Magnetic disk• Based on the concept of magnetism.• A disk is a circular platter constructed of nonmagnetic material,called the substrate, coated with a magnetizable material like ironoxide. Particles are magnetized to store data.• Substrate used to be aluminum.• Nowadays, glass is used because: improved magnetic film surface uniformity, thus reliability. reduction in surface defects, helps to reduce read/write errors. Support lower fly heights.
Magnetic disk- write mechanism• Data recorded and retrieve via a conducting coil, called a head.• Head, consists of a core, with an air gap and a set of coils.• May be separate read and write head.• During read/write, the head is stationary, the platter rotatesbeneath it .• For writing data onto the medium, the data is converted intocurrent and is passed through the read/write head coils. Thisgenerates a magnetic flux. This magnetic flux, is used tomagnetize the particles on the surface.
Magnetic disk- read mechanism• During the read operation, when the flux transitions pass under ahead gap, a voltage is introduced in the read/write coils. Thisvoltage is converted into the data pulses.• New method: separate read head, positioned close to write head forconvenience. Read head consists of a partially magnetoresistive (MR) sensor. MR material has an electrical resistance, which depends on thedirection of magnetic field moving under it.
Read mechanism contd…• By passing current through MR sensor, resistancechanges are detected as voltage levels.• Allows greater storage density , high frequencyoperation and high speeds.
Disk organization and formatting• Concentric Rings or tracksEach track has the width same as of disk head.Tracks separated by gaps, minimizes errors in case of interference orhead misalignment.• tracks divided into sectors- pie shaped segments, data transferred toand from the disk sectors.• Minimum block size is one sector; 512 bytes in most systems.• May have more than one sector per block.
Disk Velocity• Bit near centre of rotating disk passes fixed point slower than bit onoutside of disk• Increase spacing between bits in different tracks• Rotate disk at constant angular velocity (CAV) Gives pie shaped sectors and concentric tracks Individual tracks and sectors addressable Move head to given track and wait for given sector Waste of space on outer tracksLower data density• Can use zones to increase capacity Each zone has fixed bits per track More complex circuitry
Multiple Platter• One head per side• Heads are joined and aligned• Aligned tracks on each platter form cylinders• Data is striped by cylinder reduces head movement Increases speed (transfer rate)• Cylinder- set of all tracks in same relative position on theplatter.
Magnetic disk types• Floppy disks• Hard diskso Zip/bernoulli disk- on platter, 100 MB capacityo Winchester disk- two or more platters, capacity in GBs.
Speed/ Access time• Seek time: time taken by head to move to a specific track• Latency/Rotational delay: time required to spin the desired sectorunder the head.• Transfer time: time taken to transfer data.• Access time=seek time + latency+ transfer time
RAID•Redundant array of independent disks.•Coined at University of Berkeley in 1987 as redundant arrayof inexpensive disks.•Simultaneous data access by using multiple heads.•Separate physical drives viewed by os a single drive.
Stripping:Data is segmented into equal-size partitions (thestriping unit), which are transparently distributedacross multiple disks.Mirroring:Data is replicated across multiple redundant disks
Compact Disk• First made in 1983.• Compact Disk types• CR-Read only• CD-Recordable• CD-R/W• Data is encoded and read optically with a laser• Can store around 600MB to 700MB data
Data representation in CD-ROM• Digital data is represented as a series of Pits andLands:– Pit = a little depression, forming a lower level in the track– Land = the flat part between pits, or the upper levels in thetrack
Organization of data• Reading a CD is done by shining a laser at the disc anddetecting changing reflections patterns.– 1 = change in height (land to pit or pit to land)– 0 = a “fixed” amount of time between 1’sLAND PIT LAND PIT LAND...------+ +-------------+ +---...|_____| |_______|..0 0 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 0 1 0 0 ..• Note : we cannot have two 1’s in a row!
CD-ROM• Because of the heritage from CD audio, the data is storedas a single spiral track on the CD-ROM, contrary tomagnetic hard disk’s discrete track concept.• Thus, the rotation speed is controlled by CLV-ConstantLinear velocity. The rotational speed at the center ishighest, slowing down towards the outer edge. Because,the recording density is the same every where.• Note that with CLV, the linear speed of the spiral passingunder the R/W head remains constant.• CLV is the result for the poor seek time in CD-ROMs• The advantage of CLV is that the disk is utilized at its bestcapacity, as the recording density is the same every where.
CD-ROM• Note that: Since 0s are represented by the length oftime between transitions, we must travel at constantlinear velocity (CLV)on the tracks.• Sectors are organized along a spiral• Sectors have same linear length• Advantage: takes advantage of all storage spaceavailable.• Disadvantage: has to change rotational speed whenseeking (slower towards the outside)
Digital Versatile Disc•The DVD (Digital Video Disc or Digital Versatile Disc)technology is based on CD technology withincreased storage density.• The DVD’s come with a storage capacity of up to17GB• Multi-layer• Very high capacity (4.7GB to 17GB)• Has same three types as CD