Upload
kenneth-gregory
View
227
Download
1
Tags:
Embed Size (px)
Citation preview
L/O/G/Owww.themegallery.com
External MemoryChapter 3 (C)Chapter 3 (C)
CS.216 Computer Architecture and OrganizationCS.216 Computer Architecture and Organization
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 angu
lar 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 f
ormatting process
Magnetic Disks (4/9)
Gap1 Id Gap2 Data Gap3 Gap1 Id Gap2 Data Gap3
TrackSyncByte
Head Sector CRC SyncByte
Data CRC
Track & Sector Format
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 tr
ansfer to another machine
Magnetic Disks (9/9)
•Data accessing times– Seek time -- position the head over the correct t
rack– 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 (se
ctor) 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: 0 -- 5
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 (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 -- disk
s 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 exp
ensive– Number of additional disks is proportional to log of number of data di
sks
RAID Technology (8/11)
– RAID 3• Like RAID 2 however only a single redunda
nt disk is used -- the parity drive• Parity bit is computed for the set of individu
al bits in the same position on all disks• If a drive fails, parity information on the red
undant 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)
PXXP
XXXXXXP
XXXXXXP
XXXXP
XXXXP
1'1'
32101'1'
1132'10'
32'10'
3210
Write Penalty RAID 4 & 5
RAID Technology (11/11)
–Raid 5• Parity information is distributed on dat
a 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 rat
e -- 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 mediu
m power laser• Can be read many times just like a normal CD RO
M• Permits archival storage of user information, distrib
ution of large amounts of information by a user
Optical Disks (5/5)
– Erasable optical disk• Combines laser and magnetic technology to p
ermit information storage• Laser heats an area that can then have an e-fi
eld 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 physic
al air gaps between records• One words is stored across the width of t
he tape and read using multiple read/write heads
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