaces. As pointed out by Roger Klein, CMD's vice president of marketing, "There's no question that Fibre Channel offers significant benefits over SCSI in SANs [storage area networks]. It removes many of the physical constraints of SCSI, such as distance and addressing. With Fibre Channel, you can build remote, scaleable pools of RAID storage rather than the more traditional direct-attached subsystems, which is a significant benefit."

As many as 126 drives can be hosted on a single Fibre Channel loop, compared to 15 devices per channel with SCSI. And Fibre Channel also allows for dual-loop/dual-port cable configurations, providing redundant connectivity. Although SCSI integrators can add more devices via additional or multi-channel SCSI controllers, both of these approaches add complexity and cost to RAID systems.

"The main advantage of a Fibre Channel drive array is the number of drives you can attach," says Chaparral's Morris. He points out that you can attach up to 126 drives on a Fibre Channel loop, although 64 drives per controller is a more practical figure. Of course, drive count translates into capacity-per-box advantages.

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Reliability and cabling simplicity are other areas where Fibre Channel offers advantages over SCSI. According to ICP vortex's Koepf, "Many of the failures in SCSI RAID arrays are really cable failures. With SCSI, there are far more wires, terminations, connectors, etc, and if only one connector goes bad you can lose the entire bus."

Koepf also points out that, unlike SCSI, Fibre Channel is protocol independent. This is an important consideration, particularly for clustered servers, LANs, and SAN environments. "In these architectures, you're not only transmitting protocols such as SCSI, but others as well, such as IP, and Fibre Channel allows you to do multiple protocols on one cable."

Fibre up front

One place where Fibre Channel is taking hold over SCSI is in the front end (host/network connection) of high-performance RAID systems. As pointed out by CMD's Klein, "The front end is where you get the most benefit from Fibre Channel technology," and as a result, most RAID arrays are available with Fibre Channel host/network connections.

Pat McGarrah, director of strategic and technical marketing at Quantum, agrees. McGarrah partitions the RAID market into three broad segments and, "at the high-end, where data is located some distance away from the systems, Fibre Channel is clearly the best way to go."

However, the transition to Fibre Channel disk drives is moving at a slower-than-expected pace. CMD, for example, still sells a substantial number of SCSI interface subsystems and Klein expects this to continue. "SCSI will continue to have an important place in the market, particularly where price/performance is the primary determinant. Not everyone needs the attributes of Fibre Channel, such as longer distance capabilities."

Walt Hinton, StorageTek's general manager of storage networking, says that although "theoretically you can build larger configurations of drives on Fibre Channel, which should reduce the number of controllers and hence costs, right now SCSI backends are frequently less expensive. However, as array sizes become larger, it's harder to do cost-effective SCSI implementations. For smaller arrays, SCSI still offers very good price/performance."

Xiotech's Magnitude disk array currently implements Fibre Channel up front and SCSI on the back-end. Although Dick Blaschke, executive vice president of marketing, says that Xiotech expects to offer a Fibre Channel back-end with their next-generation system, he points out that familiar issues continue to haunt that approach. "There are still questions about the availability of Fibre Channel drives, their cost, and their real added value to users. Because of Fibre Channel's dual porting, it definitely offers some intrinsic data-availability benefits on the back-end of RAID systems, but whether people need these, or will use them, is debatable."

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Although Quantum's McGarrah agrees that in transaction-processing environments, the number of drives that can be connected per Fibre Channel controller vs. SCSI is an advantage, he points out that this isn't always true for other application environments. Noting that video is still one of the main applications for Fibre Channel today, McGarrah says "in these high-throughput streaming applications, you can't add more drives to either SCSI or Fibre Channel, because just a few drives will saturate the bus." McGarrah adds that "people rarely put more than four to five drives behind a controller anyway."

Mid-range RAID

SCSI and Fibre Channel are drawing battle lines in the mid-range RAID market. "Cost/performance becomes a greater factor in the mid-range market, and the use of Fibre Channel drives and controllers adds additional costs to any RAID system," says McGarrah. "For example, you have to have bypass circuits to do hot plugging, and it's also harder to do a Fibre Channel drive backplane, because you have maximum clock speeds in the GHz range vs. an 80MHz clock for SCSI."

Another back-end cost issue for Fibre Channel in the mid-range market is the lack of backward drive compatibility. Balancing the performance impact of SCSI's asynchronous command set, McGarrah points out that because Fibre Channel drives retime all signals, the slowest drive on the bus will regulate the speed of all other drives. "As a multi-drop bus, this is not the case with SCSI, and old drives won't impact the performance of newer units," says McGarrah. "This can be an issue in the mid-range RAID market where users generally want to start with a few drives and add more capacity in the future."

In the mid-range RAID market, Fibre Channel's dominance at the front end is not uncontested. ICP vortex manufactures host-based RAID controllers for both PCI-to-SCSI and PCI-to-Fibre Channel, and although Koepf says they had expected the Fibre Channel line to take off rapidly when it was launched in early 1998, in fact, their SCSI products have remained the majority seller by far. "There's still a large void of knowledge relative to the advantages of Fibre Channel, and many customers simply feel more comfortable with SCSI," says Koepf.

Putting 'inexpensive' back into RAID

The low-end segment of the RAID market is clearly the most dynamic in terms of alternative solutions, with interface options colliding with intense price/performance pressure. New products delivering RAID redundancy using inexpensive IDE/ATA drives are setting new price points in the market. "You don't get the same performance as SCSI drives, but ATA drives will still have a niche in the market," says Quantum's McGarrah. "You can use 7,200rpm or even 4,400rpm drives and still get adequate performance and reliability."

Although 3Ware Inc. is pursuing Fibre Channel and SCSI product development, Ray Cosyn, senior director of product marketing, says the company is also focusing on providing SCSI performance using IDE/Ultra-ATA drives. "Our goal is to put 'inexpensive' back into RAID," says Cosyn. (RAID originally stood for Redundant Array of Inexpensive Drives, although 'Inexpensive' was later replaced by 'Independent.')

3Ware's strategy essentially applies packet-switching technology to a RAID controller, enabling up to eight IDE drives with a direct path to each drive. "By eliminating the arbitration associated with the SCSI bus, we achieve higher performance and roughly a 2:1 price advantage over SCSI per drive," Cosyn claims.

3Ware's DiskSwitch packet-switching architecture connects a dedicated AccelerATA data channel to each drive, with all channels in turn connected into a high-speed packet-switching controller to transport data from individual drives to the host. The company's Escalade RAID controllers support 2-, 4-, and 8-port configurations.

Cosyn sees future market opportunities for high-bandwidth performance with data protection using RAID 10. "When you move to lower-cost drives and employ packet-switching technology, you get tremendous bandwidth while still being able to protect the data through RAID 10 parity. In applications with a very low hit ratio out of cache, where you have to repeatedly go to the drives, RAID 10 is optimum."

Meanwhile, Ultra-DMA (UDMA) drives are also being proposed for low-end RAID arrays. Fujitsu, for example, offers a 6.4GB UDMA hard drive for around $100.

ICP vortex's Koepf says there's a potential low-end market for UDMA RAID, but it has the same limitations as ATA-based RAID. "One problem with UDMA or IDE drives is that there's still no multi-I/O capabilities. The drives are single-I/O devices, and with RAID it's very important to overlap commands as much as possible."

RAID on a chip

The emergence of motherboard-hosted, single-chip RAID is indicative of how broad the RAID marketplace has become. "Chip-level solutions are just a progression of what has been offered in software RAID, without the CPU penalty, and this will be attractive to many users," says CMD's Klein.

3Ware, for example, is looking at shrinking its controller board to a chip-level product. "We see opportunities for workgroups that want to add additional storage to a workstation, or where a small server is supporting the workgroup," says Cosyn.

However, ICP vortex's Koepf points out that not every desktop system needs a RAID controller, and "the cost of implementing additional speed and redundancy on the motherboard would make them prohibitively expensive. I have my doubts whether this really is the future."

Back to JBOD

Given the high cost of some RAID controllers and the continually dropping cost of drives, some implementers are beginning to question whether they really need hardware-based RAID at all. According to StorageTek's Hinton, most 24x7 enterprise-class users want mirrored arrays to ensure data availability, and with the relatively low cost of drives, perhaps this could be more cost-effectively implemented via a switched fabric; for example, each write operation is multi-pathed to three drives (two local and one remote). "As you scale up into terabyte configurations, and with the arrival of 72GB drives, it's not clear that hardware RAID will always be required. With all the intelligence already being put into networks located between servers and storage, there's no reason why an intelligent storage switch couldn't do multi-pathing to multiple JBODs [Just a Bunch of Disks]."

Hinton observes that, because of the high cost of controllers in high-end configurations, "the cost of storage may exceed the cost of the server." In addition, he points out that although RAID-related performance degradation may be insignificant for individual users, "when you're in a large database environment dealing with hundreds of thousands of I/Os per second, the impact can be big."

Quantum's McGarrah says the real question is whether software RAID can deliver the same performance as hardware RAID. "In mirroring and striping the answer is 'yes,' but it's not clear whether this is true in RAID 5."

At the same time that RAID systems are viewed as mature and proven technology, the market is enjoying a resurgence of expansion and innovation. At least at this point, Fibre Channel is the performance leader on the front-end, but that may not always be the case as implementers look down the road to other approaches, such as IP and InfiniBand.

In the meantime, competing approaches continue to jockey for old and new price/performance niches, and the industry maintains the quest for new levels of RAID capabilities to open up even broader opportunities.

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