1. Introduction
1.1. A disk is a circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetizable material
1.2. Benefits of the glass substrate:
1.2.1. - Improvement in the uniformity of the magnetic film surface to increase disk reliability - A significant reduction in overall surface defects to help reduce read-write errors Ability to support lower fly heights - Better stiffness to reduce disk dynamics - Greater ability to withstand shock and damage
2. Magnetic Read and Write Mechanisms
2.1. Data are recorded on and later retrieved from the disk via a conducting coil named the head
2.2. The write mechanism exploits the fact that electricity flowing through a coil produces a magnetic field
2.3. Electric pulses are sent to the write head and the resulting magnetic patterns are recorded on the surface below, with different patterns for positive and negative currents
2.4. The write head itself is made of easily magnetizable material and is in the shape of a rectangular doughnut with a gap along one side and a few turns of conducting wire along the opposite side
2.5. An electric current in the wire induces a magnetic field across the gap, which in turn magnetizes a small area of the recording medium
2.6. Reversing the direction of the current reverses the direction of the magnetization on the recording medium
3. Data Organization and Formatting
3.1. The head is a relatively small device capable of reading from or writing to a portion of the platter rotating beneath it. This gives rise to the organization of data on the platter in a concentric set of rings, called tracks. Each track is the same width as the head. There are thousands of tracks per surface.
3.2. A bit near the center of a rotating disk travels past a fixed point (such as a read– write head) slower than a bit on the outside.
3.3. The information can then be scanned at the same rate by rotating the disk at a fixed speed, known as the constant angular velocity
3.4. Because the density, in bits per linear inch, increases in moving from the outermost track to the innermost track, disk storage capacity in a straightforward CAV system is limited by the maximum recording density that can be achieved on the innermost track. To increase density, modern hard disk systems use a technique known as multiple zone recording, in which the surface is divided into a number of concentric zones (16 is typical). Within a zone, the number of bits per track is constant.
3.5. Zones farther from the center contain more bits (more sectors) than zones closer to the center. This allows for greater overall storage capacity at the expense of somewhat more complex circuitry. As the disk head moves from one zone to another, the length (along the track) of individual bits changes, causing a change in the timing for reads and writes.
4. Physical Characteristics
4.1. Fixed-head disk
4.1.1. One read-write head per track
4.1.2. Heads are mounted on a fixed ridged arm that extends across all tracks
4.2. Movable-head disk
4.2.1. One read-write head
4.2.2. Head is mounted on an arm
4.2.3. The arm can be extended or retracted
4.3. Non-removable disk
4.3.1. Permanently mounted in the disk drive
4.3.2. The hard disk in a personal computer is a non-removable disk
4.4. Removable disk
4.4.1. Can be removed and replaced with another disk
4.4.2. Advantages: - Unlimited amounts of data are available with a limited number of disk systems - A disk may be moved from one computer system to another
4.4.3. Floppy disks and ZIP cartridge disks are examples of removable disks
4.5. Double sided disk
4.5.1. Magnetizable coating is applied to both sides of the platter
5. Disk Classification
5.1. The head mechanism provides a classification of disks into three types
5.1.1. - Head positioned a fixed distance above platter - Head comes into physical contact with platter - The head must generate or sense an electromagnetic field of sufficient magnitude to write and read properly
5.2. The narrower the head, the closer it must be to the platter surface to function
5.2.1. A narrower head means narrower tracks and therefore greater data density
5.3. The closer the head is to the disk the greater the risk of error from impurities or imperfections
6. Winchester Heads
6.1. - Used in sealed drive assemblies that are almost free of contaminants - Designed to operate closer to the disk’s surface than conventional rigid disk heads, thus allowing greater data density Is actually an aerodynamic foil that rests lightly on the platter’s surface when the disk is motionless
7. Disk Performance Parameters
7.1. - When the disk drive is operating the disk is rotating at constant speed - To read or write the head must be positioned at the desired track and at the beginning of the desired sector on the track
7.2. Seek time
7.2.1. On a movable–head system, the time it takes to position the head at the track
7.3. Rotational delay (rotational latency)
7.3.1. The time it takes for the beginning of the sector to reach the head
7.4. Access time
7.4.1. - The sum of the seek time and the rotational delay - The time it takes to get into position to read or write
7.5. Transfer time
7.5.1. - Once the head is in position, the read or write operation is then performed as the sector moves under the head - This is the data transfer portion of the operation
8. RAID
8.1. Consists of 7 levels
8.2. Levels do not imply a hierarchical relationship but designate different design architectures that share three common characteristics:
8.2.1. - Set of physical disk drives viewed by the operating system as a single logical drive - Data are distributed across the physical drives of an array in a scheme known as striping - Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure
9. Optical Memory
9.1. Optical Disk Products
9.1.1. CD(all variety) and DVD(all variety)
9.2. Compact Disk - CD-ROM
9.2.1. Audio CD and the CD-ROM share a similar technology
9.2.2. Production: - The disk is formed from a resin such as polycarbonate - Digitally recorded information is imprinted as a series of microscopic pits on the surface of the polycarbonate This is done with a finely focused, high intensity laser to create a master disk - The master is used, in turn, to make a die to stamp out copies onto polycarbonate - The pitted surface is then coated with a highly reflective surface, usually aluminum or gold - This shiny surface is protected against dust and scratches by a top coat of clear acrylic - Finally a label can be silkscreened onto the acrylic
9.2.3. CD-ROM is appropriate for the distribution of large amounts of data to a large number of users - Because the expense of the initial writing process it is not appropriate for individualized applications - The CD-ROM has two advantages: - The optical disk together with the information stored on it can be mass replicated inexpensively - The optical disk is removable, allowing the disk itself to be used for archival storage - The CD-ROM disadvantages: It is read-only and cannot be updated It has an access time much longer than that of a magnetic disk drive
9.3. Compact Disk - CD Recordable (CD-R)
9.3.1. - Write-once read-many - Accommodates applications in which only one or a small number of copies of a set of data is needed - Disk is prepared in such a way that it can be subsequently written once with a laser beam of modest-intensity - Medium includes a dye layer which is used to change reflectivity and is activated by a high-intensity laser - Provides a permanent record of large volumes of user data
9.4. Compact Disk - CD Rewritable (CD-RW)
9.4.1. - Can be repeatedly written and overwritten - Phase change disk uses a material that has two significantly different reflectivities in two different phase states - A beam of laser light can change the material from one phase to the other - Disadvantage is that the material eventually and permanently loses its desirable properties - Advantage is that it can be rewritten
9.5. Digital Versatile Disk
9.5.1. - The DVD has replaced the videotape used in video cassette recorders (VCRs) and replace the CD-ROM in personal computers and servers. - Vast volumes of data can be crammed onto the disk, currently seven times as much as a CD-ROM. - DVD’s huge storage capacity and vivid quality, PC games have become more realistic and educational software incorporates more video.
9.5.2. The DVD’s greater capacity is due to three differences from CDs:
9.5.2.1. Bits are packed more closely on a DVD. The spacing between loops of a spiral on a CD is 1.6 μm and the minimum distance between pits along the spiral is 0.834 μm. The DVD uses a laser with shorter wavelength and achieves a loop spacing of 0.74 μm and a minimum distance between pits of 0.4 μm. The result of these two improvements is about a seven-fold increase in capacity, to about 4.7 GB.
9.5.2.2. The DVD employs a second layer of pits and lands on top of the first layer. A dual-layer DVD has a semi-reflective layer on top of the reflective layer, and by adjusting focus, the lasers in DVD drives can read each layer separately. This technique almost doubles the capacity of the disk, to about 8.5 GB. The lower reflectivity of the second layer limits its storage capacity so that a full doubling is not achieved.
9.5.2.3. The DVD-ROM can be two sided, whereas data are recorded on only one side of a CD. This brings total capacity up to 17 GB.
9.6. High-Definition Optical Disks
9.6.1. - Designed to store high-definition videos and to provide significantly greater storage capacity compared to DVDs. - The higher bit density is achieved by using a laser with a shorter wavelength, in the blue-violet range. - Two competing disk formats and technologies initially competed for market acceptance: HD DVD and Blu-ray DVD. - - The Blu-ray scheme ultimately achieved market dominance. - The HD DVD scheme can store 15 GB on a single layer on a single side. Blu-ray can store 25 GB on a single layer. - Three versions are available: read only (BD-ROM), recordable once (BD-R), and re-recordable (BD-RE).
9.7. Magnetic Tape
9.7.1. - Tape systems use the same reading and recording techniques as disk systems - Medium is flexible polyester tape coated with magnetizable material - Coating may consist of particles of pure metal in special binders or vapor-plated metal films - Data on the tape are structured as a number of parallel tracks running lengthwise - Serial recording - Data are laid out as a sequence of bits along each track - Data are read and written in contiguous blocks called physical records - Blocks on the tape are separated by gaps referred to as inter-record gaps