[Storage Networking Fundamentals Pdf Free 21
Amancio Mccrae
Storage devices perform the fundamental function of storage networks, whichis the reading and writing of data stored on nonvolatile media. They operate inthe microscopic realm, combining advanced magnetic physics, chemistry, andelectronics. Demands to increase capacity and performance continue to force theindustry to conduct fundamental scientific research on the microscopiccharacteristics of materials.
Storage devices are the building blocks of storage in disk subsystems as wellas being used as standalone products in server systems. This chapter mostlyexamines disk drive technology as the device that is used far more than anyother. Tape drives are looked at at the end of the chapter.
Several types of storage devices are used in storage networking, but by farthe most important is the disk drive. If Harry Truman worked in the storagenetwork industry, he would have said the buck stops at the disk drive. By now wehave almost come to take disk drives for granted as ubiquitous gadgets that arereadily available at volume discounts. The fact is, disk drive technology is asimpressive as any other technology in all of IT, with amazing capabilities andphysical characteristics.
In the sections that follow, we'll examine the various subassembliesthat make up a disk drive, discuss the strengths and limitations of theseamazing machines, and point out what they mean for storage networkapplications.
The physical media where data is stored in a disk drive is called aplatter. Disk platters are rigid, thin circles that spin under the powerof the drive spindle motor. Platters are built out of three basic layers:
Substrates are made from a variety of materials, including aluminum/magnesiumalloys, glass, and ceramic materials. Considering the microscopic nature of diskrecording and how close the heads are to the surface, they must be amazinglyflat and relatively inelastic to thermal expansion and contraction. In addition,they have to be almost completely uniform in density and free from materialdefects that could result in balance imperfections, which cause vibration andfriction (heat) problems when spinning at high revolutions per minute (rpm).
The magnetic layer in most disk drives today uses thin film technology, whichis very smooth and only a few millionths of an inch in thickness. The thin filmlayer is made by spraying vapor molecules of the magnetic materials on thesurface of the substrate. The magnetic characteristics of the magnetic materialsgive the platter its areal densitythe measurement of how many bits can bewritten per square inch.
The protective overcoat layer provides protection from microscopic elementssuch as dust and water vapor, as well as from disk head crashes. Considering thephysics involved in high-speed disk drives, this coating is necessarily thin andprovides, at best, light-duty protection. The best way to protect disk driveplatters is to operate them in clean, dust-free, temperature-controlledenvironments.
Disk drives are usually made by arranging multiple platters on top of eachother in a stack where the platters are separated by spacers to allow the diskarms and heads to access both sides of the platters, as shown in Figure 4-2.
The recording heads used for transmitting data to and from the platter arecalled read and write heads. Read/write heads are responsible forrecording and playing back data stored on the magnetic layer of disk platters.When writing, they induce magnetic signals to be imprinted on the magneticmolecules in the media, and when reading, they detect the presence of thosesignals.
The performance and capacity characteristics of disk drives depend heavily onthe technology used in the heads. Disk heads in most drives today implementgiant magnetoresistive (GMR) technology, which uses the detection of resistancevariances within the magnetic layer to read data. GMR recording is based onwriting very low-strength signals to accommodate high areal density. This alsoimpacts the height at which the heads "fly" over the platter.
The distance between the platter and the heads is called the flyingheight, or head gap, and is measured at approximately 15 nanometers in mostdrives today. This is much smaller than the diameter of most microscopic dustparticles. Considering that head gap tolerances are so incredibly close, it isobviously 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 datacenter. Disk drives can run in a wide variety of environments, but thereliability numbers improve with the air quality: in other words, relativelycool and free from humidity and airborne contaminants.
The reference to "flying" with disk heads comes from theaerodynamic physics at work in disk drives: air movement caused by the rapidlyspinning platters passes over the heads, providing lift to the heads in much thesame way airplane wings are lifted by the difference in air pressure above andbelow them.
While we tend to think about data purely in the digital realm, the physicalrecording is an analog signal. Somehow, the 0s and 1s of digital logic have tobe converted to something that makes an impression on magnetic media. In otherwords, data on disk does not resemble written language at all but is expressedby the pattern of a magnetic signal on moving media. The read/writechannel is the disk drive subassembly that provides a specializeddigital/analog conversion.
The read/write channel is implemented in small high-speed integrated circuitsthat utilize sophisticated signal processing techniques and signal amplifiers.The magnetoresistive phenomenon that is detected by the read heads is very faintand requires significant amplification. Readers might find it interesting toponder how data read from disk is not actually based on detecting the magneticsignal that was written to media. Instead, it is done by detecting minutedifferences in the electrical resistance of the media, caused by thepresence of different magnetic signals. Amazingly, the resistance is somehowdetected by a microscopically thin head that does not make contact with themedia but floats over it at very high speeds.
The read and write heads have to be precisely positioned over specifictracks. As heads are very small, they are connected to disk arms that are thin,rigid, triangular pieces of lightweight alloys. Like everything else inside adisk drive, the disk arms are made with microscopic precision so that theread/write heads can be precisely positioned next to the platters quickly andaccurately.
The disk arms are connected at the base to the drive actuator, which isresponsible for positioning the arms. The actuator's movements arecontrolled by voice-coil drivers; the name is derived from voice coil technologyused to make audio speakers. Considering that some speakers have to vibrate atvery high frequencies to reproduce sounds, it's easy to see how diskactuators can be designed with voice coils to move very quickly. The clickingsounds you sometimes hear in a disk drive are the sounds of the actuator beingmoved back and forth.
The drive platters rotate under power of the drive spindle motor, which isdesigned to maintain constant speeds with minimal vibration over long periods oftime, sometimes measured in the tens of thousands of hours.
Most drive failures are related to motor failures. This is not to say themotors are poorly designed or designed to fail, because they clearly are not.However, they are always moving toward higher speeds with less power consumptionand less noise, and the tolerances are thin.
The actual spindle that the platters connect to is directly fixed to themotor's drive shaft. The spindle looks a bit like the inner core of someold 45-rpm record players, except the platters do not drop or slide over thecore, but are fixed in place. Separator rings are used to space the plattersprecisely so their surfaces can be traversed by the disk arms and headsaccurately.
Among the many parts of a disk drive, the bearings in the motor see constantwear and tear. While many other things can make a disk drive fail, it isinevitable that the bearings will eventually wear out.
The speed of the spindle motor must be constantly monitored to make sure itremains consistent hour after hour, day after day, month after month. The typeof technology used to maintain constant speed is called a servo-controlledclosed loop, and it is used for many different applications to fine-tuneautomated systems. Disk drives are designed with sophisticated feedback controlcircuits that detect minute speed variations in the rotating platter by readingtracking and timing data on the disk. If the speed varies too far one way oranother, the servo feedback circuit slightly changes the voltage supplied to thespindle motor to counteract the change.
The mechanical nature of reading and writing data on rotating platters limitsthe performance of disk drives to approximately three orders of magnitude (1000times) less than the performance of data transfers to memory chips. For thatreason, disk drives have internal buffer memory to accelerate data transmissionsbetween the drive and the storage controller using it.
Buffer memory might not have a significant performance impact for a singledisk drive system, such as a desktop or laptop system, but buffer memory canmake a big difference in storage subsystems that support high-throughoutapplications. When multiple drives are assembled together in an array, thecontroller, such as a subsystem controller, can overlap I/Os across multipledrives, using buffer memory transfers whenever possible. The drive can makeinternal transfers of data between buffer memory and its media platters whilethe subsystem controller is working with another drive. In general, buffermemory in disk drives can improve I/O performance for applications that read andwrite small chunks of randomly accessed data. Alternatively, streamingapplications with large files stored in contiguous storage locations do notrealize many benefits from buffer memory.
795a8134c1