Electronic Design

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.

Several Protocols Supported
The EIDE controllers support a number of handshaking protocols, including the Ultra DMA (UDMA) supported by the present hard-disk drives. UDMA speeds have progressed from 33 MHz through 66 MHz, and up to the current 100 MHz, also known as UDMA100, Ultra ATA/100, or ATA/100.

A minor change took place when the UDMA66 drives appeared. The connector remained the same, but the cable that was used changed because the higher-speed data transfers were more susceptible to noise. The new cable doubles the number of wires and provides twisted-pair support for each signal. Most end-user drive products are shipped with the new cables, and most motherboards with UDMA66 and UDMA100 support come with both kinds of cables.

In the past, designers recommended that hard disks and slower-speed ATA/IDE devices, such as CD-ROM drives, be placed on separate controllers because the slower devices would delay the hard-disk data transfers. This is especially true in the cases of UDMA66 and UDMA100 drives.

The ATA interface is less expensive to implement than SCSI and Fibre Channel, but it has a major shortcoming. ATA only processes a single operation at a time, and just one drive on a dual-drive controller can be accessed at once. This isn't a major issue on single-user PCs, although the current crop of multitasking operating systems can benefit from the multiple-operation support available with SCSI and Fibre Channel.

SCSI's Capabilities
SCSI supports multiple-command processing per drive and per subsystem. An SCSI controller can issue a command and disconnect from the drive before the command completes, enabling the controller to access another drive. This works by a drive notifying the controller when the command is complete, and then the controller reconnecting to the drive if data must be transferred to the controller.

The latest incarnation of the SCSI standard is UltraSCSI. It comes in many different forms, including UltraSCSI 2. The high end is currently dominated by drives that handle the 160-Mbyte/s Ultra160 SCSI interface.

UltraSCSI interfaces support up to 15 devices. Furthermore, SCSI is bus-oriented like ATA, versus the Fibre Channel Arbitrated Loop (FCAL) architecture.

The rise in Fibre Channel is due to the high demand for large disk farms and for high-availability disk subsystems. Both are critical to Internet e-business support. Fibre Channel drives are showing up in storage-area networks (SANs), which are often used for these environments.

Fibre Channel disk drives usually have two Fibre Channel connections per drive. This type of redundancy isn't available on ATA or SCSI hard-disk drives. But, drives with single Fibre Channel connections also are available.

Dual-channel Fibre Channel disks are normally connected to a single controller supporting two counter-rotating loops or two controllers with independent loops. The former provides redundant access to drives, so a cable or driver failure won't take down the entire system. The latter provides similar support except that the drives are accessible through the second controller should either the first controller or a drive interface fail.

The Fibre Channel Industry Association is a good source of information on Fibre Channel. The SCSI Trade Association is a useful source of information on SCSI. Although slated to replace ATA, Serial ATA will compete against SCSI and Fibre Channel.

The Works Of ATA
The Serial ATA Working Group was started by a large number of hard-disk and disk-controller vendors, including APT Technologies Inc., Intel Corp., Seagate, and Adaptec Inc. The group is working toward a standard whose implementation will cost the same as that of parallel ATA interfaces, but provide performance growth for the next ten years. It's designed to work with existing systems without any change to support software. Parallel ATA is expected to top out at the current 100-Mbyte/s data rate with possibly one more speed jump.

In August of this year, APT, Intel, Seagate, and Vitesse Semiconductor Corp. demonstrated the viability of a 1.5-Gbit/s Serial ATA interface. Seagate supplied the drive and Vitesse provided the Serial ATA Link and Transport-layer logic, along with a 1.5-Gbit/s CMOS transceiver.

Serial ATA's point-to-point architecture is a significant departure from the bus-oriented ATA and SCSI and the loop-oriented Fibre Channel. A Serial ATA drive will connect directly to the controller, and switch-oriented controller configurations are possible.

The Serial ATA definition begins with a 1.5-Gbit/s transfer rate that's effectively a 150-Mbyte/s transfer rate. Higher speeds are expected in future 3- and 6-Gbit/s versions. These higher-speed versions will let Serial ATA compete with 160-Mbyte/s SCSI and 200-Mbyte/s Fibre Channel.

Some interest has been expressed in dual-channel Serial ATA drives because the smaller 6-wire Serial ATA connections would permit the placement of two connectors on a single drive. These dual-channel hard-disk drives would be implemented where dual-channel Fibre Channel drives are now used, permitting lower-cost Serial ATA drives to compete with the more-expensive Fibre Channel drives.

Better Caching And Monitoring
One thing that all 3.5-in. drives have in common, regardless of the interface type, is a built-in cache. Low-end products have smaller 2-Mbyte caches, but higher-end products are pushing 16-Mbyte caches.

A large cache provides significant benefits for large-file applications, such as audio and video editing, and CAD. It's useful in disk farms that have high transaction rates, too. Most large-cache drives are SCSI or Fibre Channel types where multiple operations can be active at one time.

In addition, larger caches also are included due to the lower cost of memory and the added processing power found in the on-board disk controller. This improved intelligence has allowed self-monitoring to flourish.

Most 3.5-in. hard-disk drives in-clude Self Monitoring Analysis and Reporting Technology (SMART) support. Pioneered by IBM and Compaq Computer Corp., this technology requires matching software support in the controller and the motherboard. Most current motherboards support SMART, which allows hard-disk detection systems to warn users about potential impending failures. This, in turn, provides data backup and replacement of the drive before a hard failure occurs.

Hard-disk reliability improvement has also occurred in the area of system packaging. For example, Seagate's SeaShell technology wraps the drive to increase resistance to external shock. While this brings greater benefits to laptops with smaller drives, the 3.5-in. drive can especially aid in removable RAID environments.

Fluid-bearing motors have started showing up in high-end SCSI and Fibre Channel drives. These not only help improve a drive's resistance to shock, but also provide a more stable platform for the rotating platters.

Fluid bearings are starting to show up in ATA drives, but some manufacturers have concerns about reliability and potential leakage problems. The migration to fluid bearings may prove to be quicker if high quality can be maintained while costs are reduced.

Another benefit of fluid bearings is a quieter drive. In general, hard-disk drive vendors have made significant acoustic reductions to their products over the past year. Even the regular bearing motors are getting quieter.

Quieter hard-disk drives are in demand for two reasons. One is the elimination of noisier or the use of quieter cooling fans in desktop PCs. In this case, the overall noise of the system is dominated by the hard disk. Soundproofing the hard disk ensures that the entire system becomes quieter.

The second reason is that new hard-disk applications also are driving the quest for a quieter operation. Audio recording markets demand quiet hard-disk drives to prevent induced noise when recording. Plus, digital recorders in set-top boxes and in modular audio and video equipment require quiet hard disks to prevent the equipment from contributing noise to the listening experience.

Moving embedded hard disks into other areas of the home has lead to the nebulous "3AM bedroom acoustics" requirement. Although not a specific standard, it essentially means that a device shouldn't be heard even when you wake up in the middle of the night. Smaller hard disks may be seen in clock radios, but 3.5-in. drives will likely show up in the digital recorders found in TVs/recorders or set-top boxes.

Acoustic improvements will continue, as will other technology avenues. Some of the mechanical improvements that can be expected over the next year or two are shown in the figure.

Fluid bearing as well as glass platter use has already been mentioned, but the smoothness of the coating on top of the platters will continue to improve. This will allow for a reduction in the gap size between the platter and the read/write head. Moving the head closer permits the detection of weaker signals occurring as the areal density is increased.

Current GMR heads are likely to be replaced by tunneling GMR heads, with low-strength field-sensing support. The tunneling GMR support requires a slightly different layering in the platter coating to support spin-dependent tunneling (SDT).

Another recording alternative that has been around for a while is vertical magnetic recording. This effectively flips the way that information is stored by rotating the stored magnetic field 90°. Vertical recording is a more radical change compared to tunneling GMR, but it promises to provide significantly higher densities.

Head Placement Is Critical
As areal densities grow and heads move closer to the platter, the accurate placement of the read/write head over a track becomes even more critical. Today's designs employ a basic motor-driven actuator. In the future, the tip of the actuator will probably have a microactuator for fine tuning the head's position because the primary actuator's positioning motor won't have sufficient resolution to accurately position the head over a track.

A number of microactuator designs have been created, and it's likely that more than one design will be implemented by different vendors. Some technologies employ a rotating actuator, while others use picosliders. These small machines move the head from only a fraction of a track up to a few tracks. They perform the move after the primary actuator has provided the gross positioning. Typically, the microactuator will use embedded servo information for its fine-tuned positioning.

For even finer positioning in the future, microelectromechanical systems (MEMS) might be used. But they're currently too small for the next generation of hard-disk head-positioning systems. MEMS may even wind up on the microactuator.

Another advancement in actuator control will be the use of active damping. When the actuator is stopped over a track, it has a tendency to vibrate. Active damping uses the actuator motor to suppress the vibration, allowing the head to settle more quickly and accurately. Essentially, the control processor implements the motor to slow down the actuator in the direction of each vibration cycle.

The 3.5-in. drive market is dominated by a number of large companies, including Fujitsu, Hitachi America Ltd., IBM, Maxtor, Seagate Technology, Toshiba Corp., and Western Digital. Some, such as Maxtor, work exclusively in the 3.5-in. space. This type of drive addresses a large space from consumer applications through enterprise uses. Single drives are found in desktop PCs and embedded applications, and multiple drives are seen in various redundant-array-of-inexpensive-disks (RAID) configurations (see "An Introduction To RAID," p. 110).

Interfaces for hard-disk drives span an equally wide range. They vary from those that support low-end parallel ATA ports for two drives per controller, through high-end interfaces like Fibre Channel that handle up to 125 devices per controller.

One area in which 3.5-in. drives aren't dominant is laptops and embedded applications where size is critical. Smaller drives come into play here. For example, 2.5-in. drives are the most popular on regular laptops. Furthermore, while 1.8-in. drives are making some inroads here, these even smaller drives are finding a better home in embedded applications where space, but not capacity, is critical.

Other specialized drives exist with different form factors, like IBM's MicroDrive. This has a CompactFlash form factor, letting it be used in a variety of portable applications including digital cameras. The 1-Gbyte version is very popular with photography professionals, because high-end digital cameras generate multimegabit image files.

Like 3.5-in. disk drives, capacity and performance is increasing for these smaller form-factor drives. But, they're still far short of the 3.5-in. drives in terms of capacity and performance. The small disk sizes simply don't have the area necessary to compete with the 3.5-in. drives.

Interestingly, 2.5-in. drives are showing up in rack-mount RAID arrays. The reason is that more drives can fit into the same amount of space, providing better redundancy and higher performance due to the parallel operation of more disks in comparison to a similar 3.5-in. solution. Still, most installations have sufficient space for 3.5-in. RAID arrays, which have higher disk and system capacities.

In all likelihood, the yearly doubling of hard-disk capacity should continue with the low-end capacity reaching 20 Gbytes. The Serial ATA interface may become a factor in late 2001, but parallel ATA, SCSI, and Fibre Channel will support the bulk of the drives shipped in 2001.

Companies Mentioned In This Report
APT Technologies Inc.
(831) 429-7260

Adaptec Inc.
(408) 945-8600

Compaq Computer Corp.
(281) 370-0670

Fibre Channel Industry
(714) 447-4993

(408) 432-6333

Hitachi America Ltd.
(972) 488-3824

(408) 256-1600

Intel Corp.
(408) 765-8080

Maxtor Corp.
(408) 432-1700

Seagate Technology
(510) 353-4600

Serial ATA Working Group

SCSI Trade Association
(415) 750-8351

Toshiba Corp.
(650) 872-2722

Vitesse Semiconductor Corp.
(805) 388-3700

Western Digital Corp.
(949) 932-7530

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