CH06 External Memory Types of External Memory

CH06 External Memory
Types of External Memory
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• Magnetic Disk
Magnetic Disk
RAID
Optical Memory
Magnetic Tape
4RAID
4Removable
• Optical
4CD-ROM
4CD-Writable (WORM)
4CD-R/W
4DVD
• Magnetic Tape
TECH
Computer Science
CH05
Magnetic Disk
Data Organization and Formatting
• Metal or plastic disk coated with magnetizable
material (iron oxide…rust)
• Range of packaging
• Concentric rings or tracks
4Floppy
4Winchester hard disk
4Removable hard disk
Disk Data Layout
4Gaps between tracks
4Reduce gap to increase capacity
4Same number of bits per track (variable packing
density)
4Constant angular velocity
• Tracks divided into sectors
• Minimum block size is one sector
• May have more than one sector per block
Fixed/Movable Head Disk
• Fixed head
4One read write head per track
4Heads mounted on fixed ridged arm
• Movable head
4One read write head per side
4Mounted on a movable arm
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Fixed and Movable Heads
Removable or Not //
• Removable disk
4Can be removed from drive and replaced with another
disk
4Provides unlimited storage capacity
4Easy data transfer between systems
• Nonremovable disk
4Permanently mounted in the drive
Floppy Disk
Winchester Hard Disk (1)
• 8”, 5.25”, 3.5”
• Small capacity
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4Up to 1.44Mbyte (2.88M never popular)
• Slow (disk rotate at 300 and 600 rpm, average delay
100/2 and 200/2 ms.)
• Universal
• Cheap
Winchester Hard Disk (2)
• Universal
• Cheap
• Fastest external storage (typically rotate 3600 rpm,
newer faster, average rotational delay 8.3 ms.)
• Getting larger all the time
4Multiple Gigabyte now usual
Developed by IBM in Winchester (USA)
Sealed unit
One or more platters (disks)
Heads fly on boundary layer of air as disk spins
(crash into disk!)
• Very small head to disk gap
• Getting more robust
Removable Hard Disk
• ZIP
4Cheap
4Very common
4Only 100M
• JAZ
4Not cheap
41G
• L-120 (a: drive)
4Also reads 3.5” floppy
4Becoming more popular?
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Finding Sectors
ST506 format (old!)
• Must be able to identify start of track and sector
• Format disk
4Additional information not available to user
4Marks tracks and sectors
Gap1 Id
Sync
Byte
Gap2 Data Gap3 Gap1 Id
Track Head Sector CRC
Gap2 Data Gap3
Sync
Byte
Data CRC
• Foreground reading
4Find others
Characteristics
Multiple Platter
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Fixed (rare) or movable head
Removable or fixed
Single or double (usually) sided
Single or multiple platter
Head mechanism
4Contact (Floppy)
4Fixed gap
4Flying (Winchester)
Speed
One head per side
Heads are joined and aligned
Aligned tracks on each platter form cylinders
Data is striped by cylinder
4reduces head movement
4Increases speed (transfer rate)
Sequential organization vs. random access e.g.
• Seek time
4Moving head to correct track
• (Rotational) latency
4Waiting for data to rotate under head
• Access time = Seek + Latency
• Transfer rate T = (number of bytes to be
transferred)/(rotation speed)/(number of bytes on a
track) = b/(rN)
• total access time Ta = Ts + 1/(2r) + b/(rN)
4e.g. a hard disk has average seek time of 20 ms, a
transfer rate of 1 M byte/s, and 512 byte sectors with 32
sectors per track. Need to read a file consisting 256
sectors for a total of 128 K bytes. What is the total time
for the transfer?
• Case 1: Sequential Organization (256 sectors on 8
tracks x 32 sectors/tracks)
4Average seek time = 20.0 ms
4Rotational delay = 8.3 ms
4Read 32 sections (one track) = 16.7 ms
4total time to read first track = 45 ms
4Total time = 45 ms + 7*(8.3 + 16.7) ms = 0.22 s
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Time required for random access on highly fragmented organization
Optical Storage CD-ROM //
• Case 2: random access rather than sequential access
4Average seek time = 20.0 ms
4Rotational delay = 8.3 ms
4Read 1 sector = 16.7/32 = 0.5 ms
4time to read one sector = 28.8 ms
4Total time = 256 * 28.8 ms = 7.37 s
• Originally for audio
• 650Mbytes giving over 70 minutes audio
• Polycarbonate coated with highly reflective coat,
usually aluminum
• Data stored as pits
• Read by reflecting laser
• Constant packing density
• Constant linear velocity
• De-fragment you hard disk!
Constant Angular Velocity vs. Constant
Linear Velocity
CD-ROM Drive Speeds
• Audio is single speed
4Constant linear velocity
41.2 ms-1
4Track (spiral) is 5.27km long
4Gives 4391 seconds = 73.2 minutes
4Date 176.4 K bytes/s total capacity 774.57 M Bytes
• Other speeds are quoted as multiples
• e.g. 24x ~= 4 M Bytes/s (data transfer rate)
• The quoted figure is the maximum the drive can
achieve
Random Access on
CD-ROM
12 byte
Sync
4 byte
Id
Mode
Sector
FF
00 x 10 00
Min
Sec
CD-ROM Format
Data
2048 byte
Layered
ECC
288 byte
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Difficult
Move head to rough position
Set correct speed
Read address
Adjust to required location
(Yawn!)
2352 byte
• Mode 0=blank data field
• Mode 1=2048 byte data+error correction
• Mode 2=2336 byte data
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CD-ROM for & against
Other Optical Storage
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• CD-Writable
Large capacity (?)
Easy to mass produce
Removable
Robust
• Expensive for small runs
• Slow
• Read only
4WORM
4Now affordable
4Compatible with CD-ROM drives
• CD-RW
4Erasable
4Getting cheaper
4Mostly CD-ROM drive compatible
DVD - what’s in a name?
DVD - technology
• Digital Video Disk
• Multi-layer
• Very high capacity (4.7G per layer)
• dual-layer (single-sided ?) hold 8.5 Gbytes ~> 4hr
movie
• Full length movie on single disk
4Used to indicate a player for movies
f Only plays video disks
• Digital Versatile Disk
4Used to indicate a computer drive
f Will read computer disks and play video disks
• Dogs Veritable Dinner
• Officially - nothing!!!
4Using MPEG compression
• Finally standardized (honest!)
• Movies carry regional coding
• Players only play correct region films
DVD - Writable
Foreground Reading
• Loads of trouble with standards
• First generation DVD drives may not read first
generation DVD-W disks
• First generation DVD drives may not read CD-RW
disks
• Wait for it to settle down before buying!
• Check out optical disk storage options
• Check out Mini Disk
5
Magnetic Tape
Digital Audio Tape (DAT)
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• Uses rotating head (like video)
• High capacity on small tape
Serial access
Slow
Very cheap
Backup and archive
RAID
44Gbyte uncompressed
48Gbyte compressed
• Backup of PC/network servers
RAID Levels 0, 1, 2
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Redundant Array of Independent Disks
Redundant Array of Inexpensive Disks
6 levels in common use
Not a hierarchy
Set of physical disks viewed as single logical drive
by O/S
• Data distributed across physical drives
• Can use redundant capacity to store parity
information
RAID Levels 3, 4
RAID Levels 5, 6
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RAID 0
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No redundancy
Data striped across all disks
Round Robin striping
Increase speed
4Multiple data requests probably not on same disk
4Disks seek in parallel
4A set of data is likely to be striped across multiple disks
RAID 1
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Mirrored Disks
Data is striped across disks
2 copies of each stripe on separate disks
Read from either
Write to both
Recovery is simple
4Swap faulty disk & re-mirror
4No down time
• Expensive
RAID 2
• Disks are synchronized
• Very small stripes
4 Often single byte/word
• Error correction calculated across corresponding bits on disks
• Multiple parity disks store Hamming code error correction in
corresponding positions
• Lots of redundancy
RAID 3
• Similar to RAID 2
• Only one redundant disk, no matter how large the
array
• Simple parity bit for each set of corresponding bits
• Data on failed drive can be reconstructed from
surviving data and parity info
• Very high transfer rates
4 Expensive
4 Not used
RAID 4
RAID 5
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Each disk operates independently
Good for high I/O request rate
Large stripes
Bit by bit parity calculated across stripes on each disk
Parity stored on parity disk
Like RAID 4
Parity striped across all disks
Round robin allocation for parity stripe
Avoids RAID 4 bottleneck at parity disk
Commonly used in network servers
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RAID 6
• Two different parity calculations are carried out and
• stored in separate blocks on different disks.
• Able to regenerate data even if two disks containing
user data fail
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