auxilary memory

8/7/2019 auxilary memory

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Jatinder singhJatinder singh

Secondary Storage Devices:Secondary Storage Devices:Magnetic DisksMagnetic Disks

Optical DisksOptical Disks

Floppy DisksFloppy Disks

Magnetic TapesMagnetic Tapes


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Secondary Storage DevicesSecondary Storage Devices

Two major types of secondary storageTwo major types of secondary storagedevices:devices:

1.1. Direct Access Storage Devices (DASDs)Direct Access Storage Devices (DASDs)

Magnetic DiscsMagnetic DiscsHard disks (high capacity, low cost, fast)Hard disks (high capacity, low cost, fast)Floppy disks (low capacity, lower cost,Floppy disks (low capacity, lower cost,

slow)slow) Optical DisksOptical Disks

CDCD--ROM = (Compact disc, readROM = (Compact disc, read--onlyonly

memorymemory2.2. Serial DevicesSerial Devices

Magnetic tapes (very fast sequential access)Magnetic tapes (very fast sequential access)


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Storage and FilesStorage and Files

Storage has major implications for DBMS design!Storage has major implications for DBMS design!

READREAD: transfer data from disk to main memory (RAM).: transfer data from disk to main memory (RAM).

WRITEWRITE: transfer data from RAM to disk.: transfer data from RAM to disk.

BothBoth operationsoperations areare highhigh--costcost operations,operations, relativerelative toto inin--memorymemory operations,operations, soso DBDB mustmust bebe plannedplanned carefully!carefully!

Why Not Store Everything in Main Memory?Why Not Store Everything in Main Memory?

CostsC

osts t

oot

oomuchmuch:: C

ostC

ost

ofofRAMRAM ab

outab

out 100100 timestimes thethe c

ostc

ost

ofof

thethe samesame amountamount ofof diskdisk space,space, soso relativelyrelatively smallsmall sizesize..

MainMain memorymemory isis volatilevolatile..

TypicalTypical storagestorage hierarchyhierarchy::

MainMain memorymemory (RAM)(RAM) ((primaryprimary storage)storage) fforor currentlycurrentlyusedused datadata..

DiskDiskfo

rfo

r thethe mainmain databasedatabase ((secondarysec

ondary st

orage)st

orage)..

TapesTapes forfor archivingarchiving olderolder versionsversions ofof thethe datadata ((tertiarytertiarystorage)storage)..


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SStoragetorage HHierarchyierarchy

SSecondary storageecondary storage: magnetic disk/: magnetic disk/optical devices/ tape systemsoptical devices/ tape systems

typicaltypical capacitycapacity aa numbernumber ofof 100100GBGB forforfixedfixed mediamedia;; gg forfor removableremovable

costcost perper MBMB $$00..0101 forfor fixedfixed mediamedia,,moremore forfor removableremovable

typicaltypical accessaccess timetime 88msms toto 1212msms forfor

fixedfixed media,media, largerlarger forfor removableremovable


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Magnetic DisksMagnetic Disks

Bits of data (0s and 1s) are storedBits of data (0s and 1s) are storedon circular magnetic platters calledon circular magnetic platters calleddisksdisks..

A disk rotates rapidly (& neverA disk rotates rapidly (& neverstops).stops).

AA disk headdisk head reads and writes bits ofreads and writes bits of

data as they pass under the head.data as they pass under the head. Often, several platters are organizedOften, several platters are organized

into ainto a disk packdisk pack (or(or disk drivedisk drive).).


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A Disk DriveA Disk Drive

Disk drive with 4 platters and 8 surfaces and 8 RW heads

surfaces

Spindle Boom

Read/Write heads


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Looking at a surfaceLooking at a surface

Surface of disk showing tracks and sectors

sector

tracks


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Organization of DisksOrganization of Disks

Disk contains concentricDisk contains concentric tracks.tracks.

Tracks are divided intoTracks are divided intosectorssectors

AA sectorsector is the smallest addressableis the smallest addressableunit in a disk.unit in a disk.


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Components of a DiskComponents of a Disk

Platters

Spindle

Disk head

Arm movement

Arm assembly

Tracks

Sector

The platters spin (say, 90rps).

The arm assembly ismoved in or out to position

a head on a desired track.Tracks under heads makea cylinder(imaginary!).

Only one head

reads/writes at anyone time.

Block size is a multipleof sector size (which is often

fixed).


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Disk ControllerDisk Controller

DiskDisk controllerscontrollers:: typicallytypically embeddedembedded inin thethe diskdiskdrivedrive,, whichwhich actsacts asas anan interfaceinterface betweenbetween thetheCPUCPU andand thethe diskdisk hardwarehardware..

TheThe controllercontroller hashas anan internalinternal cachecache (typically(typically aanumbernumber ofof MBs)MBs) thatthat itit usesuses toto bufferbuffer datadata forfor

read/writeread/write requestsrequests..


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Accessing DataAccessing Data

When a program reads a byte from theWhen a program reads a byte from thedisk, the operating system locates thedisk, the operating system locates thesurface, track and sector containing thatsurface, track and sector containing thatbyte, and reads the entire sector into abyte, and reads the entire sector into a

special area in main memory calledspecial area in main memory calledbufferbuffer..

The bottleneck of a disk access is movingThe bottleneck of a disk access is movingthe read/write arm.the read/write arm.

So

it makes sense to

store a

file in tracks thatS

oit makes sense t

ost

ore a

file in tracks thatare below/above each other on differentare below/above each other on different

surfaces, rather than in several tracks on thesurfaces, rather than in several tracks on thesame surface.same surface.


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CylindersCylinders

AA cylindercylinder is the set of tracks at ais the set of tracks at agiven radius of a disk pack.given radius of a disk pack.

i.e. a cylinder is the setof

tracks thati.e. a cylinder is the setof

tracks thatcan be accessed without moving thecan be accessed without moving thedisk arm.disk arm.

All the information on a cylinder canAll the information on a cylinder can

be accessed without moving thebe accessed without moving theread/write arm.read/write arm.


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CylindersCylinders


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Estimating CapacitiesEstimating Capacities

Track capacity = # of sectors/track *Track capacity = # of sectors/track *bytes/sectorbytes/sector

Cylinder capacity = # of tracks/cylinder *Cylinder capacity = # of tracks/cylinder *track capacitytrack capacity

Drive capacity = # of cylinders * cylinderDrive capacity = # of cylinders * cylinder

capacitycapacity Number of cylinders = # of tracks in aNumber of cylinders = # of tracks in a

surfacesurface


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ClustersClusters

UsuallyUsually File manager,File manager, under the operatingunder the operatingsystem, maintains the logical viewof a file.system, maintains the logical viewof a file.

File managerFile manager views the file as a series ofviews the file as a series ofclusters,clusters, eacheach of a number of sectors. Theof a number of sectors. The

clusters are ordered by their logical order.clusters are ordered by their logical order. Files can be seen in the form of logical sectors orFiles can be seen in the form of logical sectors or

blocks, which needs to be mapped to physicalblocks, which needs to be mapped to physicalclusters.clusters.

File manager uses aFile manager uses afile allocation ta

ble (FAT)

file allocation ta

ble (FAT)to map logical sectors of the file to the physicalto map logical sectors of the file to the physical

clusters.clusters.


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ExerciseExercise

Consider aConsider a blockblock--addressableaddressable disk with thedisk with thefollowing characteristics:following characteristics: Size of track 20,000 bytes.Size of track 20,000 bytes. Nondata overhead per block = 300 bytes.Nondata overhead per block = 300 bytes. Record size = 100 byte.Record size = 100 byte.

Q) How many records can be storedQ) How many records can be storedper track ifblocking factor is 10 orper track ifblocking factor is 10 or60?60?

a) 10 (20000/1300*10=150)a) 10 (20000/1300*10=150)b) 60 (20000/6300*60=180)b) 60 (20000/6300*60=180)


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The Cost of a Disk AccessThe Cost of a Disk Access T

heT

he timetime to

to

accessaccess aa secto

rsecto

r inin aa tracktracko

no

n aasurfacesurface isis divideddivided intointo 33 componentscomponents::

TimeTimeComponentComponent

ActionAction

Seek TimeSeek Time Time to move the read/writeTime to move the read/writearm to the correct cylinderarm to the correct cylinder

Rotational delay (orRotational delay (orlatency)latency)

Time it takes for the disk toTime it takes for the disk torotate so that the desiredrotate so that the desiredsector is under thesector is under theread/write headread/write head

Transfer timeTransfer time Once the read/write head isOnce the read/write head ispositioned over the data, thispositioned over the data, thisis the time it takes foris the time it takes for

transferring datatransferring data


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Seek timeSeek time

Seek time is the time required to moveSeek time is the time required to movethe arm to the correct cylinder.the arm to the correct cylinder.

Largest in cost.Largest in cost.

Typically:Typically:

5 ms (miliseconds) to move from one track to5 ms (miliseconds) to move from one track tothe next (trackthe next (track--toto--track)track)

50 ms maximum (from inside track tooutside50 ms maximum (from inside track tooutsidetrack)track)

30 ms average (from one30 ms average (from one randomrandom track totrack toanotheranother randomrandom track)track)


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Average Seek Time (Average Seek Time (ss))--11 It is usually impossible to know exactly how manyIt is usually impossible to know exactly how many

tracks will be traversed in every seek,tracks will be traversed in every seek, we usually try to determine thewe usually try to determine the average seek timeaverage seek time

((ss) required for a particular file operation.) required for a particular file operation.

If the starting positions for each access are random, itIf the starting positions for each access are random, itturns out that the average seek traversesturns out that the average seek traverses one third ofone third ofthe total number of cylindersthe total number of cylinders.. Why? There are more ways to travel short distanceWhy? There are more ways to travel short distance

than to travel long distancethan to travel long distance

Manufacturers specifications for disk drives often listManufacturers specifications for disk drives often list

this figure as thethis figure as the average seek timeaverage seek time for the drives.for the drives. Most hard disks today haveMost hard disks today have ss under 10under 10 ms, andms, and

highhigh--performance disks haveperformance disks have ss as lowas lowas 7.5 ms.as 7.5 ms.


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Average Seek Time (Average Seek Time (ss))--22 Seek time depends only on the speedSeek time depends only on the speed

with which the head rack moves, andwith which the head rack moves, andthe number of tracks that the headthe number of tracks that the headmust move across to reach its target.must move across to reach its target.

Given the following (which are constantGiven the following (which are constantfor a particular disk):for a particular disk): HHss = the time for the I/ O head to start= the time for the I/ O head to start

movingmoving HHtt = the time for the I/ O head to move= the time for the I/ O head to move

fr

om

one track t

othe next

fr

om

one track t

othe next

Then the time for the head to move nThen the time for the head to move ntracks is:tracks is:

Seek(n)= HSeek(n)= Hss+ H+ Htt*n*n


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Latency (Rotational Latency)Latency (Rotational Latency)--

11 LatencyLatency is the time needed for the disk to rotate so theis the time needed for the disk to rotate so thesector we want is under the read/write head.sector we want is under the read/write head.

Hard disks usually rotate at about 5000Hard disks usually rotate at about 5000--7000 rpm,7000 rpm,

1212--8 msec per revolution.8 msec per revolution.

Note:Note:

Min latency = 0Min latency = 0

Max latency = Time for one disk revolutionMax latency = Time for one disk revolution

Average latency (Average latency (rr)) = (min + max) / 2= (min + max) / 2

= max / 2= max / 2= time for disk revolution= time for disk revolution

Typically 6Typically 6 4 ms, at average4 ms, at average


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Rotational LatencyRotational Latency--22

Given the following:Given the following: R = the rotational speed of the spindleR = the rotational speed of the spindle

(in rotations per second)(in rotations per second)

UU = the number of radians through= the number of radians throughwhich the track must rotatewhich the track must rotate

then the rotational latencythen the rotational latency UU radiansradians is:is:

Latency= (Latency= (UU/2/244))**(1000/(1000/R), in msR), in ms


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Transfer TimeTransfer Time--11

Transfer time is the time for the read/write head to passTransfer time is the time for the read/write head to passover a block.over a block.

The transfer time is given by the formula:The transfer time is given by the formula:

number of sectorsnumber of sectors

Transfer time =Transfer time = ------------------------------------------------------------------ xx rotationrotationtimetimetrack capacity in number of sectorstrack capacity in number of sectors

e.g. if there are Se.g. if there are Stt sectors per track, the time to transfersectors per track, the time to transfer

one sector would be 1/ Sone sector would be 1/ Stt of a revolutionof a revolution..


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Transfer TimeTransfer Time--22

The transfer time depends only on the speed at which theThe transfer time depends only on the speed at which thespindle rotates, and the number of sectors that must bespindle rotates, and the number of sectors that must beread.read.

Given:Given:

SStt = the total number of sectors per track= the total number of sectors per track the transfer time for n contiguous sectors on the samethe transfer time for n contiguous sectors on the same

track is:track is:

Transfer Time =(Transfer Time =(n/Sn/Stt)*()*(1000/1000/R), inR), in msms


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Fast Sequential ReadingFast Sequential Reading We assume that blocks are arranged so thatWe assume that blocks are arranged so that

there is no rotational delay in transferringthere is no rotational delay in transferringfrom one track to another within the samefrom one track to another within the samecylinder. This is possible if consecutive trackcylinder. This is possible if consecutive trackbeginnings are staggered (like running racesbeginnings are staggered (like running raceson circular race tracks)on circular race tracks)

We also assume that the consecutive blocksWe also assume that the consecutive blocksare arranged so that when the next block is onare arranged so that when the next block is onan adjacent cylinder, there is no rotationalan adjacent cylinder, there is no rotationaldelay after the arm is moved to new cylinderdelay after the arm is moved to new cylinder

Fast sequential readingFast sequential reading: no rotational delay: no rotational delayafter finding the first block.after finding the first block.


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Assuming Fast Reading,Assuming Fast Reading,

ConsequentlyConsequently

ReadingReading bb blocks:blocks:

i.i. Sequentially:Sequentially:

s + r +s + r + bb * btt* btt

bb * btt* btt

ii.ii. Randomly:Randomly:bb * (s + r + btt)* (s + r + btt)

insignificant for large files, where b is very large:


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Secondary Storage Devices:Secondary Storage Devices:

Magnetic TapesMagnetic Tapes


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CharacteristicsCharacteristics

No direct access, but very fastNo direct access, but very fastsequential access.sequential access.

Resistant to

different envir

onmentalResistant t

odi

fferent envir

onmentalconditions.conditions.

Easy to transport, store, cheaperEasy to transport, store, cheaperthan disk.than disk.

Before it was widely used to storeBefore it was widely used to storeapplication data; nowadays, itsapplication data; nowadays, itsmostly used for backups or archives.mostly used for backups or archives.


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MT CharacteristicsMT Characteristics--22

A sequence of bits are stored onA sequence of bits are stored onmagnetic tape.magnetic tape.

For st

orage, the tape is

wound

on aF

or st

orage, the tape is

wound

on areel.reel.

To access the data, the tape isTo access the data, the tape isunwound from one reel to another.unwound from one reel to another.

As the tape passes the head, bits ofAs the tape passes the head, bits ofdata are read from or written ontodata are read from or written ontothe tape.the tape.


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Reel 1 Reel 2

tape

Read/write head


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TracksTracks

Typically data on tape is stored in 9Typically data on tape is stored in 9separate bit streams, orseparate bit streams, or trackstracks..

Each track is a sequenceof

bits.Each track is a sequenceof

bits. Recording density = # of bits perRecording density = # of bits per

inch (bpi). Typically 800 or 1600 bpi.inch (bpi). Typically 800 or 1600 bpi.30000 bpi on some recent devices.30000 bpi on some recent devices.


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MT recording in detailMT recording in detail

0

1

10

1

1

0

1

0

0

1

10

1

1

0

1

0

0

1

10

1

1

0

1

0

0

1

10

1

1

0

1

0

parity bit

8 bits = 1 byte


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Tape OrganizationTape Organization

2400

logical record

BOT

marker

Header block

(describes data blocks)

Data blocks Interblock gap(for acceleration &

deceleration of tape)

EOT

marker


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Data Blocks and RecordsData Blocks and Records

Each data block is a sequence ofEach data block is a sequence ofcontiguouscontiguous recordsrecords..

A record is the unit of data that a usersA record is the unit of data that a users

program deals with.program deals with.

The tape drive reads an entire block ofThe tape drive reads an entire block ofrecords at once.records at once.

Unlike a disk, a tape starts and stops.Unlike a disk, a tape starts and st

ops.

When stopped, the read/write head is overWhen stopped, the read/write head is overan interblock gap.an interblock gap.


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Secondary Storage Devices:Secondary Storage Devices:

CDCD--ROMROM


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Physical Organization ofCDPhysical Organization ofCD--ROMROM

Compact DiskCompact Disk read only memory (writeread only memory (writeonce), R/W is also available.once), R/W is also available.

Data is encoded and read optically with aData is encoded and read optically with a

laserlaser Can store around +600MB dataCan store around +600MB data

Digital data is represented as a series ofDigital data is represented as a series ofPitsPits andand Lands:Lands:

PitPit = a little depression, forming a lower level= a little depression, forming a lower levelin the trackin the track

LandLand = the flat part between pits, or the= the flat part between pits, or theupper levels in the trackupper levels in the track


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Organization of dataOrganization of data

Reading a CD is done by shining a laser at theReading a CD is done by shining a laser at thedisc and detecting changing reflections patterns.disc and detecting changing reflections patterns.

1 = change in height (land to pit or pit to land)1 = change in height (land to pit or pit to land)

0 = a fixed amount of time between 1s0 = a fixed amount of time between 1s

LANDLAND PITPIT LANDLAND PITPIT LANDLAND......------------+ ++ +--------------------------+ ++ +------......

|_____| |_______||_____| |_______|

..0 0 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 0 1 0 0 ....0 0 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 0 1 0 0 ..

Note : we cannot have two 1s in a row!Note : we cannot have two 1s in a row!=> uses Eight to Fourteen Modulation (EFM) encoding=> uses Eight to Fourteen Modulation (EFM) encodingtable.table. Usually, a pattern of 8 bits is translated to/from aUsually, a pattern of 8 bits is translated to/from apattern of 14pattern of 14 pitspits andand landslands..


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CDCD--ROMROM

yy WhileWhile thethe speedspeed ofof CDCD--ROMROM readersreaders isisrelativelyrelatively higher,higher, suchsuch asas 2424X(X(2424 timestimes CDCDaudioaudio speed),speed), thethe speedspeed ofof writingwriting isis muchmuchslower,slower, asas lowlow asas 22XX..

NoteNote thatthat thethe speedspeed ofof thethe audioaudio isis aboutabout150150KBKB perper secondsecond..

yyTheThe DVDDVD (Digital(Digital VideoVideo DiscDisc oror DigitalDigitalVersatileVersatile Disc)Disc) technologytechnology isis basedbased onon CDCDtechnologytechnology withwith increasedincreased storagestorage densitydensity..

TheThe DVDDVD technologytechnology allowsallows twotwo--sidesidemedium,medium, withwith aa storagestorage capacitycapacity ofof upup toto1010GBGB..


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CDCD--ROMROM

BecauseBecause ofof thethe heritageheritage fromfrom CDCD audio,audio, thethe datadata isisstoredstored asas aa singlesingle spiralspiral tracktrack onon thethe CDCD--ROM,ROM, contrarycontrarytoto magneticmagnetic hardhard disksdisks discretediscrete tracktrack conceptconcept..

Thus,Thus, thethe rotationrotation speedspeed isis controlledcontrolled byby CLVCLV--ConstantConstantLinearLinear velocityvelocity.. TheThe rotationalrotational speedspeed atat thethe centercenter isis

highest,highest, slowingslowing downdown towardstowards thethe outerouter edgeedge.. Because,Because,thethe recordingrecording densitydensity isis thethe samesame everyevery wherewhere..

NoteNote thatthat withwith CLV,CLV, thethe linearlinear speedspeed ofof thethe spiralspiral passingpassingunderunder thethe R/WR/W headhead remainsremains constantconstant..

CLVCLV isis thethe culpritculprit forfor thethe poorpoor seekseek timetime inin CDCD--ROMsROMs

TheThe advantageadvantage ofof CLVCLV isis thatthat thethe diskdisk isis utilizedutilized atat itsitsbestbest capacity,capacity, asas thethe recordingrecording densitydensity isis thethe samesame everyevery

wherewhere..


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CDCD--ROMROM

Note that: Since 0's are represented by theNote that: Since 0's are represented by the length of timelength of timebetween transitions, we must travel atbetween transitions, we must travel at constant linearconstant linearvelocityvelocity ((CLVCLV)on the tracks.)on the tracks.

Sectors are organized along a spiralSectors are organized along a spiral

Sectors have same linear lengthSectors have same linear length

Advantage: takes advantage of all storage space available.Advantage: takes advantage of all storage space available.

Disadvantage: has to change rotational speed whenDisadvantage: has to change rotational speed whenseeking (slower towards the outside)seeking (slower towards the outside)


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AddressingAddressing

1 second of play time is divided up into 751 second of play time is divided up into 75sectorssectors..

Each sector holds 2KBEach sector holds 2KB

60 min CD:60 min CD:60min * 60 sec/min * 75 sectors/sec =60min * 60 sec/min * 75 sectors/sec =270,000 sectors = 540,000 KB ~ 540 MB270,000 sectors = 540,000 KB ~ 540 MB

AA sectorsector is addressed by:is addressed by:Minute:Second:SectorMinute:Second:Sectore.g. 16:22:34e.g. 16:22:34


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File Structures forCDFile Structures forCD--ROMROM

One of the problems faced in using CDsOne of the problems faced in using CDsfor data storage is acceptance of afor data storage is acceptance of acommon file system, with the followingcommon file system, with the followingdesired design goals:desired design goals: Support for hierarchical directory structure,Support for hierarchical directory structure,

with access ofone or two seekswith access ofone or two seeks Support for generic file names (as in file*.c),Support for generic file names (as in file*.c),

during directory accessduring directory access

If implement UNIX file system on CDIf implement UNIX file system on CD--ROM, it will be a catastrophe! The seekROM, it will be a catastrophe! The seektime per access is from 500 msec to 1 sec.time per access is from 500 msec to 1 sec.


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File Structures forCDFile Structures forCD--ROMROM In this case, one seek may be necessary perIn this case, one seek may be necessary per

subdirectory. For examplesubdirectory. For example/usr/home/mydir/ceng351/exam1/usr/home/mydir/ceng351/exam1

will require five seeks to locate the file exam1 onlywill require five seeks to locate the file exam1 only

SolutionSolution

One approach place the entire directory structure inOne approach place the entire directory structure inone file, such that it allows building a left child rightone file, such that it allows building a left child rightsibling structure to be able to access any file.sibling structure to be able to access any file.

For a small file structure file, the entire directoryFor a small file structure file, the entire directory

structure can be kept in the memory all the time, whichstructure can be kept in the memory all the time, whichallows method towork.allows method towork.


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The second approach is to create an indexThe second approach is to create an indexto the file locations by hashing the fullto the file locations by hashing the fullpath names of each file.path names of each file.

This method will not work for generic fileThis method will not work for generic fileor directory searches.or directory searches.

A third method may utilize both aboveA third method may utilize both abovemethods, one can keep the advantage ofmethods, one can keep the advantage of

Unix likeone

file per direct

ory scheme, atUnix like

one

file per direct

ory scheme, atthe same time allows building indexes forthe same time allows building indexes for

the subdirectories.the subdirectories.


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A forth method, assume directories as filesA forth method, assume directories as filesas well and use a special index thatas well and use a special index thatorganizes the directories and the files intoorganizes the directories and the files intoa hierarchy where a simple parental indexa hierarchy where a simple parental index

indicates the relationship between allindicates the relationship between allentries.entries.Rec NumberRec Number File or dir nameFile or dir name ParentParent00 RootRoot11 Subdir1Subdir1 0022 Subdir11Subdir11 11

33 Subdir12Subdir12 1144 File11File11 1155 FileFile 0066 Subdir2Subdir2 00


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auxilary memory

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