Small Form Factor Disks Go Mainstream, the System i Has Gone Fishin'

Published: July 9, 2007

by Timothy Prickett Morgan

I am not that old, but I am old enough to remember a time when the AS/400 was always at the cutting-edge of storage technology innovations, whether one was talking about cache memory, main memory, disk drives, or tape drives. Those were the days, when IBM's Rochester labs were at the forefront of memory and disk technology, inventing the most dense memory chips and the most capacious disk drives in the IT sector.

Nothing could make the current System i stand in such stark contrast with its peers in the server space and the attitude of the AS/400 Division of days gone by than the fact that this product line still does not have support for small form factor Serial Attached SCSI (SAS) disk drives. Last year, as these disks entered the market, IBMers I spoke with said that they were not "enterprise grade" yet, but this has clearly changed in 2007 and it is time to bring the benefits of low-power, 2.5-inch SAS drives to the System i platform.

According to a report from the SCSI Trade Association, an industry association comprised of the big players in the SCSI peripherals market that was founded in 1995, SAS disk drive shipments grew exponentially in 2006, and a report from Gartner suggests that SAS drives with the 3 Gb/sec interface will replace the Ultra320 SCSI interface (commonly used on 3.5-inch disks) in 2007 as the dominant disk drive interface type. (The System i currently supports Ultra320 SCSI disks in a 3.5-inch form factor that spin at 10K RPM or 15K RPM and that offer capacities of 35 GB, 70 GB, and 140 GB.) Gartner's research indicates that in 2006, the installed base of SAS disks numbered about 4.5 million units, or about 11 percent of all disks out there in the field, and the company believes that during 2007 another 12 million SAS disks will ship. (Not all SAS disks are in the small form factor, by the way. Some are still using the 3.5-inch form factor.) Gartner expects that the exponential growth for 3 Gb/sec SAS disks will continue through 2008, and in late 2008, Gartner is anticipating that SAS disks with a 6 Gb/sec interface will start hitting the market in volume; shipments will actually begin late this year for this faster SAS interface. Gartner projects that shipments for 6 Gb/sec SAS disks will grow at a compound annual growth rate of 210 percent between 2007 and 2011.

"The server market has successfully transitioned from parallel SCSI, which dominated the market for over 20 years, to 3Gb/s SAS," says Harry Mason, who is president of the SCSI Trade Association and who is also directory of industry marketing for disk array maker LSI Logic. "We anticipated the dramatic growth in SAS shipments witnessed in the fourth quarter of 2006, and expect volumes to significantly increase throughout 2007. STA members are shipping more small form factor (SFF) HDDs to enterprise environments. Emerging SAS designs for blade servers, as well as networked storage solutions, promise to fuel new growth as SAS makes its way deeper into the enterprise. STA believes SFF drives will become the dominant form factor in multi-user applications, due to the concern over power usage costs of 3.5-inch drives."

SAS drives are only just now beginning to run at 15K RPM, which is one of the reasons why some server makers (namely IBM) have been reluctant to ship them inside servers. (X64 servers from Hewlett-Packard, Sun Microsystems, Dell, and others come with small form factor SAS and SATA drives as either standard or optional these days.) For many applications--particularly online transaction processing workloads with random data accesses--having more disk arms and reasonable data access and transfer speeds is more important than the rotational speed of the disks, so this argument seems kind of moot. If you are, like IBM with the System i product line, trying to sell a fewer number of expensive but fast 15K RPM disk drives, it comes as no surprise that IBM is also trying to peddle SCSI disk controllers with big read and write cache memories. But it is a mistake to believe that this is the only way to design a System i disk subsystem.

IBM's old disk drive business, which it sold off to Hitachi two years ago and which has an UltraStar product line that draws its heritage from the excellent disk engineering work done by IBM Rochester two decades ago, has just launched 2.5-inch, 10K RPM disks that would be perfect for the System i line. The UltraStar C10K147 disks made by Hitachi Global Storage Technologies come in 73 GB and 147 GB raw capacities (one two or four platters with one or two recording heads, respectively). The drives have an average seek time of 3.7 milliseconds and an average latency of 3 milliseconds, and include a 16 MB data buffer. In typical usage, the drives are rated 8.4 watts for the 73 GB unit and 9 watts for the 147 GB unit.

Compare this to the UltraStar 15K300, a 3.5-inch drive that comes with UltraSCSI 320, 2 Gb/sec Fibre Channel, and 3 Gb/sec SAS interfaces that spins at 15K RPM and that has capacity of 73 GB, 147 GB, or 300 GB. (That is accomplished using one, two, or four disk platters, with two heads per platter.) The average seek time on this drive ranges from 3.4 milliseconds to 3.6 milliseconds (the larger number is for the 300 GB unit) and the average latency is only 2 milliseconds. The latency is obviously lower by 33 percent because the spin rate on the platters is 50 percent higher than on the 10K RPM disks. But look at average power consumption. That extra 50 percent in platter speed in the fatter disk (which has bigger platters, and therefore more friction generating more heat inside the disk) means that average power consumption during typical server workloads ranges from 13.2 watts for the skinny, single-platter UltraStar 15K300 to 14.9 watts for the 147 GB disk, to 18.1 watts for the 300 GB disk.

When you have a few dozen to a few hundred disk drives, the noise and the heat start adding up on these faster, larger disks. Which is why it is long since time to bring small form factor SAS disks to the System i.

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