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Chapter 19

Database RecoveryTechniques

Copyright 2004 Pearson Education, Inc.

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Copyright 2004 Ramez Elmasri and Shamant !a"athe

Elmasri/Navathe, Fundamentals o Database !ystems, Fourth Edition Chapter 19-3

Chapter 19 Outline

Databases Recovery# Purpose o$ %ata&ase Reco"ery

2 'ypes o$ (ailure

) 'ransaction *og

4 %ata +pdates %ata Caching

- 'ransaction Roll&ac /+ndo and Roll(or1ard

Checpointing

3 Reco"ery schemes

5RIES Reco"ery Scheme

#0 Reco"ery in 6ultidata&ase System

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Database Recovery

1 Purpose of Database Recovery

7 'o &ring the data&ase into the last consistent state,1hich e8isted prior to the $ailure.

7 'o preser"e transaction properties /5tomicity,Consistency, Isolation and %ura&ility.

E8ample9 I$ the system crashes &e$ore a $und trans$ertransaction completes its e8ecution, then either one or &othaccounts may ha"e incorrect "alue. 'hus, the data&asemust &e restored to the state &e$ore the transaction modi$iedany o$ the accounts.

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Database Recovery

2 Types of Failure

'he data&ase may &ecome una"aila&le $or use due to

7'ransaction $ailure9 'ransactions may $ail &ecause o$incorrect input, deadloc, incorrect synchronization.

7 System $ailure9 System may $ail &ecause o$ addressing

error, application error, operating system $ault, R56

$ailure, etc.

7 6edia $ailure9 %is head crash, po1er disruption, etc.

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Database Recovery

3 Trasactio !o"

(or reco"ery $rom any type o$ $ailure data "alues prior tomodi$ication /:(I6 :e(ore Image and the ne1 "alue a$termodi$ication /5(I6 ; 5(ter Image are re

?

6

!

> @ 200

? @ #00

6 @ 200

! @ 400

> @ #00

? @ ,0

6 @ 200

! @ 400

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Elmasri/Navathe, Fundamentals o Database !ystems, Fourth Edition Chapter 19-,

Database Recovery

4 Data p.ate

7 '))e.iate p.ate9 5s soon as a data item ismodi$ied in cache, the dis copy is updated.

7 Deferre. p.ate9 5ll modi$ied data items in thecache is 1ritten either a$ter a transaction ends itse8ecution or a$ter a $i8ed num&er o$ transactionsha"e completed their e8ecution.

7 /ha.o0 up.ate9 'he modi$ied "ersion o$ a dataitem does not o"er1rite its dis copy &ut is 1ritten at

a separate dis location.7 '-place up.ate9 'he dis "ersion o$ the data item is

o"er1ritten &y the cache "ersion.

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Elmasri/Navathe, Fundamentals o Database !ystems, Fourth Edition Chapter 19-

Database Recovery

5 Data Cachi"

%ata items to &e modi$ied are $irst stored into data&ase

cache &y the Cache 6anager /C6 and a$ter

modi$ication they are $lushed /1ritten to the dis. 'he$lushing is controlled &y *o.ifie.and Pi-pi&its.

Pi-pi9 Instructs the operating system not to $lush

the data item.

*o.ifie.9 Indicates the 5(I6 o$ the data item.

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Database Recovery

6 Trasactio Roll-bac$ .o a. Roll-For0ar. Re.o

'o maintain atomicity, a transactionAs operations are re.oe

or u.oe.

.o9 Restore all :(I6s on to dis /Remo"e all 5(I6s.

Re.o9 Restore all 5(I6s on to dis.

%ata&ase reco"ery is achie"ed either &y per$orming only

+ndos or only Redos or &y a com&ination o$ the t1o. 'hese

operations are recorded in the log as they happen.

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Database Recovery

Roll-bac$

=e sho1 the process o$ roll&ac 1ith the help o$ the $ollo1ing three transactions'#, and '2 and ').

'# '2 ')

readBitem /5 readBitem /: readBitem /C

readBitem /% 1riteBitem /: 1riteBitem /:

1riteBitem /% readBitem /% readBitem /51riteBitem /5 1riteBitem /5

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Database Recovery

Roll-bac$ (e e&ecutio of T1 T2 a. T3 as recor.e. i the lo"7+ # C D

3 15 4 2

8starttrasactio T3:

8rea.ite) T3 C:

; 80riteite) T3 # 15 12: 12

8starttrasactioT2:8rea.ite) T2 #:

;; 80riteite) T2 # 12 1: 1

8starttrasactioT1:

8rea.ite) T1 +:

8rea.ite) T1 D:

80riteite) T1 D 2 25: 25

8rea.ite) T2 D:

;; 80riteite) T2 D 25 26: 26

8rea.ite) T3 +:

---- syste) crash ----

; T3 is rolle. bac$ because it .i. ot reach its co))it poit7

;; T2 is rolle. bac$ because it rea.s the value of ite) # 0ritte by T37

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Database RecoveryRoll-bac$ (e e&ecutio of T1 T2 a. T3 as recor.e. i the lo"7

READ(C) READ(A)

READ(D)

WRITE(B)T3

BEGINREAD(B) WRITE(D)WRITE(B)T2

BEGIN WRITE(D)READ(A) READ(D)T1

BEGIN

syste !rashTime

'llustrati" casca.i" roll-bac$

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Database Recovery

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Database Recovery

, Chec$poiti"

'ime to time /randomly or under some criteria the data&ase

$lushes its &u$$er to data&ase dis to minimize the tas o$

reco"ery. 'he $ollo1ing steps de$ines a checpoint operation9#. Suspend e8ecution o$ transactions temporarily.

2. (orce 1rite modi$ied &u$$er data to dis.

). =rite a checpointD record to the log, sa"e the log to dis.

4. Resume normal transaction e8ecution.

%uring reco"ery re.o or u.o is re

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Database Recovery

/teal=%o-/teal a. Force=%o-Force

Possi&le 1ays $or $lushing data&ase cache to data&ase dis9

/teal9 Cache can &e $lushed &e$ore transaction commits.

%o-/teal9 Cache cannot &e $lushed &e$ore transaction commit.

Force9 Cache is immediately $lushed /$orced to dis.

%o-Force9 Cache is de$erred until transaction commits.

'hese gi"e rise to $our di$$erent 1ays $or handling reco"ery9Steal!o(orce /+ndoRedo, Steal(orce /+ndo!oredo,

!oSteal!o(orce /Redo!oundo and !oSteal(orce /!o

undo!oredo.

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Database Recovery

Deferre. p.ate %o .o=Re.o

'he data update goes as $ollo1s9

#. 5 set o$ transactions records their updates in the log.

2. 5t commit point under =5* scheme these updates are

sa"ed on data&ase dis.

5$ter re&oot $rom a $ailure the log is used to redo all the

transactions a$$ected &y this $ailure. !o undo is re

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Elmasri/Navathe, Fundamentals o Database !ystems, Fourth Edition Chapter 19-1,

Database Recovery

#. 5 set o$ transactions records their updates in the log.

2. 5t commit point under =5* scheme these updates are

sa"ed on data&ase dis.

5$ter re&oot $rom a $ailure the log is used to redo all the

transactions a$$ected &y this $ailure. !o undo is re

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Database Recovery

Deferre. p.ate i a si"le-user syste)

/a '# '2

readBitem /5 readBitem /:

readBitem /% 1riteBitem /:

1riteBitem /% readBitem /%

1riteBitem /5

/&

startBtransaction, '#D

1riteBitem, '#, %, 20D

commit '#D

startBtransaction, '#D

1riteBitem, '2, :, #0D1riteBitem, '2, %, 2Dsystem crash

'he 1riteBitem, FD operations o$ '# are redone.

'2 log entries are ignored &y the reco"ery manager.

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Database RecoveryDeferre. p.ate 0ith cocurret users

'his en"ironment re

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b 8starttrasactio T1:

80riteite) T1 D 2:

8co))it T1:8chec$poit:

8starttrasactio T4:

80riteite) T4 # 15:

80riteite) T4 + 2:

8co))it T4:

8starttrasactio T2:

80riteite) T2 # 12:8starttrasactio T3:

80riteite) T3 + 3:

80riteite) T2 D 25: syste) crash

T2 a. T3 are i"ore. because they .i. ot reach their co))it poits7

T4 is re.oe because its co))it poit is after the last chec$poit7

a T1 T2 T3 T4

rea.ite) + rea.ite) # rea.ite) + rea.ite) #

rea.ite) D 0riteite) # 0riteite) + 0riteite) #

0riteite) D rea.ite) D rea.ite) C rea.ite) +

0riteite) D 0riteite) C 0riteite) +

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'1o ta&les are re

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Database Recovery

Recovery Techi>ues #ase. o '))e.iate p.ate

In this algorithm 5(I6s o$ a transaction are $lushed to the data&ase disunder =5* &e$ore it commits. (or this reason the reco"ery manager

u.oesall transactions during reco"ery. !o transaction is re.oe. It is

possi&le that a transaction might ha"e completed e8ecution and ready to

commit &ut this transaction is also u.oe.

.o=%o-re.o +l"orith)

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Database RecoveryRecovery Techi>ues #ase. o '))e.iate p.ate

Reco"ery schemes o$ this category apply u.o and also re.o $or

reco"ery. In a singleuser en"ironment no concurrency control isre

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Database RecoveryRecovery Techi>ues #ase. o '))e.iate p.ate

Reco"ery schemes o$ this category applies undo and also redo to reco"er

the data&ase $rom $ailure. In concurrent e8ecution en"ironment a

concurrency control is re

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Database Recovery/ha.o0 Pa"i"

'he 5(I6 does not o"er1rite its :(I6 &ut recorded at another place on

the dis. 'hus, at any time a data item has 5(I6 and :(I6 /Shado1

copy o$ the data item at t1o di$$erent places on the dis.

" #

Database

"$ #$

> and ?9 Shado1 copies o$ data items

> and ?9 Current copies o$ data items

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Database Recovery/ha.o0 Pa"i"

'o manage access o$ data items &y concurrent transactions t1o

directories /current and shado1 are used. 'he directory arrangement is

illustrated &elo1. Jere a page is a data item.

%a&e ' (old)

%a&e 1

%a&e *

%a&e + (old)

%a&e

%a&e -

%a&e + (ne.)

%a&e ' (ne.)

1

+

*

'

-

Current Directory

(ater updatin& pa&es +, ')

!hado. Directory

(not updated)

1

+

*

'

-

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Elmasri/Navathe, Fundamentals o Database !ystems, Fourth Edition Chapter 19-2,

Database Recovery

9 The +R'?/ Recovery +l"orith)

'he 5RIES Reco"ery 5lgorithm is &ased on9

#. =5* /=rite 5head *ogging

2. Repeating history during redo9 5RIES 1ill retrace all actions o$

the data&ase system prior to the crash to reconstruct the data&ase

state 1hen the crash occurred.

). *ogging changes during undo9 It 1ill pre"ent 5RIES $rom

repeating the completed undo operations i$ a $ailure occurs

during reco"ery, 1hich causes a restart o$ the reco"ery process.

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Database RecoveryThe +R'?/ Recovery +l"orith)

'he 5RIES reco"ery algorithm consists o$ three steps9

17 +alysis9 step identi$ies the dirty /updated pages in the &u$$er

and the set o$ transactions acti"e at the time o$ crash. 'he

appropriate point in the log 1here redo is to start is alsodetermined.

27 Re.o9 necessary redo operations are applied.

37 .o9 log is scanned &ac1ards and the operations o$

transactions acti"e at the time o$ crash are undone in re"erseorder.

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The !o" a. !o" /e>uece %u)ber !/%

5 log record is 1ritten $or /a data update, /& transaction commit,

/c transaction a&ort, /d undo, and /e transaction end. In the case

o$ undo a compensating log record is 1ritten.

5 uni

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The !o" a. !o" /e>uece %u)ber !/%

5 log record stores9

#. Pre"ious *S! o$ that transaction9 It lins the log record o$

each transaction. It is lie a &ac pointer points to the pre"ious

record o$ the same transaction.

2. 'ransaction I%). 'ype o$ log record.

(or a 1rite operation the $ollo1ing additional in$ormation is logged9

4. Page I% $or the page that includes the item

. *ength o$ the updated item

-. Its o$$set $rom the &eginning o$ the page

. :(I6 o$ the item

3. 5(I6 o$ the item

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The Trasactio table a. the Dirty Pa"e table

(or e$$icient reco"ery $ollo1ing ta&les are also stored in the log during

checpointing9

Trasactio table9 Contains an entry $or each acti"e transaction,

1ith in$ormation such as transaction I%, transaction status and the

*S! o$ the most recent log record $or the transaction.

Dirty Pa"e table9 Contains an entry $or each dirty page in the

&u$$er, 1hich includes the page I% and the *S! corresponding to the

earliest update to that page.

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Chec$poiti"

5 checpointing does the $ollo1ing9

#. =rites a begin_checkpointrecord in the log

2. =rites an end_checkpointrecord in the log. =ith this record the

contents o$ transaction ta&le and dirty page ta&le are appended tothe end o$ the log.

). =rites the *S! o$ the begin_checkpointrecord to a special $ile.

'his special $ile is accessed during reco"ery to locate the last

checpoint in$ormation.

'o reduce the cost o$ checpointing and allo1 the system to

continue to e8ecute transactions, 5RIES uses G$uzzy

checpointingH.

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'he $ollo1ing steps are per$ormed $or reco"ery17 +alysis phase9 Start at the &eginBchecpoint record and

proceed to the endBchecpoint record. 5ccess transaction ta&le

and dirty page ta&le are appended to the end o$ the log. !ote

that during this phase some other log records may &e 1ritten to

the log and transaction ta&le may &e modi$ied. 'he analysisphase compiles the set o$ redo and undo to &e per$ormed and

ends.

27 Re.o phase9 Starts $rom the point in the log up to 1here all dirty

pages ha"e &een $lushed, and mo"e $or1ard to the end o$ the

log. 5ny change that appears in the dirty page ta&le is redone.37 .o phase9 Starts $rom the end o$ the log and proceeds

&ac1ard 1hile per$orming appropriate undo. (or each undo it

1rites a compensating record in the log.

'he reco"ery completes at the end o$ undo phase.

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Database Recovery+ e&a)ple of the 0or$i" of +R'?/ sche)e

&'N &A'T&'N TRANID T*E *AGEID Other In+%,

1 - T1 up.ate C

2 - T2 up.ate B

3 1 T1 !%it

" /e0in !he!$p%int

# en. !he!$p%int

- T3 up.ate A

2 T2 up.ate C

T2 !%it

(a)

TRAN'ACTION ID &A'T &'N 'TAT4' *AGE ID &'N

T1 3 !%it C 1

T2 2 in pr%0ress B 2

TRAN'ACTION ID &A'T &'N 'TAT4' *AGE ID &'N

T1 3 !%it C 1

T2 !%it B 2

T3 in pr%0ress A

(/)

TR/N!/CT0N T/23E D0RT# %/4E T/23E

(!)

TR/N!/CT0N T/23E D0RT# %/4E T/23E

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El i/N th F d t l D t b ! t F th Editi Ch t 19 35

Database Recovery

1 Recovery i )ulti.atabase syste)

5 multidata&ase system is a special distri&uted data&ase system

1here one node may &e running relational data&ase system under

+ni8, another may &e running o&Kectoriented system under

1indo1 and so on. 5 transaction may run in a distri&uted $ashion

at multiple nodes. In this e8ecution scenario the transactioncommits only 1hen all these multiple nodes agree to commit

indi"idually the part o$ the transaction they 1ere e8ecuting. 'his

commit scheme is re$erred to as Gtwo-phase commitH /2PC. I$ any

one o$ these nodes $ails or cannot commit the part o$ the

transaction, then the transaction is a&orted. Each node reco"ers thetransaction under its o1n reco"ery protocol.

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