External Memory. Types of External Memory. Magnetic Disk. Optical. Magnetic Tape. RAID Removable. CD-ROM CD-Recordable (CD-R) CD-R/W DVD

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1 External Memory 1 Types of External Memory Magnetic Disk RAID Removable Optical CD-ROM CD-Recordable (CD-R) CD-R/W DVD Magnetic Tape 2 1

2 3 Magnetic Disk Disk substrate coated with magnetizable material (iron oxide rust) Substrate used to be aluminium Now glass Improved surface uniformity Increases reliability Reduction in surface defects Reduced read/write errors Lower flight heights Better stiffness Better shock/damage resistance 4 2

3 Read and Write Mechanisms Recording & retrieval via conductive coil called a head May be single read/write head or Multiple heads During read/write, head is stationary, platter rotates Write Current through coil produces magnetic field Pulses sent to head Magnetic pattern recorded on surface below Read (traditional) Magnetic field moving relative to coil produces current Coil is the same for read and write Read (contemporary) Separate read and write heads Partially shielded magneto resistive (MR) sensor Electrical resistance depends on direction of magnetic field High frequency operation Higher storage density and speed 5 Inductive Write MR Read 6 3

4 Data Organization and Formatting Concentric tracks Gaps between tracks Reduce gap to increase capacity Same number of bits per track (variable packing density) Constant angular velocity Tracks divided into sectors A number of sectors make up a Block (Cluster, Disk Allocation Unit) Minimum block size is one sector 7 8 4

5 Disk Data Layout 9 Disk Velocity Bits near centre of rotating disk pass fixed point slower than bits on outside of disk Increased spacing between bits in different tracks Disk rotates at a constant angular velocity (CAV) Gives rise to pie shaped sectors across tracks and concentric tracks Individual tracks and sectors are addressable Move head to given track and wait for given sector to pass under head Space underused on outer tracks Lower data density Zones are used to increase capacity Each zone has fixed bits per track More complex circuitry 10 5

6 Disk Layout Methods Diagram 11 Finding Sectors Must be able to identify start of track and sector Format disk Additional information not available to user Marks tracks and sectors 12 6

7 Winchester Disk Format Seagate ST Characteristics Fixed (rare) or movable head Removable or fixed Single sided or double sided (usual) Single or multiple platter Head mechanism Contact (Floppy) Fixed gap Flying (Winchester) 14 7

8 Fixed/Movable Head Disk Fixed head One read write head per track Heads mounted on fixed ridged arm Movable head One read write head per side Mounted on a movable arm 15 Removable or Not Removable disk Can be removed from drive and replaced with another disk Provides unlimited storage capacity Easy data transfer between systems Nonremovable disk Permanently mounted in the drive 16 8

9 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) 17 Multiple Platters 18 9

10 In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three platters and six read/write heads: Tracks and Cylinders 20 10

11 Storing the Data Data are stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a track: A typical track is shown in yellow; a typical sector is shown in blue. A sector contains a fixed number of bytes -- for example, 256 or 512. Either at the drive or the operating system level, sectors are often grouped together into clusters The process of low-level formatting a drive establishes the tracks and sectors on the platter. The starting and ending points of each sector are written onto the platter. This process prepares the drive to hold blocks of bytes. High-level formatting then writes the filestorage structures, like the file-allocation table, into the sectors. This process prepares the drive to hold files