L/O/G/O External Memory Chapter 3 (C) CS.216 Computer Architecture and Organization

External Memory Magnetic disks Compact Disk Tape

Magnetic Disks (1/9) –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

Magnetic Disks (2/9) –Tracks are separated by gaps –Disk rotates at a constant speed -- constant angular velocity Number of data bits per track is a constant 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

Magnetic Disks (3/9)

Magnetic Disks (4/9) Gap1 IdGap2DataGap3 Gap1 IdGap2DataGap3 Track Sync Byte HeadSectorCRC Sync Byte DataCRC Track & Sector Format

Magnetic Disks (5/9) Disk Cylinder

Magnetic Disks (6/9) –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

Magnetic Disks (9/9) 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

RAID Technology (1/11) –Disk drive performance has not kept pace with improvements 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

RAID Technology (2/11) –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 Technology (3/11)

RAID Technology (4/11) –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 Can be used to support high data transfer rates by having block transfer size be in multiples of the strip Can support low response time by having the block transfer size equal a strip -- support multiple strip transfers in parallel

RAID Technology (5/11)

RAID Technology (6/11) –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 Technology (7/11) –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 Technology (8/11) –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 the data from the failed disk “on the fly”

RAID Technology (9/11) –RAID 4 Access is to individual strips rather than to all disks at once (RAID 3) Bit-by-bit parity is calculated across corresponding strips on each disk Parity bits stored in the redundant disk Write penalty –For every write to a strip, the parity strip must also be recalculated and written –Thus 1 logical write equals 2 physical disk accesses –The parity drive is always written to and can thus be a bottleneck Write-Write Concurrent Access is not possible

RAID Technology (10/11) Write Penalty RAID 4 & 5

RAID Technology (11/11) –Raid 5 Parity information is distributed on data disks in a round-robin scheme No parity disk needed Write-Write Concurrent Access may be possible

Optical Disks (1/5) –Advent of CDs in the early 1980s revolutionized the audio and computer industries –Basic operation 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, –Mechanical pressing by a Master Disk –a laser is used to burn pits into the track -- write once (CD-R)!

Optical Disks (2/5) –Basic operation (cont.) 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

Optical Disks (3/5) –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 –Capacity 650Mbytes giving over 70 minutes audio –Only economical for production of large quantities of disks –Disks are removable and thus archival –Slower than magnetic disks

Optical Disks (4/5) –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

Optical Disks (5/5) –Erasable optical disk Combines laser and magnetic technology to permit information storage Laser heats an area that can then have an e- field orientation changed to alter information storage “State of the e-field” can be detected using polarized light during reads

Magnetic Tape (1/2) –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

Magnetic Tape (2/2)

Summary Goal of the memory hierarchy is to produce a memory system that has an average access time of roughly the L1 memory and an average cost per bit roughly equal to the lowest level in the hierarchy Range of performance spans 10 orders of magnitude! Components / levels discussed –Cache –Main memory –Secondary memory

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