8/10/2019 Ch9_Disks_06_2

1/33

1

The Bare Basics

Storing Data on Disks and Files

Chapter 9

8/10/2019 Ch9_Disks_06_2

2/33

2

Disks and Files

DBMS stores information on (hard) disks.

This has major impliations for DBMS desi!n"

#$%D& transfer data from disk to main memor'(#%M).

#T$& transfer data from #%M to disk.

Both are hi!h*ost operations+ relati,e to in*memor'operations+ so m-st e planned aref-ll'"

8/10/2019 Ch9_Disks_06_2

3/33

/

Why Not Store Everything in MainMemory?

Costs too much. Same amo-nt of mone' 0ill -' 'o- sa' either 12MB of

#%M or 23B of disk.

Main memory is volatile. e 0ant data to e sa,ed et0een r-ns. (4,io-sl'")

T'pial stora!e hierarhies& Main memor' (#%M) for -rrentl' -sed data (primar'stora!e) .

Disk for the main dataase (seondar' stora!e). Tapes for arhi,in! older ,ersions of data (tertiar' stora!e

8/10/2019 Ch9_Disks_06_2

4/33

5

Disks Seondar' stora!e de,ie of hoie. Main ad,anta!e o,er tapes&

random access ,s. sequential.

Data is stored and retrie,ed in -nits & alled disk locks or!ages.

6nlike #%M+ time to retrie,e a disk pa!e,aries dependin! -pon loation on disk. Therefore+ relati,e plaement of pa!es on

disk has major impat on DBMS

performane"

8/10/2019 Ch9_Disks_06_2

5/33

8/10/2019 Ch9_Disks_06_2

6/33

;

$ccessing a Disk %age

Time to aess (read80rite) a disk lok& seek time (mo,in! arms to position disk head on trak) rotational delay (0aitin! for lok to rotate -nder head)

trans"er time (at-all' mo,in! data to8from disk s-rfae) Seek time and rotational dela' dominate.

Seek time ,aries from ao-t 1 to 2mse #otational dela' ,aries from to 1mse

Transfer rate is ao-t 1mse per 5

8/10/2019 Ch9_Disks_06_2

7/33>

$rranging %ages on Disk

?Ne&t@ lok onept& loks on same trak+ follo0ed ' loks on same 'linder+ follo0ed ' loks on adjaent 'linder

Bloks in a le sho-ld e arran!ed seA-entiall'on disk (' ?ne:t@)+to minimie seek androtational dela'.

or a seA-ential san+!re'"etchingse,eralpa!es at a time is a i! 0in"

8/10/2019 Ch9_Disks_06_2

8/33

($)D *(edundant $rray o")nde!endent Disks+

Disk %rra'& %rran!ement of se,eral disks that !i,esastration of a sin!le lar!e disk.

3oals& nrease performane and reliailit'.

T0o main tehniA-es& Data stripin!&

Data is partitionedSie of a partition is alled the stripin! -nit.Eartitions are distri-ted o,er se,eral disks.

#ed-ndan'&More disks FG more reliale.#ed-ndant information allo0s reonstr-tion of data if a disk

fails.

8/10/2019 Ch9_Disks_06_2

9/339

($)D ,evels

=e,el & Ho red-ndan' Best 0rite performane

Hot est in readin!. (h'I)

=e,el 1& Mirrored (t0o idential opies) $ah disk has a mirror ima!e (hek disk) Earallel reads+ a 0rite in,ol,es t0o disks. Ma:im-m transfer rate F transfer rate of

one disk

8/10/2019 Ch9_Disks_06_2

10/331/

Disk S!ace Management

=o0est la'er of DBMS soft0are mana!esspae on disk.

Ji!her le,els all -pon this la'er to& alloate8de*alloate a pa!e read80rite a pa!e

Ji!her le,els don@t need to kno0 ho0 thisis done+ or ho0 free spae is mana!ed.

8/10/2019 Ch9_Disks_06_2

11/3315

Bu-er Management in a

DBMS

Data must e in ($M "or DBMS to o!erate on it. Tale o" /"rame01 !ageid2 !airs is maintained3

DB

MAIN MEMORY

DISK

disk pa!e

free frame

Ea!e #eA-ests from Ji!her =e,els

B6$# E44=

hoie of frame ditated' replacement policy

8/10/2019 Ch9_Disks_06_2

12/3317

When a %age is (equested 333

f reA-ested pa!e is not in -Ker pool& Choose a frame for re!lacement f frame is dirt'+ 0rite it to disk #ead reA-ested pa!e into hosen frame

%in the pa!e and ret-rn its address.

)" requests can e !redicted *e3g31 sequenti

!ages can e!re'"etched *several !ages at

8/10/2019 Ch9_Disks_06_2

13/331;

More on Bu-er Management

#eA-estor of pa!e m-st -npinit+ and indiate0hether pa!e has een modied& dirtyit is -sed for this.

Ea!e in pool ma' e reA-ested man' times+ a!in count is -sed. % pa!e is a andidate for replaement iK!in count

F .

CC L reo,er' ma' entail additional 84 0hena frame is hosen for replaement. (Write'

$head ,og protool more later.)

8/10/2019 Ch9_Disks_06_2

14/331>

Bu-er (e!lacement %olicy

rame is hosen for replaement ' are!lacement !olicy4 =east*reentl'*-sed (=#6)+ Clok+ M#6 et.

Eoli' an ha,e i! impat on of 84@sdepends on access !attern.

Sequential 5ooding& Hast' sit-ationa-sed ' =#6 N repeated seA-ential

sans. -Ker frames O pa!es in le means eah

pa!e reA-est a-ses an 84. M#6m-h etter in this sit-ation (-t not in all

sit-ations+ of o-rse).

8/10/2019 Ch9_Disks_06_2

15/33

1

DBMS vs3 6S File System

4S does disk spae L -Ker m!mt alread'"

So 0h' not let 4S mana!e these tasksI

DiKerenes in 4S s-pport& Eortailit' iss-es Some limitations+ e.!.+ les don@t span m-ltiple disk

de,ies. B-Ker mana!ement in DBMS reA-ires ailit' to&

pin a pa!e in -Ker pool+ fore a pa!e to disk (important for implementin! CC L

reo,er')+ adj-st re!lacement !olicy1 and pre*feth pa!es ased

on aess patterns in t'pial DB operations.

8/10/2019 Ch9_Disks_06_2

16/33

19

Structure o" a DBMS

% t'pial DBMS has a la'eredarhitet-re.

Disk Stora!e hierarh'+ #%D

Disk Spae Mana!ement#oles+ ree loks

B-Ker Mana!ement

B-Ker Eool+ #eplaement poli'

iles and %ess Methods

ile or!aniation(heap les+ sortedle+ inde:es)

ile and pa!e le,el stora!e(olletion

of pa!es or reords)

Query Optimization

and Execution

Relational Operators

iles and Access !ethods

"u##er !ana$ement

Disk Space !ana$ement

D"

These layers

must consider

concurrency

control andrecovery

Index FilesSystem Catalog

Data Files

8/10/2019 Ch9_Disks_06_2

17/33

2

Files o" (ecords

Ea!e or lokis the !ran-larit' for doin! 84 Ji!her le,els of DBMSoperate on &

records+ and

7les omposed of reords. =$& % olletion of pa!es+ eah ontainin!

a olletion of reords. ile m-st s-pport&

insert8delete8modif'reord reada parti-lar reord (speied -sin! record

id) sanall reords (possil' 0ith some onditions

on the reords to e retrie,ed)

8/10/2019 Ch9_Disks_06_2

18/33

21

8nordered Files *9ea! Files+

Simplest le str-t-reontains reords in noparti-lar order.

%s le !ro0s and shrinks+ disk pa!es arealloated and de*alloated.

To s-pport reord le,el operations+ 0e m-st& keep trak of the!agesin a le keep trak of "ree s!aceon pa!es keep trak of the recordson a pa!e

There are man' alternati,es for keepin! trak

of this.

8/10/2019 Ch9_Disks_06_2

19/33

22

$lternative :4

9ea! File )m!lemented as,ist

Maintain a tale ontainin! pairs of&OheapPlePname+ headPpa!ePaddressG

$ah pa!e ontains 2 ?pointers@ (rid) pl-s data.

JeaderEa!e

DataEa!e

DataEa!e

DataEa!e

DataEa!e

DataEa!e

DataEa!e

Ea!es 0ithree Spae

-ll Ea!es

8/10/2019 Ch9_Disks_06_2

20/33

2/

9ea! File )m!lemented as a

,ist nsert a ne0 pa!e into heap le Disk mana!er adds a ne0 free spae pa!einto link

Delete a pa!e from heap le #emo,edfrom the list

Disk mana!er dealloatesit

Disad,anta!es& f reords are of ,ariale len!th+ all pa!es 0ill e in

free list.

#etrie,e and e:amine se,eral pa!esfor eno-!hspae.

8/10/2019 Ch9_Disks_06_2

21/33

25

$lternative ;4 9ea! File 8sing %ageDirectory

n diretor'+ eah entr' for a pa!e inl-desn-mer of free 'teson pa!e.

The diretor' is a olletion of pa!es(linked list implementation is j-st one alternati,e). Much smaller than linked list o" all 9F !ages"

Data

Ea!e 1

DataEa!e 2

DataEa!e H

HeaderPage

DIRECTORY

8/10/2019 Ch9_Disks_06_2

22/33

27

$lternative ;4

9ea! File 8sing a %ageDirectory

%d,anta!e of Ea!e Diretor'&

The sie of diretor' is ,er' small(m-hsmaller than heap le.)

Searhin! spae is ,er' eQient+ ea-send free spae 0itho-t lookin! at at-al

heap data pa!es.

8/10/2019 Ch9_Disks_06_2

23/33

2;

%age Formats

Ea!e & astration is -sed for 84 #eord & data !ran-larit' for hi!her le,el of

DBMS

Jo0 to arran!e reords in pa!esI dentif' a reord&

R Opa!ePid+ slotPn-merG+ 0here slotPn-mer F rid

R Most ases+ -se Opa!ePid+ slotPn-merG as rid.

%lternati,e approahes to mana!e slotson apa!e

Jo0 to s-pport insert8deletin!8searhin!I

8/10/2019 Ch9_Disks_06_2

24/33

2>

(ecords Formats4 Fi&ed ,ength(ecord

nformation ao-t eld t'pes same for allreords in a le

Stored reord format in systemcatalogs3

N indin! i

8/10/2019 Ch9_Disks_06_2

25/33

2

%age Formats4 Fi&ed ,ength(ecords

(ecord id = /!age id1 slot 023

Note4 )n 7rst alternative1 moving records "or "ree s!acemanagement changes rid> may not e acce!talei"e&isting e&ternal re"erences to the record that is moved3

Slot 1

Slot 2

Slot H

. . . . . .

H M1. . .

M ... / 2 1

E%C

8/10/2019 Ch9_Disks_06_2

26/33

29

(ecord Formats4 ariale

,engthT0o alternati,e formats ( elds is :ed)&

N SeondoKers diret aess to i@th eld

N eQient stora!e of nulls* small diretor' o,erhead.

5

ieldCo-nt

ields Delimited ' Speial S'mols

1 2 / 5

1 2 / 5

%rra' of ield 4Ksets

8/10/2019 Ch9_Disks_06_2

27/33

/

%age Formats4 ariale ,ength(ecords

Slot diretor' F OreordPoKset+reordPlen!thGU

Ea!e i#id F (i+H)

#id F (i+2)

#id F (i+1)

Eointer

to startof freespae

S=4T D#$CT4#V

H . . . 2 1

2 1; 25 H

slots

4Kset ofreordfrom startof dataarea

Length = 24

Length = 16

Length = 20

8/10/2019 Ch9_Disks_06_2

28/33

/1

%age Formats4 ariale ,ength(ecords

Slot diretor' F OreordPoKset+ reordPlen!thGU Dis8%d,anta!es&

N Mo,in!& rid is not han!ed

N Deletion& oKset F *1 (rid han!edI

Can 0e delete slotI h'I)

N nsertion& #e-se deleted slot.

4nl' insert if none a,ailale.

ree spaeI ree spae pointerI #e'le afterdeletionI

8/10/2019 Ch9_Disks_06_2

29/33

/2

System Catalogs

Meta information stored in s'stem atalo!s.

or eah inde:& str-t-re (e.!.+ BN tree) and searh ke' elds

or eah relation& name+ le name+ le str-t-re (e.!.+ Jeap le) attri-te name and t'pe+ for eah attri-te inde: name+ for eah inde: inte!rit' onstraints

or eah ,ie0& ,ie0 name and denition

El-s statistis+ a-thoriation+ -Ker pool sie+ et.

Catalogs are themselves stored as relation

8/10/2019 Ch9_Disks_06_2

30/33

//

$ttr@Cat*attr@name1 rel@name1 ty!e1!osition+

attrPname relPname t'pe positionattrPname %ttri-tePCat strin! 1relPname %ttri-tePCat strin! 2t'pe %ttri-tePCat strin! /

position %ttri-tePCat inte!er 5sid St-dents strin! 1name St-dents strin! 2lo!in St-dents strin! /a!e St-dents inte!er 5!pa St-dents real 7d a-lt' strin! 1fname a-lt' strin! 2sal a-lt' real /

8/10/2019 Ch9_Disks_06_2

31/33

/5

Summary

Disks pro,ide heap+ non*,olatile stora!e. #andom aess+ -t ost depends on loation

of pa!e on disk mportant to arran!e data seA-entiall'to

minimie seekand rotation dela's.

B-Ker mana!er rin!s pa!es into #%M. Ea!e sta's in #%M -ntil released ' reA-estor.

ritten to disk 0hen frame hosen forreplaement. rame to replae ased on replaement poli'3 Tries to pre*feth se,eral pa!es at a time.

8/10/2019 Ch9_Disks_06_2

32/33

/7

More Summary

DBMS ,s. 4S ile S-pport DBMS needs feat-res not fo-nd in man' 4Ss.R forin! a pa!e to diskR ontrollin! the order of pa!e 0rites to diskR les spannin! disks

R ailit' to ontrol pre*fethin! and pa!e replaementpoli' ased on preditale aess patterns

ormats for #eords and Ea!es & Slotted pa!e format& s-pports ,ariale len!th

reords and allo0s reords to mo,e on pa!e.

Wariale len!th reord format& eld oKsetdiretor' oKers s-pport for diret aess to i@theld and n-ll ,al-es.

8/10/2019 Ch9_Disks_06_2

33/33

Even More Summary

ile la'erkeeps trak of pa!es in a le+ ands-pports astration of a olletion ofreords. Ea!es 0ith free spae identied -sin! linked list

or diretor' str-t-re

nde:ess-pport eQient retrie,al ofreords ased on the ,al-es in some elds.

Catalo! relationsstore information ao-trelations+ inde:es and ,ie0s.

nformation ommon to all reords in olletion