Mid 2 preparation for  Computer Architecture and Organization., Exercises for Computer Architecture and Organization

Mid 2 preparation for Computer Architecture and Organization., Exercises for Computer Architecture and Organization

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07 External Memory

Lecture - 5 External Memory

Computer Architecture Course Code: CSE360

Types of External Memory • Magnetic Disk

—RAID —Removable

• Optical —CD-ROM —CD-Recordable (CD-R) —CD-R/W —DVD

• Magnetic Tape

Magnetic Disk • A disk is a circular platter constructed of non-

magnetic material, called substrate, coated with magnetizable material (iron oxide…rust)

• Traditionally, substrate has been an aluminium • Now, glass substrate have been introduced

—Improved surface uniformity – Increases disk reliability

—Reduction in surface defects – Reduces read/write errors

—Lower flight heights (See later) —Better stiffness —Better shock/damage resistance

Read and Write Mechanisms • Recording & retrieval of data via conductive coil called a head In many systems, there are two heads, a read head and a write

head During read/write, head is stationary, platter rotates Write

Electricity flowing through coil produces magnetic field Electric pulses sent to write head Resulting magnetic patterns are recorded on surface below,

with different patterns for positive and negative currents Read (traditional)

Magnetic field moving relative to coil produces current in the coil

Coil is the same for read and write Read (contemporary)

Different read mechanism, requires separate read head, positioned close to write head

The read head consists of a partially shielded magneto resistive (MR) sensor

The MR material has an electrical resistance that depends on direction of magnetic field

By passing current through MR sensor, resistance changes are detected as voltage signals

The MR design allows high frequency operation which equates to higher storage density and speed

Inductive Write MR Read

Inductive Write An electric current in the wire induces a magnetic field, which

in turn magnetizes a small area of the recording medium. Reversing the direction of current reverses the direction of

magnetization on the recoding medium MR Read

As discussed in previous slide

Data Organization and Formatting • The head is a small device capable of reading from or

writing to a portion of the platter • This gives organization of data on the platter in a

concentric set of rings, called tracks (there are thousands of tracks per surfaces)

• Adjacent tracks are separated by gaps which prevents or reduces errors due to misalignment of head or simplify interference of magnetic fields

• Reducegap to increase capacity • Data are transferred to or from the disk in sectors • There are hundreds of sectors per track, these may be

either fixed or variable length • In most contemporary systems, fixed length sectors are

used, with 512 bytes being the universal sector size • To avoid imposing unreasonable precision on system,

adjacent sectors are separated by intratrack (intersector) gaps

Disk Data Layout

Disk Velocity • A bit near centre of rotating disk travels a fixed (such as

read-write head) point slower than a bit on outside of disk • Need to compensate for the variation in speed so that head

can read all bits at the same rate • This can be done by increasing spacing between bits in

different tracks • The information can then be scanned at the same rate by

rotating disk at fixed speed, known as constant angular velocity (CAV) — Disk is divided into pie-shaped sectors and concentric

tracks — Individual blocks of data can be directly addressed by

track and sector — To move the head from its current location to a specific

address, it only takes short movement of the head to a specific track and short wait for given sector

Waste of space on long outer tracks – Lower data density (the amount of data stored on long outer tracks

is same as what can be stored on short inner tracks)

Disk Layout Methods Diagram

• To increase density, modern hard disk systems use a technique known as multiple zone recording, in which the surface is divided into number of concentric zones (16 is typical) — Each zone has fixed bits per track Zones farther from centre contain more bits (more

sectors) — This allows greater storage capacity at the expense of

somewhat more complex circuitry

Finding Sectors • Some means is needed to locate sector

positioned within a track • Clearly, there must be some starting point

on the track and a way of identifying the start and end of each sector

• Thus, the disk is formatted with some extra data used only by disk drive and not available to user

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 information useful

to the disk controller - The ID field a unique identifier or address used to locate a particular

sector - The SYNCH byte is a special bit pattern that delimits the

beginning of the field - The track number identifies a track on a surface - The head number identifies a head (because disk has multiple

surfaces) - The ID and data fields each contain error-detecting code.

Physical Characteristics of Disk Systems

Fixed/Movable Head Disk • The head may be either fixed or movable

with respect to the radial direction of the platter

• Fixed head —One read write head per track —Heads mounted on fixed ridged arm (rare

today) • Movable head

—One read write head per side —Mounted on a movable arm (because, the

head must be able to be positioned above any track)

Removable or Not • The disk itself is mounted in a disk drive, which

consists of arm, a spindle that rotates the disk and electronics needed for I/O of binary data

• Removable disk —Can be removed from drive and replaced with

another disk —Provides unlimited storage capacity —Easy data transfer between systems —e.g. Floppy disk

• Nonremovable disk —Permanently mounted in the drive —e.g. hard disk in a personal computer is a

nonremovable disk

Sides • For most disks, the magnetizable coating

is applied to both sides of platter, which is then referred to as double sided.

• Some less expensive disk systems use single-sided disks

Multiple Platter • Some disk drives accommodate multiple platters stacked

vertically a fraction of an inch apart. • Multiple-platter disks employ a movable head, with

one read-write head per platter surface All of the heads are mechanically fixed so that all are

at the same distance from the centre of the disk and move together

Thus, at any time, all of the heads are positioned over tracks that are of equal distance from the centre of the disk

The set of all the tracks in the same relative position on the platter is referred to as a cylinder.

• Data is striped by cylinder — reduces head movement — Increases speed (transfer rate)

Multiple Platters • The read-write head has

been positioned a fixed distance above the platter, allowing air gap

• The head actually comes into physical contact with the physical medium during read or write operation (e.g. this mechanism is used with the floppy disk)

• The narrower the head is, the closer it to the platter surface

• Narrower head means narrower tracks, therefore greater data density

• The closer the head to the disk, the greater the risk of error from imperfections

• To push the technology further, Winchester disk was developed (see later)

Tracks and Cylinders • All of the shaded

tracks are part of one cylinder

Floppy Disk • 8”, 5.25”, 3.5” • Small capacity

—Up to 1.44Mbyte (2.88M never popular) • Slow • Universal • Cheap • Obsolete?

Winchester Hard Disk (1) • Developed by IBM in Winchester (USA) • Winchester heads are used in sealed-unit drive with

aerodynamic head design One or more platters (disks) • They are designed to operate closer to the disk’s

surface (very small head to disk gap) than conventional rigid disk heads, thus allowing greater data density

• The resulting noncontact system can be engineered to use narrower heads that operate closer to the platter’s surface than conventional rigid disk heads.

Getting more robust

Winchester Hard Disk (2) • Universal (Winchester disk is commonly found in

personal computers an workstations, where it is referred to as a hard disk)

Cheap Fastest external storage Getting larger all the time

—250 Gigabyte now easily available

Disk Performance Parameters: Speed • 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 that track.

Seek time — On a movable-head system, the time it takes to position the

head at the track is known as seek time(Rotational) latency

— Once the track is selected, the disk controller waits until the appropriate sector rotates to line up with the head. The time it takes for the beginning of the sector to reach the head is known as rotational delay.

Access time The sum of the seek time, if any, and the rotational delay equals

the access time (Access time = Seek + Latency), which is the time it takes to get into position to read or write.

Transfer rate 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 and the time for the transfer is the transfer time.

Timing of Disk I/O Transfer

• The actual disk I/O operation depend on the computer system, operating system, and nature of I/O channel and disk controller hardware.

• In addition to access time and transfer time, there are several queuing delays associated with I/O operation.

• When a process issues an I/O request, it must wait first for the device to be available.

• At that time, the device is assigned to the process. • If the device shares a single I/O channel or a set of I/O channels

with other disk drivers, there may be an additional wait for the channel to be available.

• At that point, the seek is performed to begin disk access.

RAID • Redundant Array of Independent Disks • Redundant array of inexpensive disks • 7 levels in common use • RAID is a 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. • Redundant disk capacity is used to store parity

information, which guarantees data recoverability in case of a disk failure.

• A parity bit is a bit that is added to a group of source bits to ensure that the number of set bits (i.e., bits with value 1) in the outcome is even or odd. It is a very simple scheme that can be used to detect single or any other odd number (i.e., three, five, etc.) of errors in the output.

RAID 0 • No redundancy Data striped across all disks

Increase speed Multiple data requests probably not on

same disk Disks seek in parallel A set of data is likely to be striped across

multiple disks

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