SCSI Failing to Drive Out Satan

Tuesday Sep 16th 2003 by Drew Robb
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Is Serial ATA the work of the devil? Some SCSI storage vendors would have you think so. However, succumbing to this temptation can lead to a slice of heaven in your datacenter.

If SATA is short for Serial ATA, and SAN is the acronym for a Storage Area Network, does it follow that a SATA Network is a 'SATAN'? Certainly high-end SCSI-based storage hardware vendors can be forgiven for labeling SATA as the work of the devil.

As recently as January 2002, however, with a second roll-out forecast delayed, it looked like SATA would end up as yet another technology promise without support or substance. Since then, though, the adoption of SATA standards has been described by some as unprecedented. SATA has rapidly established itself as a serious rival to parallel technology in enterprise storage environments.

"More and more we are seeing that ATA is taking over from SCSI," says John McArthur, Group Vice President Storage Research at IDC. "We are seeing greater usage of slower ATA and SATA drives for various uses due to its low cost and high capacity. SCSI on the other hand tends to be faster and more reliable, but more expensive."

So what exactly is this disruptive technology? Serial ATA, short for Serial Advanced Technology Attachment, is a standard for connecting disk drives and systems. SATA is based on serial signaling technology, whereas traditional ATA (also called Parallel ATA or PATA) uses parallel signaling.

SATA has several practical advantages over parallel signaling. SATA cables are thinner and more flexible than the ribbon cables required for conventional PATA drives, which results in easier installation and a reduction in space required for SATA-based hardware. SATA cables can also be considerably longer than PATA ribbon cables, another factor that enables greater leeway in the physical arrangement of the storage network.

Because there are fewer conductors in serial signaling technology, crosstalk and electromagnetic interference (EMI) are less liable to cause problems. The signal voltage is much lower as well (250 mV for SATA as compared with 5 V for PATA). Together these technical advantages ensure far greater efficiency in integration for SATA over PATA.

Page 2: A Rapid Ride to Acceptance

A Rapid Ride to Acceptance

The reasons for SATA's rapid progress are simple — SATA technology is improving in performance and capacity while continuing to benefit from a reduced cost. This has created an enticing sweet spot of price/performance that is particularly attractive in delivering storage solutions for non-mission-critical data.

SATA applications are wide ranging as well. The technology can be used, for example, as a near-term repository for data that will eventually be archived to tape. Additionally, SATA can be used in snapshot repositories, in remote volume mirroring destinations, or for electronic vaulting. SATA is well suited to tiered storage uses as well, especially for applications that require storage with varying performance, availability, and cost characteristics.

SATA is also gaining acceptance in low cost, entry-level SANs targeted to small and medium-sized businesses (SMBs), especially when coupled with iSCSI. According to some analysts, SATA storage solutions could cut costs by as much as 60%.

Even without this cost benefit, from a technology viewpoint there are compelling drivers for utilizing SATA. As well as bandwidth and flexibility demands putting increasing pressure on parallel systems, there are inherent problems with traditional SCSI and ATA, including incompatible cables and connectors, different software, simple physical space problems with bulky SCSI cables, and restrictions on the lengths of cables due to the need for eliminating the possibility of signal errors. In solving these issues, SATA improves integration efficiency and creates long-term scalability and cost benefits that are vital for forward progress.

Where SCSI has excelled to date has been in speed and reliability. ATA and SATA drives typically operate at speeds of 5,000 to 10,000 rpm (revolutions per minute), whereas SCSI generally operates around the 15,000 rpm mark. And MTBF (Mean Time Between Failure) for ATA/SATA desktop drives has usually been pegged in the range of a few hundred thousand hours, while SCSI drives generally are rated at well over a million hours. The tradeoff, of course, is the higher cost.

Perhaps the most interesting development, though, is the melding together of the best of both worlds through the integration of SATA and SCSI via Serial Attached SCSI (SAS). This next-generation evolution of SCSI leverages proven technology while enabling integration with SATA and all of its inherent benefits.

One of the crucial features is the enablement of one of more SAS host controllers to connect to a large number of drives. Using an expander, a controller can connect to other host connections and expanders. This architecture enables massive storage network topologies, as well as the balance of lower-cost/lower-performance SATA drives where they are appropriate with higher-cost/more reliable SCSI devices in areas where they are needed.

According to IDC's McArthur, "SATA and SAS is a marriage made in heaven."

This story originally appeared on Enterprise IT Planet.

» See All Articles by Drew Robb

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