Disk Drives

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Several types of storage devices are used in storage networking, but by far the most important is the disk drive. If Harry Truman worked in the storage network industry, he would have said the buck stops at the disk drive. By now we have almost come to take disk drives for granted as ubiquitous gadgets that are readily available at volume discounts. The tact is, disk drive technology is as impressive as any other technology in all of IT, with amazing capabilities and physical characteristics.

In the sections that follow, we'll examine the various subassemblies that make up a disk drive, discuss the strengths and limitations of these amazing machines, and point out what they mean for storage network applications.

Major Parts of a Disk Drive
Disk drives are constructed from several highly specialized parts and subassemblies designed to optimally perform a very narrowly defined function within the disk drive. These components are:

  • Disk platters Read and write heads
  • Read/write channel
  • Arms and actuators
  • Drive spindle motor and servo control electronics
  • Buffer memory
  • Disk controller

Now we'll discuss each of these subassemblies briefly.

 

Disk Platters
The physical media where data is stored in a disk drive is called a platter. Disk platters are rigid, thin circles that spin under the power of the drive spindle motor. Platters are built out of three basic layers:

  • The substrate, which gives the platter its rigid form
  • The magnetic layer, where data is stored
  • A protective overcoat layer that helps minimize damage to the disk drive from microscopically sized dust particles

The three different layers within a disk platter are illustrated in Figure 4-1, which shows both the top and bottom sides of a platter.

 

Figure 4-1 Material Layers in a Disk Media Platter
Figure 4-1

Substrates are made from a variety of materials, including aluminum/magnesium alloys, glass, and ceramic materials. Considering the microscopic nature of disk recording and how close the heads are to the surface, they must be amazingly flat and relatively inelastic to thermal expansion and contraction. In addition, they have to be almost completely uniform in density and free from material defects that could result in balance imperfections, which cause vibration and friction (heat) problems when spinning at high revolutions per minute (rpm).

The magnetic layer in most disk drives today uses thin film technology, which is very smooth and only a few millionths of an inch in thickness. The thin film layer is made by spraying vapor molecules of the magnetic materials on the surface of the substrate. The magnetic characteristics of the magnetic materials give the platter its areal density--the measurement of how many bits can be written per square inch.

The protective overcoat layer provides protection from microscopic elements such as dust and water vapor, as well as from disk head crashes. Considering the physics involved in high-speed disk drives, this coating is necessarily thin and provides, at best, light-duty protection. The best way to protect disk drive platters is to operate them in clean, dust-free, temperature-controlled environments.

Storage capacity of a single platter varies from drive to drive, but recent developments in commercially available products have resulted in platter capacities in excess of 100 GB per platter.

Disk drives are usually made by arranging multiple platters on top of each other in a stack where the platters are separated by spacers to allow the disk arms and heads to access both sides of the platters, as shown in Figure 4 2.

Figure 4-2 Disk Platters Connected to the Spindle Motor in a Stack
Figure 4-2

Read and Write Heads
The recording heads used for transmitting data to and from the platter are called read and write heads'. Read/write heads are responsible for recording and playing back data stored on the magnetic layer of disk platters. When writing, they induce magnetic signals to be imprinted on the magnetic molecules in the media, and when reading, they detect the presence of those signals.

The performance and capacity characteristics of disk drives depend heavily on the technology used in the heads. Disk heads in most drives today implement giant magnetoresistive (GMR) technology, which uses the detection of resistance variances within the magnetic layer to read data. GMR recording is based on writing very low strength signals to accommodate high areal density. This also impacts the height at which the heads "fly" over the platter.

The distance between the platter and the beads is called the flying height or head gap, and is measured at approximately 15 nanometers in most drives today. This is much smaller than the diameter of most microscopic dust particles. Considering that head gap tolerances are so incredibly close, it is obviously a good idea to provide a clean and stable environment for the tens, hundreds, or thousands of disk drives that are running in a server room or data center. Disk drives can run in a wide variety of environments, but the reliability numbers improve with the air quality: in other words, relatively cool and free from humidity and airborne contaminants.

The reference to "flying" with disk heads comes from the aerodynamic physics at work in disk drives: air movement caused by the rapidly spinning platters passes over the heads, providing lift to the heads in much the same way airplane wings are lifted by the difference in air pressure above and below them.

Read/Write Channel
While we tend to think about data purely in the digital realm, the physical recording is an analog signal. Somehow, the 0s and Is of digital logic have to be converted to something that makes an impression on magnetic media. In other words, data on disk does not resemble written language at all but is expressed by the pattern of a magnetic signal on moving media. The read/ write channel is the disk drive subassembly that provides a specialized digital/analog conversion.

The read/write channel is implemented in small high-speed integrated circuits that utilize sophisticated signal processing techniques and signal amplifiers. The magnetoresistive phenomenon that is detected by the read heads is very faint and requires significant amplification. Readers might find it interesting to ponder how data read from disk is not actually based on detecting the magnetic signal that was written to media. Instead, it is done by detecting minute differences in the electrical resistance of the media, caused by the presence of different magnetic signals. Amazingly, the resistance is somehow detected by a microscopically thin head that does not make contact with the media but floats over it at very high speeds.

Arms and Actuators
The read and write heads have to be precisely positioned over specific tracks. As heads arc very small, they are connected to disk arms that are thin, rigid, triangular pieces of lightweight alloys. Like everything else inside a disk drive, the disk arms are made with microscopic precision so that the read/write heads can be precisely positioned next to the platters quickly and accurately.

The disk arms are connected at tile base to tile drive actuator, which is responsible for positioning the arms. The actuator's movements are controlled by voice-coil drivers; the name is derived from voice coil technology used to make audio speakers. Considering that some speakers have to vibrate at very high frequencies to reproduce sounds, it's easy to see bow disk actuators can be designed with voice coils to move very quickly. The clicking sounds you sometimes hear in a disk drive are the sounds of the actuator being moved back and forth.

Drive Spindle Motor and Servo Control Electronics
The drive platters rotate under power of the drive spindle motor, which is designed to maintain constant speeds with minimal vibration over long periods of time, sometimes measured in the tens of thousands of hours.

Most drive failures are related to motor failures. This is not to say the motors are poorly designed or designed to fail, because they clearly are not. However, they are always moving toward higher speeds with less power consumption and less noise, and the tolerances are thin.

The actual spindle that the platters connect to is directly fixed to the motor's drive shaft. The spindle looks a bit like the inner core of some old 45-rpm record players, except the platters do not drop or slide over the core, but are fixed in place. Separator rings arc used to space the platters precisely so their surfaces can be traversed by the disk arms and heads accurately.

Among the many parts of a disk drive, the bearings in the motor see constant wear and tear. While many other things can make a disk drive fail, it is inevitable that the bearings will eventually wear out.

The speed of the spindle motor must be constantly monitored to make sure it remains consistent hour after hour, day after day, month after month. The type of technology used to maintain constant speed is called a servo-controlled closed loop, and it is used for many different applications to fine-tune automated systems. Disk drives arc designed with sophisticated feedback control circuits that detect minute speed variations in the rotating platter by reading tracking and timing data on the disk. If the speed varies too far one way or another, the servo feedback circuit slightly changes the voltage supplied to the spindle motor to counteract the change.

Buffer Memory
The mechanical nature of reading and writing data on rotating platters limits the performance of disk drives to approximately three orders of magnitude ( 1000 times) less than the performance of data transfers to memory chips. For that mason, disk drives have internal buffer memory to accelerate data transmissions between the drive and the storage controller using it.

Buffer memory might not have a significant performance impact for a single disk drive system, such as a desktop or laptop system, but buffer memory can make a big difference in storage subsystems that support high-throughout applications. When multiple drives are assembled together in an array, the controller, such as a subsystem controller, can overlap l/Os across multiple drives, using buffer memory transfers whenever possible. The drive can make internal transfers of data between buffer memory and its media platters while the subsystem controller is working with another drive. In general, buffer memory in disk drives can improve 1/O performance for applications that read and write small chunks of randomly accessed data. Alternatively, streaming applications with large files stored in contiguous storage locations do not realize many benefits from buffer memory.

Buffer memory sizes have increased over time, although not at the same rate as the areal density of platters. Today, disk drives typically have buffer capacities between 2 MB and 16 MB.

Disk Controller
Disk drives all have internal target controllers that respond to commands from host or subsystem initiators. In addition to interoperating with the external initiator, the storage controller in a disk drive is responsible for executing the command within the drive. The software component of a disk drive controller is referred to as firmware and is typically stored in e-prom chips in the drive's circuit board.

Processor chips used as disk drive controllers are constantly being improved with faster cores and more memory. Strangely enough, one of the challenges for disk drive manufacturers is how to make the best use of the additional intelligence at their disposal. It's a tougher question than it first appears because disk drives have traditionally been used as relatively stupid slave devices responding to I/O requests from host and subsystem controllers. Storage applications such as NAS can be added to disk drives, but that creates competition between the disk drive manufacturers and their customers--the system and subsystem vendors. So far, the disk drive manufacturers have been at the wrong end of the pecking order and their attempts to integrate higher-level storage functions in the drives have mostly failed.

Instead, what has been successful is the use of processor intelligence to increase reliability and ease of use. If you stop to consider how much easier it is to install disk drives today than it was ten years ago, the improvement has been remarkable. When you think about these improvements in terms of installed capacity, the advancements have been truly incredible. For example, the reliability provided by a handful of today's high capacity disk drives used together to form a terabyte (TB) of storage is far, far better than the reliability of the approximately 500 disk drives that were needed ten years ago to build the same terabyte of capacity. Obviously, improvements in areal density make this possible, but a significant amount of work has also been done in the drive's internal controllers.

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