Input/Output Organization: Secondary Storage
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Transcript of Input/Output Organization: Secondary Storage
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Input/Output Organization:Secondary Storage
CE 140 A1/A28 August 2003
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Side Bar: Binary Prefixes Before: Since 2^10 = 1024 ≈ 1000 = 1
kilo, 1024 bytes was referred to as 1 KB – OK at first
Problem: Confusion 1 MB = 220 = 1,048,576 bytes 1 MB = 106 = 1,000,000 bytes (storage industry) 1 MB = 1,024,000 bytes (floppy disk) For 1 GB, difference is about 7%
Solution: standardize the prefixes
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Side Bar: Binary Prefixes December 1998: International
Electrotechnical Commission (IEC), organization for worldwide standardization in electrotechnology approved a standard on binary prefixes
IEEE now also uses a similar standard on a trial basis
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Side Bar: Binary PrefixesFactor Name Symbol Origin Derivation
210 kibi Ki kilobinary: (210)1 kilo: (103)1
220 mebi Mi megabinary: (210)2 mega: (103)2
230 gibi Gi gigabinary: (210)3 giga: (103)3
240 tebi Ti terabinary: (210)4 tera: (103)4
250 pebi Pi petabinary: (210)5 peta: (103)5
260 exbi Ei exabinary: (210)6 exa: (103)6
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Side Bar: Binary Prefixes Examples
1 kibibit = 1 Kibit (IEEE 1 Kb) = 210 bit = 1024 bits
1 kilobit = 1 kbit (IEEE 1 KB) = 1010 bit =
1000 bits 1 mebibyte = 1 MiB = 220
byte = 1,048,576 bytes
1 megabyte = 1 MB = 1020 byte =
1,000,000 bytes
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Secondary Storage Primary or main memory cannot
accommodate all programs at once Also, much of main memory is volatile Need for cheap, large, non-volatile
storage secondary storage Examples
Magnetic hard disks, optical disks, floppy disks, tape systems
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Magnetic Hard Disks One more disks mounted on a
common spindle Disks are placed in a rotary drive Disks rotate at uniform angular speed Read/Write head – magnetic yoke/coil
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Magnetic Hard Disk Writing
current pulses applied to magnetic coil Magnetization of film in underneath head
switches direction parallel to applied field Reading
Change in magnetic field in vicinity of head induces voltage/current in coil
Only changes can be monitored
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Magnetic Hard Disks
Source: PC Guide <http://www.pcguide.com/ref/hdd/op/index-c.html>
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Magnetic Hard Disk
Source: Structured Computer Organization by Tanenbaum
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Magnetic Hard Disk If magnetization states are presented by
1’s and 0’s, a string of 1’s or 0’s will only induce voltage at start and end of string
Clock is needed to synchronize information
Before: clock stored on a separate track Now: clock encoded together with data
Simple example: Manchester encoding
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Manchester encoding
Phase encoding Change in magnetization for each bit Space for each bit must accommodate two
changes in magnetization
0 1 0 1 1 1 0
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Winchester Technology Disks and heads are placed in sealed,
air-filtered enclosures This allows read/write heads to
operate closer to magnetic surface allowing for increased density
All modern drives use Winchester Technology
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Three Parts of a Disk System Disk – disk platters Disk Drive – electromechanical
mechanism, includes rotary drive and read/write heads
Disk Controller – controls disk operation, may or may not be part of the disk enclosure
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Physical Disk Geometry
TRACK
SECTOR
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Physical Disk Geometry Previous figure shows each track has same
number of sectors, the number of sectors in each track is the same as the maximum number of sectors that can be placed in the innermost track
Zoned Bit Recording (ZBR) – varies the number of sectors per track so that each track can be utilized more efficiently
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Physical Disk Geometry Track – concentric divisions of the surface Sector – divisions in the track Cylinder – set of corresponding tracks on all
surfaces; data on same cylinder can be accessed without moving read/write head
Three coordinates to locate data: surface number, track number, sector number
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Physical Disk Geometry CHS Geometry
C – number of cylinders (tracks per surface) H – number of heads (number of surfaces) S – number of sectors per track OK for older disks NOT OK for newer disks
Zoned Bit Recording Defect Mapping Disk controller needs translation mechanism
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Sector Usually stores 512 bytes of data Consists of preamble or sector
header Error-correcting code (ECC)
information at the end Sectors are separated by intersector
gaps
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Magnetic Hard Disks
Source: Structured Computer Organization by Tanenbaum
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Formatting a Disk Unformatted Disk: absolutely no
information Formatting divides disk into tracks
and sectors Unformatted Size > Formatted Size 15% of disk size is taken up by
formatting information
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Access Time Seek Time: time for the arm to be
moved to the right radial position Rotational latency/delay: time for the
desired sector to rotate under the head
Seek Time + Rotational Delay = Disk Access Time
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Hard Disk Example 3.5” disk (usually found in desktop
PCs) 20 data-recording surfaces 15,000 tracks per surface 400 sectors per track 512 bytes per sector Total formatted capacity:
61,440,000,000 bytes = 57.2 GiB = 61.4 GB
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Logical Disk Geometry What’s wrong with previous example:
20 data-recording surfaces = 10 platter? Most disks usually just have around 3 platters!
Because of BIOS limitations, disk controllers intentionally mis-represent their characteristics as a logical disk geometry, to overcome size barriers imposed by BIOS
Also, physical disk geometry is no longer practical with ZBR and defect mapping
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Logical Block Addressing (LBA) Alternative to CHS addressing Overcomes BIOS size limitations Each sector is just numbered from
0,1,2,3,4,… up Disk controller just translates these
LBA addresses to actual physical addresses
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Data Buffer/Cache If bus to which disk is connected is
much faster than the disk, data buffer is needed (this is a standard I/O practice)
Data buffer can also serve as a cache to disk contents
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Disk Controller Controls operation of disk Interface to the bus OS initiated Read transfer: MM
address, disk address, word count Disk controller: Seek, Read, Write,
Error Checking
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Floppy Disks Used as removable storage The read/write head touches the
magnetic surface higher failure rate
Disk is not continously spinning, only when accessed
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Commercially Available Hard Disks ATA/IDE Disks
Most popular on general-purpose PCs SCSI Disks
For high-performance servers/workstations
RAID For increased redundancy and reliability
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Integrated Drive Electronics (IDE) One of the first type of drives where
the controller is integrated with the drive itself
Term is used to refer to ATA drives which leads to confusion because SCSI drives also have disk controllers integrated with the disk package
Before: controllers were in separate modules and hard disk-specific
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ATA – Advanced Technology Attachment Primary storage interface used in PCs ATA drives are also sometimes
referred to as IDE drives ATA/IDE drives
ATA is the correct term for the interface
Different versions of the ATA standard different speeds
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ATA Size Barrier 28 bits are used to specify a sector 228 = 268,435,456 sectors 512 bytes/sector 137.4 GB or 128
GiB Solution: new ATA standard ATA-6
allows 48 bits to specify a sector 248 = 281,474,976,710,656 sectors 128 PiB or 144 PB
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SCSI Disks SCSI – Small Computer System
Interface Usually used as a storage interface in
high-end machines Generally provides better
performance than ATA disks Higher Cost: 9500 pesos for 18.4 GB
SCSI (10K RPM) versus 5000 pesos for 80 GB ATA (7200 RPM)
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RAID Disk Arrays RAID – Redundant Array of
Independent (previously Inexpensive Disks)
Original idea by Patterson et al - Patterson was same guy who started RISC
“Villain”: SLED – single large expensive disk
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RAID Disk Arrays Rationale: Provide more redundancy
with the use of “cheap” disks Reliability is highly important when
with storage devices Realiability versus Availability Realiablity – is something broken? Availability – is the system still
available to the user?
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RAID Disk Arrays Increasing redundancy will not improve
reliability, it can only improve availability Increased performance: throughput is
increased by having multiple drives/read-write heads accessing data at the same time
Drawback: an array with N devices will have 1/N the reliability of a single device
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RAID Disk Arrays When a single disk fails, lost information
can be recovered from redundant information
DANGER: When another disk failure happens between a time a disk fails and the time it is replaced/repaired (MTTR – mean time to repair – measured in hours)
MTTF (mean time to failure) is measured in years
An array of disks has higher availability than a single disk
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RAID Levels RAID 0
Simply interleaves strips of data over multiple disks No redundancy! – not really a true RAID
RAID 1 Mirrored data Just copy data in backup drive in the event of a
failure RAID 2
Interleaves bytes of data over multiple disks Can use Hamming code Example: 1 byte is divided into 2 nibbles (4 bits), 3
bits of Hamming code are added total 7 bits distributed among 7 drives
Needs disk arms to be synchronized
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RAID Levels RAID 3
Similar to RAID 2 But only 1 parity bit is added to every nibble
RAID 4 Works with strips again Parity is written to 1 drive
RAID 5 Parity information is distributed among the
different disks Reduces bottleneck on parity drive in RAID 4
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RAID Disk Arrays
Source: Structured Computer Organization by Tanenbaum
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Optical Disks Data is stored using the application of
light (optical) First Generation: LaserVision from
Philips used for movies (laser discs) Next: Philips and Sony came up with
the CD (Compact Disc) used for audio CDs: 120 mm across, 1.2 mm thick,
with a 15-mm hole, supposed to last 100 years
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Cross-Section of Optical Disk
Label
Pit Land Polycarbonate Plastic
Aluminum Acrylic
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Transition from Pit to Land
Reflection NoReflection
Reflection
PitLand
SourceDetector
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CD Technology
Source: Structured Computer Organization by Tanenbaum
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CD Technology For audio to play at uniform rate, pits
and lands must stream by at a constant linear velocity
Slower angular velocity at the outside compared to the inside tracks
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CD Data Layout Data is stored in blocks called sectors Different sector formats Mode 1 format
1 byte/symbol is encoded using 14 bits (6 bits used as Hamming code for error correction)
1 frame = 42 consecutive symbols (588 bits) Each frame holds 192 data bits (24 bytes),
remaining 396 bits used for error correction 1 sector = 98 frames 16-byte header/preamble 2048 bytes of stored data in 1 sector 288 bytes for error-correction for the sector
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CD Data Reliability Three separate error-correcting
schemes Within a symbol/byte Within a frame Within a CD-ROM sector
It takes 98 frames of 588 bits (7203 bytes) to carry a single 2048-byte payload 28% Efficiency
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CD-ROM Capacity and Speed Single-Speed (1X) 75 sectors/sec
150 KiB/s 650 MiB of data for ordinary disk
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CD-ROM
Source: Structured Computer Organization by Tanenbaum
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AT Attachment Packet Interface ATAPI allows CD-ROM and tape
devices to share the ATA bus with ordinary disk drives
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CD-Recordables Layer of dye is added Initial state: dye is transparent, light
reflects off gold surface Final state after being hit by beam:
spot heats up, breaking bonds, and makes the spot opaque
Reflectivity of pits and lands are simulated
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CD-Recordables
Source: Structured Computer Organization by Tanenbaum
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CD-Recording Issues Each track must be written
continuously without stopping Hard disk must deliver data
continously Buffer underrun error occurs if the
stream of data to the CD runs dry
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Buffer Underrun Protection Eliminates buffer underrun errors by
pausing writing process (turning off the laser) when buffer becomes empty
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CD-Rewritables Instead of a dye, an alloy of silver,
indium, antimony and tellurium is used
Depending on how energy is applied, it goes into an amorphous state or into a crystalline state
When in an amorphous state, alloy does not reflect
Again, simulates pits and lands
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DVD Technology Digital Versatile (or previously Video)
Disk Same general design as CDs but with
Smaller pits (0.4 micron versus 0.8 microns for CDs)
Tighter spiral (0.74 microns between tracks versus 1.6 microns for CDs)
Red laser (at 0.65 microns versus 0.78 microns for CDs)
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DVD Technology Minimum capacity: 4.7 GiB (at least seven
times capacity of most CD-ROMs) 4.7 GB can accommodate
133 minutes of full-screen, full-motion video at high-res (720x480)
Eight soundtracks 32 subtitles
But still some applications (and movies) need more capacity!
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DVD Formats DVD-5: Single-sided, single-layer (4.7 GB) DVD-9: Single-sided, dual-layer (8.5 GB) DVD-10: Double-sided, single-layer (9.4
GB) DVD-18: Double-sided, dual-layer (17 GB) SS-DL supporters: Philips and Sony DS-SL supporters: Toshiba and Time Warner Compromise: All combinations will be
offered, and let the market decide which standards will survive.
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DVD Technology
Source: Structured Computer Organization by Tanenbaum
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DVD Video and DVD-ROM First DVD standards to hit the market DVD-ROM is essentially the same as
DVD Video with support for filesystems
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DVD-Recordables Competing standards supported by
two groups of manufacturers DVD+R and DVD+RW DVD-R, DVD-RW and DVD-RAM
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DVD+R and DVD+RW Supported by Philips, Sony, Hewlett-
Packard, Dell, Ricoh, Yamaha DVD+R – recordable DVD+RW – rewritable Can be read in most DVD-ROM drives
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DVD-R, DVD-RW and DVD-RAM Supported by Panasonic, Toshiba,
Apple, Hitachi, NEC, Pioneer, Samsung, Sharp, DVD Forum
DVD-R – recordable DVD-RW – rewritable Can be read in most DVD-ROM drives DVD-RAM – rewritable, cannot be read
by non DVD-RAM drives, housed in special cartridge
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Magnetic Tape Systems Employs similar principle used in
magnetic disk-recording Usually used for backups Different types
DLT – digital linear tape Travan DAT – digital audio tape / DDS – digital
data storage LTO – linear tape open
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Trends in Storage Technologies Increasing disk capacities through
increased areal density Areal Density = Bits/Inch = Bits
(Tracks/Inch) x (Bits/Track-Inch) 2000: 20 GB/platter 2001: 40 GB/platter 2002: 60 GB/platter 2003: 80 GB/platter Increased capacities at decreasing
cost/megabyte
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The IBM Microdrive
Source: http://www.pocketpcmag.com/Jan01/storage.stm
1 GB Capacity CF Form Factor
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Solid State Disks Alternative to magnetic and optical
disks Employs solid state semiconductor
devices (e.g. RAM) and a power source (if needed) to make storage nonvolatile
Faster performance – random access, no rotating disk
Increased physical resilience
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Solid State Disks For high performance, random-access
applications, e.g. Mail and news servers, relational DBs, etc.
Cons: Expensive! (One of the cheapest: 3$/MB) Needs power source/backup for
nonvolatility Less storage density than magnetic disks
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Solid State Disks Flash drives – another form of SSDs
Uses flash memory Not just for USB Flash Drives Now also available in 2.5” and 3.5” form-
factors suitable for notebook and desktop PCs, respectively