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Disk Drives

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Hard drive• In our original view of a computer as being

comprised of ALU, Control, Memory, Input and Output, the hard drive is a device connected either as input or output.

• But the hard drive is also sometimes viewed as a logical extension of memory.

• The hard drive is the primary storage device. – Compared to RAM, storage is non-volatile

– Compared to ROM, storage is more easily written.

Solid State Drive (SSD)

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Speed and Capacity

• The two main characteristics of a hard drive are – Its capacity: how much data can it hold– Its speed: how quickly can it be read from or

written to • Data intensive applications such as databases,

graphics and so on will require pages to swapped in and out of memory. The speed of the hard drive will be an important factor for determining how efficiently such programs run.

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Analog of Moore

• Recall that Moore’s Law concerns the exponential growth of the number of transistors on an integrated chip.

• There has been similar exponential growth in the capacity of hard drives.

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Exponential Improvement in Hard Drive Capacity

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Paper Tape and Cards

• Prior to hard drives, programs and data could be stored on cards or paper tape.

• In both cases a hole could correspond to a 1 and the absence of a hole to a 0.

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Magnetic Tape

• Magnetic tape was an improvement upon punch cards and paper tape, both in terms of speed and capacity.

• In magnetic tape, a long thin piece of plastic is covered with a ferromagnetic material, such as Ferric oxide (Fe2O3).

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Magnetic Tapes versus Hard drives

• The writing or reading of a individual bit is similar whether we are talking about a hard drive or a magnetic tape.

• The difference is one of addressing: – Magnetic tapes have sequential access – Hard drives have random access

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Magnets

• Little magnets align themselves in a particular way when in the presence of a magnetic field – E.g. a compass points North because it is a

magnet aligning itself with the Earth’s magnetic field

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Ferromagnetic• Many atoms are like tiny little magnets, but the

little magnets point in random directions and tend to cancel out any magnetic effect on a large scale.

• An external magnetic field can make these little magnets line up and produce a large-scale (macroscopic) effect.

• If the little magnets remain aligned even when the external magnetic field is removed, then the material is said to be ferromagnetic.

• The lining up of the magnets is called magnetization. • A ferromagnetic material “holds” or “remembers” its

magnetization state.

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Writing/Recording

• The signal (data changing over time) is fed to a magnet which magnetizes the material on the region of the tape that corresponds to that time. – The tape may record analog or digital

information.

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Reading/Playing

• The magnetized region produces a magnetic field of its own.

• Any device that can sense this magnetic field can read the information encoded on the tape.

• It is important to note that the reading device (head) does not have to be in physical contact with the tape in order to sense the tape region’s magnetic field – just in its vicinity.

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Floating height/Flying height

• As opposed to floppies, VCR and cassette tapes, hard disk heads do not come in contact with the medium they are reading from or writing to.

• The distance the head is from the material is one of the important design parameters and is known as the floating height or flying height.

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Reading, Sensitivity and Density• As the heads are made sensitive to smaller magnetic

fields, the region corresponding to a unit of information can be made smaller.

• Also as the heads are made to approach the material more closely (where the field is stronger) without touching it, the region corresponding to a unit of information can be made smaller.

• If the regions grow smaller, the number of such regions per area (the density) increases. – There are other ways to measure density, so this version is

sometimes called the areal density.

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Exponential Improvement in Hard Drive Density

The units for areal density are bits per square inch (BPSI) or in this case MBPSI (mega)

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Shrinking Bit Size

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IBM RAMAC• To get a sense of this improvement, consider an

early disk drive. • One of the first commercially available hard disks

was IBM's RAMAC (Random Access Method of Accounting and Control) introduced in 1956. – Capacity: about 5 MB

– Used 50 24" disks!!!!!!!!

– Areal density: 2,000 bits per square inch

– Data throughput: 8,800 bits/s.

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Form Factor

• The areal density improvement has allowed the capacity to increase while the size has decreased.

• Drive’s form factors (basically their width and height) have continued to grow smaller and smaller.

• The 5.25-inch width was a standard. It came in three standard heights– Full-height: 3.25 inch– Half-height: 1.625 inch– Third height: 1 inch

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3.5 inch Form Factor

• But now the 3.5-inch width has replaced it as the standard in PCs.

• This width comes in two standard heights – Third height: 1-inch which is standard (slim-line)

– Half-height: 1.625-inch which is used for higher capacity drives

• Besides the overall benefits of miniaturization, smaller widths allow the platters to spin faster and thus help improve the speed.

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Platters and Spindle Speed• Instead of the long plastic strip of magnetic tape,

hard disks have a collection of circular shaped aluminum or glass platters (which serve as the substrate) that are covered with magnetic material (the media layer).

• Data is accessed by having the head float over the platter as its spins. – Access speed is thus related to rotational or spindle speed

which is measured in RPM (revolutions per minute). – Spindle speeds in the thousands to tens of thousands are

typical these days.

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Hard versus floppy disk

• The disks in a hard drive are fairly rigid and hence the name “hard” disk.

• The disk in a floppy disk is made of a more flexible material.

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Anatomy of a Hard Drive• The data is stored on platters: hard, flat circular-

shaped piece of aluminum coated with magnetic material on one or both sides.

• The spindle serves as a rotational axis for the platters.

• The rotational motion is driven by the spindle motor.

• The information is accessed by a read/write head. – Typically there are two heads per platter, one on the

top, one on the bottom.

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Anatomy of a Hard Drive (Cont.)• The head is attached to a part called the slider,

which is in turn attached to the actuator arm, which is used to position the head in the desired region.

• The position of the actuator arm is controlled by the actuator.

• There is actually a parallel array of heads which are moved in unison by the actuator.

• The actuator in controlled by the logic board which communicates the rest of the PC.

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Don’t try this at home

• Reading requires the head to come very, very close to the platter without touching it.

• Hard drives should not be opened because a typical speck of dust is larger than the head-to-platter reading distance.

• Opening a drive will almost assuredly ruin it. – Head crash: when the head touches the platter

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Dust particle versus Flying Height

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Tracks• In both hard and

floppy disks, the data is written in concentric circular paths known as tracks

• Floppy disks had 80 (double-density) or 160 (high-density) tracks.

track

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Tracks (cont.)

• The density of tracks is measured in units of tracks per inch (TPI).

• Each track is further divided into sectors.

• The location of information is remembered by noting its track and sector numbers.

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Sectors

• Radial lines break the tracks up into sectors, each of which holds 512 bytes of information.

Sector

A floppy typically had 17 sectors per track

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Not all sectors are created equal

• A sector on the outer portion of the platter has a greater area than a sector on the inner portion. – More and more of the storage area is wasted as

one moves out in the radial direction. – More modern drive technologies divide the

outer tracks into more sectors to make use of this storage area.

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Zoned Bit Recording

• Zoned bit recording (ZBR), a.k.a. multiple zone recording or zone recording.

• Tracks are broken into groups called zones based on their radial position. Tracks with greater radii are broken into more sectors so that storage area is not wasted. – Requires more sophisticated controller.

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Larger Radius, Faster Access

• In ZBR, the outer tracks have more sectors and thus hold more data, but the data is accessed by the spinning of the disk. So more data goes by per revolution when reading the outer tracks.

• The outer tracks tend to be used first. So disk access performance may go down as one starts to use the inner/slower tracks.

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ZBR

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Cylinders• Each platter has tracks with the various radii. • All the tracks on different platters but with the

same radius make up a cylinder. • For example, if a hard drive has

– Four platters– And each platter has 600 tracks

• Then – There will be 600 cylinders– Each cylinder will have 8 tracks (assuming that each

platter has tracks on both sides).

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Why cylinders are important

• If the data being written does not fit in a single track, it can be spread across the cylinder.

• Activating different heads (at the same radius/cylinder) is an electronic process and thus is faster than moving the arm to a new radius, which is a mechanical process and thus slow.

Cylinder snapping

• When a drive is partitioned, the partitions are “snapped” to cylinders so that a cylinder is not shared by more than one partition.

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Defragmentation makes reading files from disk drives more efficient by keeping related data together on the drive. Together means same track, then same cylinder, then consecutive cylinders

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Accessing a disk

1. The combination of application, operating system, BIOS and possibly disk interface circuitry determine the location of the data on the hard disk.

2. The location corresponds to a geometric location on the disk. One needs to know

A. The cylinder which determines the radius or track. B. The head which determines which platter and which

side of the platter. C. The sector which determines where along the track.

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Accessing a disk (Cont.)3. See if the information is cached before starting

the slower process of accessing it on the drive. 4. If the drive is not already spinning, it must be

“spun up” to its working rotational speed. It might have been “spun down” to save energy.

5. The actuator moves the heads to the appropriate cylinder (track, radial position).

6. The actuator then selects the appropriate head and waits until the selected sector passes by the head. It then reads.

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Accessing a disk (Cont.)

7. The information is read and placed temporarily into a buffer.

8. The hard disk interface then sends the data to some other part of the PC, in most cases the memory.

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Cache on the Hard Disk

• The hard disk has a cache/buffer. – After requesting data from the hard drive, one does not

sit by idly waiting for the result. One carries on with whatever else possible. The result of the read is placed in a buffer and picked up when the processor/memory is ready to take it.

– When reading, one also grabs the data in the neighboring sectors and places it in the buffer (pre-fetching). This is the standard idea of caching something you expect to need soon based on locality of reference.

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Size and Distinction

• The cache on the hard disk is typically between 512 KB and 2 MB. – Some SCSI drives may have as much as 16

MB.

• Be careful not to confuse the cache on the hard drive with “disk cache” which refers to a section of main memory used to hold data recently read from the hard drive.

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Command overhead time

• Command overhead time is the time it takes from when the hard drive is given the read instruction to when the actuator starts positioning the head– Since this is an electronic time as opposed to

the mechanical seek time, it is much smaller and does not contribute much to the access time.

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Seek and ye shall find, but be quick about it

• Seek time is the time required to position the head to the selected cylinder. – Typical seek times are in milliseconds (ms) – Recall that processor times are in nanoseconds

(ns, approx. a million times smaller) and memory times are in microseconds (s, approx. a thousand times smaller).

• Seek time is not access time, but is probably the major part thereof.

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Versions of Seek Time

• Average: from a random track to another random track– This is what is typically reported – 8 – 10 ms

• Track-to-track: from one track to the adjacent track

• Full stroke: the full range from the innermost to outermost track

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Settle Time

• After the head has reached the appropriate cylinder (track, radius), it must take a short amount of time to stabilize before reading can occur.

• This time is called the settle time or settling time. • It is short compared to seek time and does not

vary much from manufacturer to manufacturer.

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Latency

• After the instructions have been interpreted and the actuator begins to move (command overhead) and the head has reached the selected cylinder (seek) and has stabilized (settle), it is still not ready to read.

• It must wait until the selected sector rotates past the head. This time is called the latency.

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Latency and Spindle Speed

• The time it must wait for the correct sector to swing by clearly depends on how fast the disks are rotating – the spindle speed.– If the spindles rotates at 10,000 RPM (revolutions per

minute), then it rotates at speed of 10,000/60 = 166.7 revolutions per second.

– If there are 166.7 revolutions per second, then a revolution takes 1/166.7 seconds = 0.006 s or 6 ms.

– The average latency is half of the rotation time or in this case 3 ms.

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PCGuide table

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Ordering the to-do list

• Because the hard drive is slower than the processor and memory, there may be a back up of tasks for it to perform. The order in which it performs these tasks can greatly affect its efficiency.

• One ordering is a simple FIFO (first-in, first-out) ordering. The tasks (reads and writes) are queued up and the first task requested is the first task performed.

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More Sophisticated Orderings

• Seek-Time Optimization (a.k.a. Elevator seeking):– Seek time involves the radial positioning of the

head. The tasks are ordered based on their radial positioning to minimize seek time.

• Access-Time Optimization (a.k.a. multiple command reordering):– Takes into account both radial and angular

positioning to minimize access time which includes seek time and latency.

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Ordering Comparison

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Locating data• The platters are dense with data, if the head is

even the slightest bit off target, one will be reading the wrong data.

• The actuator uses a voice coil. The voice coil uses a feedback mechanism to locate the data. – Feedback means taking some output and putting it back

in as input. In this case the head is reading information from the platter. Some of that information is telling the head where it is on the platter.

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Servo Information

• The information on the disk about location on the disk is called servo information. – It is not user’s data but data about the disk’s

operation.

• The servo information can be placed – In dedicated sectors (wedge servo)– On dedicated platter side (dedicated servo)– Spread throughout (embedded servo)

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Low-level format

• Low-level formatting (LLF), a.k.a. physical formatting, establishes the tracks, sectors, etc. and writes the servo information.

• For a hard drive, this is done once by the manufacturer.

• For a floppy disk, this can be done by the user. • Any previous data is lost when a low-level format

is performed on a disk.

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Our old enemy heat

• When objects heat up, they tend to expand (thermal expansion).

• If the heat in a hard drive is not distributed uniformly, then different platters are at different temperatures and thus have expanded different amounts.

• This is why dedicated servo is not as good as embedded servo.

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Thermal recalibration• Every once in awhile the hard disk reads

itself just to check on track positioning and so forth. – You may hear the hard drive spinning even

though the PC is neither reading it nor writing to it.

– This is called thermal recalibration. – The amount of thermal calibration necessary

was reduced by manufacturers moving to embedded servo techniques.

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A gentle breeze• While reading or writing the platters spin, which

causes a breeze. The actuator arm is designed like a wing that floats in this breeze.

• When the drive spins down (stops spinning), the breeze stops blowing and the actuator arm must come in for a landing. It comes in contact with the platter.

• We do not want there to be any data written in the region where the arm lands.

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The landing zone is for …• There are special regions on the disk called landing

zones where no data is written and where the head comes to rest when the drive spins down.

• Positioning the head into the landing zone is called head parking.

• There is a BIOS parameter that informs the system where the landing zones are. But in modern drives the drive is designed to return automatically to the landing zone (even if the power is lost).

• Some drives are designed so that the head never lands.

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Landing Zone Setting in BIOS

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Spindle Speed Revisited• In addition to accurately positioning the head, the

platter must spin at a very accurate speed. • The standard spindle speed for a long time was 3600

RPM (revolutions per minutes), which corresponds to 60 cycles per second or 60 Hz, which is same frequency as the AC power that comes out of a standard US wall socket.

• But since latency is connected to spindle speed, the spindle speeds have been raised.

• The higher speeds tend to debut with SCSI drives and then filter down to ATA/IDE drives.

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Spindle speed and drive type

Source: PCGuide.com

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Spindle Speed Issues

• Smaller platters are easier to spin faster – Think of an ice skater spinning

• Platters must spin together, so it is easier to spin fewer platters.

• The platter must spin at a steady speed and not vibrate (we don’t want a head crash).

• Heat and power: spinning faster requires more power and generates more heat.

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Power Issue

• Because of momentum (objects in motion tend to remain in motion), it takes more power to start the drive spinning than to keep it spinning. – This is one place where the 12V connection is used

– “Power management” reduces the power when a device is not being used. But spinning up a hard drive requires more power than maintaining the rotation, so continually starting and stopping could cost power.

• Also the starting and stopping could be detrimental to the drive.

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Power Issue (Cont.)

• When a computer has multiple drives, e.g. a master/slave set up, the slave may delay its spin up until the master has completed its spin up so they are not pulling on the power simultaneously. – One may have to use a particular jumper setting

to ensure this delay.

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Cover and Base Casting

Don’t open the case (unless your room is clean).

The screws in the case are unusual to prevent you from opening it.

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Yes to Air / No to Dust

• The hard drive cannot be completely sealed off, it needs air to keep it from overheating.

• On the other hand, the slightest spec of dust can cause a head crash.

• A hard drive needs an air filtration and circulation system. – The circulation occurs naturally in that the spinning

disks cause the air to flow. – There is a filter to keep dirt out. It does not need

changing.

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Heat Issues Revisited

• Recall that moving air (convection) is the primary mechanism for cooling computer devices and that the spinning platters get the air moving inside hard drives.

• Thus typical hard drives (ATA/IDE and SCSI with moderate spindle speeds) can use passive cooling.

• High speed SCSI drives may need active cooling (i.e. fans).

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Drive cooler

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Bay cooler

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Retail vs. OEM

• Retail– Hard Disk Drive, Installation Instructions,

Drivers and/or Overlay Software, Mounting Hardware, Interface Cable, Warranty Card

• OEM (original equipment manufacturer)– Hard Disk Drives and Jumpers (maybe)

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References

• PC Hardware in a Nutshell, Thompson and Thompson

• http://www.pcguide.com

• All-in-One A+ Certification, Meyers and Jernigan