Chapter4: Memory External Memory

Types of External Memory Magnetic Disk RAID (Redundant Array of Independent Disks) Removable Optical CD-ROM CD-Recordable (CD-R) CD-R/W DVD Magnetic Tape Magnetic disks – most important component of external memory range from PC to mainframe and super computer RAID disk technology used to achieve great performance and higher availability Optical storage technology become increasingly important for all types of computer systems. CD and DVD are becoming increasingly important

Disk substrate coated with magnetizable material (iron oxide…rust) Magnetic Disk Disk substrate coated with magnetizable material (iron oxide…rust) Substrate used to be aluminium or aluminium alloy material Now glass Improved surface uniformity Increases reliability Reduction in surface defects Reduced read/write errors Lower flight heights Better stiffness Reduce disk dynamics Better shock/damage resistance A disk is a circular platter constructed of nonmagnetic material called substrate Traditionally, substrate has been an aluminium or aluminium alloy material More recently, glass substrate have been reduced 5 advantages of using glass

Read and Write Mechanisms Recording & retrieval via conductive coil called a head May be single read/write head or separate ones 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 Used in floppy disk systems Read (contemporary): Separate read head, close to write head Consists of partially shielded magnetoresistive (MR) sensor Electrical resistance depends on direction of magnetic field High frequency operation Higher storage density and speed Write: electric pulses sent to write head, and the resulting magnetic pattern recorded on surface below with different pattern for + and – current Read : exploit the fact that magnetic field moving relative to a coil produces electrical current in the coil. When surface passes under the head, it generates current of the same polarity as the one already recorded Read contemporary: by passing current thru MR sensor, resistance changes are detected as voltage signals. MR Design allows high frequency operation, which equates to greater storage densities and operating speeds.

Inductive Write MR Read Inductive Write/MR Read Head Current induces magnetic field across the gap, in turn magnetizes small area of recording medium Reversing current direction reverses the direction of magnetization on the recording medium

Data Organization and Formatting Concentric rings or 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 Minimum block size is one sector May have more than one sector per block Head is relatively small device capable of reading from and writing to a portion of a platter rotating beneath it.

Disk Data Layout To avoid imposing unreasonable precision requirement on the system Error due to misalignment or simply interference of magnetic field Hundreds of sectors per track. May be fixed or variable length. Most contemporary systems, fixed-length are used Disk data layout Adjacent tracks are separated by gaps. This to prevent/minimizes error Data are transferred to and from the disk in sectors

Disk Velocity Bit near centre of rotating disk passes fixed point slower than bit on outside 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 tracks Lower data density Can use zones to increase capacity (density) Each zone has fixed bits per track More complex circuitry (allows for greater overall storage capacity) Density = in bits per linear inch Some way must found to compensate for the variation in speed so that the head can read all the bits at the same rate Information can then be scanned at the same rate by rotating the disk at the fixed speed, known as CAV Constant = fixed speed Zones = multi zone

Disk Layout Methods Diagram The layout of a disk using CAV (b) The nature of multiple zone recording

Must be able to identify start of track and sector Format disk Finding Sectors Must be able to identify start of track and sector Format disk Additional information not available to user Marks tracks and sectors

Winchester Disk Format Seagate ST506 Each track contains 30 fixed length sectors of 600 bytes each Each sector holds 512 bytes of data plus control infos useful to the disk controller ID field used to locate a particular sector & SYNCH byte delimits the beginning of the field Track# identifies a track on a surface & Head# identifies a head because this disk has multiple surfaces ID and data field, each contain an error detecting code ID field, unique identifier/address SYNCH byte, a special bit pattern

Physical Characteristics Head motion Fixed (rare) or movable head Disk portability Removable or fixed Sided Single or double (usually) sided Platter Single or multiple platter Head mechanism Contact (Floppy) Fixed gap Flying gap (Winchester)

Fixed/Movable Head Disk Fixed head One read-write head per track Heads mounted on fixed ridged arm, extends across all the tracks Movable head One read write head per side Mounted on a movable arm Arm can be extended/retracted for positioning above the track Fixed head – very rare today

Sided/Double Sided Most disks: magnetizable coating applied to both side Only cheap disk use single-sided

Removable or Not Removable disk Nonremovable disk Can be removed from drive and replaced with another disk Provides unlimited storage capacity Easy data transfer between systems Eg: floppy disk, ZIP cartridge Nonremovable disk Permanently mounted in the drive Eg: hard disk in a PC

Heads are joined and aligned 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)

Multiple Platters 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)

Tracks and Cylinders Cylinder: set of all the tracks in the same relative position on the platter All the shaded tracks are part of one Cylinder Head mechanism- physical contact with medium during read/write operation

Floppy Disk 8”, 5.25”, 3.5” Small capacity Slow Universal Cheap Up to 1.44Mbyte (2.88M never popular) Slow Universal Cheap Obsolete?

Winchester Hard Disk (1) Developed by IBM in Winchester (USA) Sealed unit One or more platters (disks) Heads fly on boundary layer of air as disk spins Very small head to disk gap Getting more robust

Winchester Hard Disk (2) Universal Cheap Fastest external storage Getting larger all the time 250 Gigabyte now easily available

Access time = Seek + Latency Transfer rate Speed Seek time Moving head to correct track (Rotational) latency Waiting for data to rotate under head Access time = Seek + Latency Transfer rate Time to/from the disk depends on the rotation speed of the disk T = b/rN (b= no. Of bytes to be transferred r = rotation speed N = no of bytes on a track)

Timing of Disk I/O Transfer Timing of a Disk I/O transfer disk I/O operation depends on the computer system, OS, nature of I/O channel and disk controller hardware

Assignment#2 – RAID What is RAID? RAID has a total of seven levels. Briefly explain for each of them. Due date: 31Oct2011