[Technology Report]
Hard-Disk Capacities For 3.5-In. Drives Are On The Rise
Improvements in recording densities, read/write heads, interfaces, and reliability ensure that rotating magnetic-media will be around for quite some time.
In the hard-disk drive landscape, 3.5-in. drives still dominate. They're able to do this by providing the right combination of power, price, and performance for the largest number of applications. While smaller form factors such as IBM's MicroDrive CompactFlash spark the interest of mobile users, the 3.5-in. drive remains entrenched in areas where battery life isn't an issue.
At the high end, SCSI and Fibre Channel drives fill servers and disk farms. At the low end, desktop PCs may be squeezed by the growing number of embedded applications, like set-top boxes. But for the majority of applications in between, the 3.5-in. disk is king. Furthermore, improvements in this disk-drive technology continue unabated, making it likely that its influence on data storage will continue for a very long time.
Capacity for the high end of 3.5-in. hard-disk drives is about 80 Gbytes per drive. One such available drive is the DiamondMax 80 from Maxtor Corp. It has four platters that spin at 5400 rpm. At 20 Gbytes per platter, the DiamondMax 80 is pushing the current level of technology while providing high reliability.
Four is about the maximum number of platters used for the current crop of 3.5-in. hard-disk drives. Of course, the cost of four-platter drives is higher than that of single-platter drives. Low-cost single-platter drives have become prevalent because areal recording density levels of rotating magnetic media have grown high enough to let these drives meet the system requirements for a variety of applications. One example of such an application is low-cost PCs.
Most current platters consist of aluminum with a magnetic coating. The platter is about 1 mm thick. As rotational speeds increase, however, the use of thicker platters on the order of 1.27 mm becomes necessary to reduce flutter. That, in turn, reduces the fine head-tracking accuracy required to achieve high density levels of areal information.
The platter substrate is the base for a multilayer collection of nonmagnetic and magnetic materials covered with a diamond-like carbon layer and a 2-nm lubricant layer. The diamond-like carbon layer is found on most read/write heads as well.
Seagate Technology and Fujitsu use a slightly different technique. These companies employ small hemispheres of DLC to provide similar protection for the head while leaving the head exposed.
Typical head-to-magnetic layer separation is 30 nm. The flying height or gap size is in the 10- to 15-nm range, with the remaining space taken up by other layers including the lubricant layer.
Glass Substrates Another alternative material to aluminum is a glass substrate. IBM is using glass on some of its hard disks, but this technology is relatively new. More vendors might move to glass platters as areal recording densities and rotational speeds increase.
In the current crop of hard disks, the useful areal recording density is on the order of 15 Gbits/in.2 Areal density is based on a number of factors including head-to-magnetic layer separation, tracking accuracy of the actuator, and the recording heads. Current high-density recording heads use Giant Magneto Resistance (GMR) technology.
A higher areal recording density improves data-transfer performance because more information passes under the read/write head every inch. The other way to increase performance is to increase rotational speed.
The 5400-rpm rotational speed is par for consumer-oriented hard disks. Faster 7200-rpm disks, like the 45-Gbyte Maxtor DiamondMax Plus 45, provide higher performance. This is just below products targeted at the server and high-end workstation market, where 10,000-rpm drives, such as the 73-Gbyte Seagate Cheetah 73 and Western Digital's WD Vantage Ultra 2 SCSI line, are found.
A few 15,000-rpm drives are just becoming available, such as the 18.3-Gbyte Seagate X15 Ultra160 SCSI drive with an average seek time of only 3.9 ms. The average seek time for drives with a lower rotational speed is in the range of 5.2 to 6 ms. Although the average seek time isn't directly related to the rotational speed, vendors tend to provide higher-performance features with higher-rotational drives.
Rotational speed and areal density determine how much information can be transferred to or from the hard disk. But, the hard-disk interface determines how this information is exchanged between the drive and the controller.
Three hard-disk interfaces dominate the 3.5-in. drive space. These include ATA, SCSI, and Fibre Channel. Serial ATA is an emerging standard, but the first products aren't expected to ship until the middle of 2001, or possibly even later.
External hard disks often have interfaces like the IEEE 1394, also known as Firewire or iLink, and the Universal Serial Bus (USB). Hard-disk subsytems that support these external interfaces normally implement a hard disk with one of the three standard interfaces. A controller provides the link between the hard-disk interface and the external interface.
The USB 1.x standard has a 12-Mbit/s bandwidth that's well below the performance of most hard disks. Though the USB 2.0 standard bumps up performance by a factor of 10, it hasn't generated much interest in terms of a hard-disk interface for internal PC use.
IEEE 1394 has sufficient bandwidth for hard-disk interfaces with support starting at 100 Mbits/s and rising to 400 Mbits/s. Yet, various technical and licensing details have prevented it from becoming a factor as a native hard-disk interface. Similar to USB, all hard-disk connections by this interface occur through bridge controllers with a native hard-disk interface and an IEEE-1394 interface.
The ATA interface, also referred to as IDE and EIDE, is by far the most popular native hard-disk interface. Most motherboards today have one or two EIDE controllers with each controller supporting up to two drives. In a two-drive system, one drive is configured as a master and the other as a slave.
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