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External Memory

Module by: Nguyen Thi Hoang Lan

1. Magnetic disks

1.1 Magnetic disks overview

The magnetic disks are the foundation of external memory on virtually all computer system. Both removable and fixed disk or hard disk are used in computer system from personal computer to mainframe or supercpomputer.
  • Principle
– The disk is a metal or plastic platter coated with magnetizable material
– Data is recorded onto and later read from the disk using a conducting coil, the head
– Data is organized into concentric rings, called tracks, on the platter
– Tracks are separated by gaps
– Disk rotates at a constant speed – constant angular velocity
The number of data bits per track is a constant
The data density is higher on the inner tracks
– Logical data transfer unit is the sector
Sectors are identified on each track during the formatting process
  • Disk characteristics
- Single vs. multiple platters per drive (each platter has its own read/write head)
- Fixed vs. movable head. Fixed head has a head per track. Movable head uses one head per platter
- Removable vs. nonremovable platters. Removable platter can be removed from disk drive for storage of transfer to another machine
- Data accessing times:
+ Seek time -- position the head over the correct track
+ Rotational latency -- wait for the desired sector to come under the head
+ Access time -- seek time plus rotational latency
+ Block transfer time -- time to read the block (sector) off of the disk and
transfer it to main memory.
Figure 1
Figure 12.1. Disk Organization

1.2 RAID Technology

The RAID (Redundant Array of Independent Disk) technology can obtain greater performance and higher availability. RAID refers to a family of techniquesfor using multiple disks as a paralle array of data storage devices with redundant built in to compensate for disk failure.
– Disk drive performance has not kept pace withimprovements in other parts of the system
– Limited in many cases by the electromechanical transport means
– Capacity of a high performance disk drive can be duplicated by operating many (much cheaper) disks in parallel with simultaneous access
– Data is distributed across all disks
– With many parallel disks operating as if they were a single unit, redundancy techniques can be used to guard against data loss in the unit (due to aggregate failure rate being higher)
– “RAID” developed at Berkeley – Redundant Array of Independent Disks
  • Six levels RAID: 0 – 5
– RAID 0
» No redundancy techniques are used
» Data is distributed over all disks in the array
» Data is divided into strips for actual storage similar in operation to interleaved
memory data storage
» RAID can be used to support high data transfer rates by having block transfer size be in multiples of the strip
» RAID can support low response time by having the block transfer size equal a strip --
support multiple strip transfers in parallel
– RAID 1
All disks are mirrored – duplicated Data is stored on a disk and its mirror Read from either the disk or its mirror Write must be done to both the disk and mirror
  • Fault recovery is easy -- use the data on the mirror
  • System is expensive!
– RAID 2
All disks are used for every access – disks are synchronized together
Data strips are small (byte)
Error correcting code computed across all disks and stored on additional disks
Uses fewer disks than RAID 1 but still expensive.Number of additional disks is
proportional to log of number of data disks
– RAID 3
Like RAID 2 however only a single redundant disk is used -- the parity drive
Parity bit is computed for the set of individual bits in the same position on all
Disks If a drive fails, parity information on the redundant disks can be used to calculate thedata from the failed disk “on the fly”

2. Optical disks

Advent of CDs in the early 1980s revolutionized the audio and computer industries
  • Basic operations
» CD is operated using constant linear velocity
» Essentially one long track spiraled onto the disk
» Track passes under the disk’s head at a constant rate -- requires the disk to change rotational speed based on what part of the track you are on
» To write to the disk, a laser is used to burn pits into the track -- write once!
» During reads, a low power laser illuminates the track and its pits
- In the track, pits reflect light differently than no pits thus allowing you to store 1s and 0s
– Characteristics:
- Master disk is made using the laser
- Master is used to “press” copies in a mass production mechanical style
- Cheaper than production of information on magnetic disks
- Storage capacity roughly 775 NB or 550 3.5” disks
- Transfer rate standard is 176 MB/second
- Only economical for production of large quantities of disks
- Disks are removable and thus archival
- Slower than magnetic disks.
  • WORMs -- Write Once, Read Many disks
» User can produce CD ROMs in limited quantities
» Specially prepared disk is written to using a medium power laser
» Can be read many times just like a normal CD ROM
» Permits archival storage of user information, distribution of large amounts of information by a user.
  • Erasable optical disk
» Combines laser and magnetic technology to permit information storage
» Laser heats an area that can then have an efield orientation changed to alter information storage
» “State of the e-field” can be detected using polarized light during reads.

3. Magnetic Tape

– The first kind of secondary memory
– Still widely used
» Very cheap
» Very slow
– Sequential access
» Data is organized as records with physical air gaps between records
» One words is stored across the width of the tape and read using multiple read/write heads.

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