ORACLE DATAGUARD ADMINISTRATION 1What is Data Guard?Data Guard,
which was introduced as the standby database in Oracle database
Version 7.3 under thename of Data Guard with Version 9i, is a data
protection and availability solution for Oracle databases.The basic
function of Oracle Data Guard is to eep a synchroni!ed copy of a
database as standby, inorder to mae provision, in case the primary
database is inaccessible to end users. These cases arehardware
errors, natural disasters, and so on. "ach new Oracle release added
new functionalities toData Guard and the product became more and
more popular with offerin#s such as data protection,hi#h
availability, and disaster recovery for Oracle databases. $sin#
Oracle Data Guard, it%s possible to direct user connections to a
Data Guard standby databaseautomatically with no data loss, in case
of an outa#e in the primary database. Data Guard also offerstain#
advanta#e of the standby database for reportin#, test, and bacup
offloadin#. &orruptions
ontheprimarydatabasemaybefi'edautomaticallybyusin#thenon(corrupteddatablocsonthestandby
database. There will be minimal outa#es )seconds to minutes* on the
primary database inplanned maintenances such as patchin# and
hardware chan#es by usin# the switchover feature ofData Guard,
which chan#es the roles of the primary and standby databases. +ll
of these features areavailable with Data Guard, which doesn%t
re,uire an installation but a clonin# and confi#uration of
theOracle database. +Data Guard confi#uration consists of two main
components- primary database and standbydatabase. The
primarydatabase is the database for whichwe want totae
precautionfor itsinaccessibility. .undamentally, chan#es on the
data of the primary database are passed throu#h thestandby database
andthese chan#es are appliedtothe standby database inorder
toeepitsynchroni!ed. The followin# fi#ure shows the #eneral
structure of Data Guard-Database/erver +Database/erver
01rimary2nstance3edo Transport/tandby2nstance1rimary/tandbyDatabase
Database$pdate )3edo$pdates+pply or /45)D06n*+pply*1rimary
/tandbyDatabase Database/tora#e + /tora#e 05et%s loo at the standby
database and its properties more closely. /tandby database
2tispossibletoconfi#ureastandbydatabasesimplybycopyin#, clonin#,
orrestorin#aprimarydatabase to a different server. Then the Data
Guard confi#urations are made on the databases in orderORACLE
DATAGUARD ADMINISTRATION 2to start the transfer of redo information
from primary to standby and also to start the apply processon the
standby database.Primary and standby databases may exist onthe same
server;however, this kind of configuration should only be used for
testing.Inaproductionenvironment,
theprimaryandstandbydatabaseservers are generally preferred to be
on separate data centers.Data Guard eeps the primary and standby
databases synchroni!ed by usin# redo information. +s youmay now,
transactions on an Oracle database produce redo records. This redo
information eeps allof the chan#es made to the database. The Oracle
database first creates redo information in memory)redo lo#
buffers*. Then they%re written into online redo lo#files, and when
an online redo lo#file is full,its content is written into an
archived redo lo#.An Oracle database can run in the A!"I#$%O&
mode or the'OA!"I#$%O&mode. In the A!"I#$%O&mode, online
redologfiles are written into archived redo logs and in
the'OA!"I#$%O& mode, redo logfiles are overwritten without
beingarchivedastheybecomefull. Ina(ata&uardenvironment,
theprimary database must be in the A!"I#$%O& mode.2n Data
Guard, transfer of the chan#ed data from the primary to standby
database is achieved by redowith no alternative. 7owever, the apply
process of the redo content to the standby database may vary.The
different methods on the apply process reveal different type of
standby databases. There were two inds of standby databases before
Oracle database Version 88g, which were- physicalstandby database
and lo#ical standby database. 6ithin Version 88g we should mention
a third type ofstandby database which is snapshot standby. 5et%s
loo at the properties of these standby databasetypes. 1hysical
standby database The 1hysical standby database is a bloc(based copy
of the primary database. 2n a physical standbyenvironment, in
addition to containin# the same database ob9ects and same data, the
primary andstandby databases are identical on a bloc(for(bloc
basis. 1hysical standby databases use 3edo +pplymethod to apply
chan#es. 3edo +pply uses Managed recovery process )MRP* in order to
mana#eapplication of the chan#e in information on redo. 2n Version
88g, a physical standby database can be accessible in read(only
mode while 3edo +pply isworin#, which is called +ctive Data Guard.
$sin# the +ctive Data Guard feature, we can offload report9obs from
the primary to physical standby database.Physical standby database
is the only option that has nolimitation on storage vendor or data
types to keep asynchroni)ed copy of the primary database.5o#ical
standby database 5o#ical standby database is a feature introduced
in Version 9i3:. 2n this confi#uration, redo data isfirst converted
into /45 statements and then applied to the standby database. This
process is called/45 +pply. This method maes it possible to access
the standby database permanently and allowsread;write while the
replication of data is active. Thus, you%re also able to create
database ob9ects onthe standby database that don%t e'ist on the
primary database. /o a lo#ical standby database can beused for many
other purposes alon# with hi#h availability and disaster recovery.
Due to the basics of /45 +pply, a lo#ical standby database will
contain the same data as the primarydatabase but in a different
structure on the diss. ORACLE DATAGUARD ADMINISTRATION 3One
discoura#in# aspect of the lo#ical standby database is the
unsupported data types, ob9ects, andDD5s. The followin# data types
are not supported to be replicated in a lo#ical standby
environment- 8 0.25" : &ollections )includin# V+33+7F and
started to wait for the ne't lo# )>78* to completelyarrive and
#et archived. 2t also indicates that the ne't lo# is in
transit,which means it iscurrently bein# received by 3./. A. 5et%s
convert the 3edo +pply mode to real(time apply and see how Data
Guard will apply theredo as it received from the primary database.
.irst we%llstop 3edo +pply on the standbydatabase and start a#ain
in the real(time apply mode- S&'( !'4/R D!4!5!S/ R/60/R M!N!G/D
S4!ND5Y D!4!5!S/ !N/'. Data#ase alteredG ORACLE DATAGUARD
ADMINISTRATION 11S&'( !'4/R D!4!5!S/ R/60/R M!N!G/D S4!ND5Y
D!4!5!S/ 1S2NG 1RR/N4'6GF2'/ D2S6NN/4 FR6M S/SS26N. Data#ase
alteredG 9. +fter a few minutes we will chec the status of the
processes- S&'( select process) status) thread;) seS
--------------------------------------- ---------------- !R=
6NN/4/D: : : : !R= '6S2NG?@B-K@K-@@C @:K MRP: !PP'Y2NG,'6G
?@BK?ACDK-C @:A9::: RFS2D'/ ::: : RFS2D'/ ?@BK?ACDADB ?? Gow it%s
obvious that =31 is applyin# the lo# as it arrives to standby. The
3./ process is transferrin#the lo# se,uence >73, which is the
current lo# on the primary side, and at the same time the
=31process is applyin# the same lo# se,uence. 5oo at the bloc
number columnD we can see that =31 isapplyin# the redo blocs that
have 9ust arrived. You should also $no" that) even there is a D/'!Y
value specified on thepri%arydata#ase. if
theapply%odeisreal-ti%eapplyonthestand#ydata#ase) the D/'!Y "ill #e
ignoredG You+ll see the follo"ing lines in thestand#y alert log in
such a caseJManaged stand#y recovery started "ith 1S2NG 1RR/N4
'6GF2'/2gnoring previously specified D/'!Y 9: %inutes/45 +pply
)lo#ical standby databases* The /45 +pply technolo#y resides on
minin# the standby redo lo#s, buildin# /45 transactions
thatapplythechan#esin,uestion,
andfinally,e'ecutin#the/45onthestandbydatabase,
whichisread;writeaccessible. Thisprocessismoree'pensiveintermsof
hardwareresourceusa#easamatter of course. The 5/1 process mana#es
the application of chan#es to a lo#ical standby database. The
#eneralpurpose of buildin# a lo#icalstandby database is reportin#
the needs with read; writeaccess re,uirement. /45 +pply is not
suitable for disaster recovery and hi#h availability as much as3edo
+pply because of the unsupported data types and lo#ically different
database infrastructure. /45 +pply offers the followin# benefits to
its users- 8 The lo#ical standby database is always read;write
accessible while /45 +pply is runnin#D sothat users may run
reports, create temporary tables and inde'es for performance
issues. +lsoit%s possible to create ob9ects and eep data on the
standby database, which do not e'ist onprimary. : The lo#ical
standby database is open for read;write activity. 0ut normally
there are no writespossible on the standby ob9ects, which e'ist on
primary. This feature maintains theconsistency of the replicated
primary data. 3 2t%s possible to up#rade the Oracle database
software version with almost no downtime usin# alo#ical standby
database. 3ole transitions 3ole transitions basically enable users
to chan#e the roles of the databases ina Data Guardconfi#uration.
There are tworole transitionoptions inDataGuard,
whichares"itchoverandfailover./witchover
2nabasicDataGuardconfi#urationwithoneprimaryandonestandbydatabase,
aswitchoveroperationchan#estherolesof thesedatabases,
andsothedirectionof theredoshippin#.
2nacorrectlydesi#nedconfi#uration,archived lo# shippin#in the
opposite direction starts immediatelyafter switchover and clients
do not need to chan#e their connection descriptions in order to
connectthe new primary database. ORACLE DATAGUARD ADMINISTRATION
122f there is more than one standby database in a Data Guard
confi#uration, it%s possible to performswitchover between
theprimary and any of the standby databases. +fter the switchover,
the newprimary database can continue to send redo to all of the
standby databases in the confi#uration. 3e#ardless of the
confi#uration of Data Guard, a switchover operation always
#uarantees !ero dataloss. Thisbrin#shi#hreliabilitytoswitchover
andthusit%swidelyusedfor plannedmaintenanceoperations, such as
hardware or operatin# system up#rades, database software rollin#
up#rade, andother infrastructure maintenances. /witchover reduces
the downtime for these maintenanceoperations by a si#nificant
amount of time..ailover .ailover is the operation of convertin# a
standby database to a primary database, because of a
failureintheori#inal primarydatabase. 2f
theflashbacdatabaseisdisabledontheprimarydatabase,failover
isanoperationwithnoreturn. 2nother words,
wehavetoflashbacthefailedprimarydatabase to a state before failover
in order to re(establish the confi#uration. 6ithout flashbac,
DataGuard confi#uration needs to be built from scratch. + manual
database failover may be performed in the case of failure with the
initiative of the databaseowner. 7owever, this will re,uire e'tra
outa#e for the decision main#. 2f fast(start failover is used,which
is a 8Fg release : feature, the failover operation will perform
automatically..ast(start failover This property of automatin# the
failover operation can only be used in Data Guard broer
enabledconfi#uration. The observer process which runs on a
different server from the primary and standbydatabases,
continuously monitors the accessibility of the primary database. 2f
both the observer andthe standby database cannot reach the
primarydatabase for a predefined len#th of time, a fully(automated
failover process is started. 6ith 88g 3elease :, we call it fully
automated, because thisprocess includes chan#in# the role of the
standby as primary, startin# the database services on thenew
primary database, disconnectin# the client from the failed primary
database, and redirectin# themto the new primary database.2f the
observer establishes the connection with the ori#inal primary
database a#ain after the failover, itinforms the database that the
failover was performed and it will automatically reinstate the
databaseusin# flashbac. 2n order to confi#ure fast(start failover,
we need to specify the fast recovery area andenable flashbac on the
primary and standby databases. Jeepinmindthat inVersion88g, Data
Guard must be on=a'imum+vailability or =a'imum1erformance mode in
order to use fast(startfailover. 2n 8Fg3elease :, only =a'imum
+vailabilitymode is supported for fast(start failover.Data Guard
bac#round processes 2n a Data Guard confi#uration we can see some
Oracle Data Guard specific bac#round processes inboth, primary and
standby databases. These processes perform the operations of redo
transport andapplyservices.
DataGuardbroeralsohassomespecificbac#roundprocesses.
6ecanseethedescription and duties of the most important Data Guard
processes as follows- MRP: )Managed Stand#y Recovery Process*
coordinates the read and apply process ofredo in a physical standby
database. RFS )Re%ote File Server* is responsible for receivin# the
redo data, which is sent by theprimary database to the standby
database. 'SP: )'ogical Stand#y oordinator Process* coordinates the
/45 +pply processes, whichare the minin# processes and apply
processes. 'SP? )'ogical Stand#y Dictionary 5uild Process* is used
on the lo#ical standby databaseswhen a switchover or failover is in
action. 'SP- )'ogical Stand#y Set Guard Process* is used to operate
Database Guard settin#s.Database Guard specifies which ob9ects will
be protected for modification in a lo#ical standbydatabase. NS!n
)Redo 4ransport NS!? Process* is used on the primary database to
ship redo data tothe standby database when +/ /ession 2D- 883
/erial number- 3 8. 6e%ve restarted the primary database in the
mount mode and chan#ed the protection mode. 7owever, when we tried
to open it, we encountered an O3+(F3883 error. 6e can see why the
database raised this error in the alert lo#file as follows- 'GWRJ
Destination '6G,!R=20/,D/S4,- is using asynchronous net"or$ 2F6
'GWRJ Mini%u% of ? synchronous stand#y data#ase
re/Y,1NF41R>/YFtraceF 41R>/Y,lg"r,-@CD@GtrcJ 6R!-?9:B-J a
%ini%u% of one stand#y data#ase destination is re/Y,lg"r,-D:-:GtrcJ
6R!-?9:B-J a %ini%u% of one stand#y data#ase destination is re-
database busy. Open, close, mount, and dismount not allowed now8.
+s you can see, it%s not possible to shut down a standby database
in the =a'imum 1rotection mode if it%s the only standby database
alive. 6e%ll see the followin# lines in the standby database alert
lo# when we try to shut it down- !tte%pt to shut do"n Stand#y
Data#ase Stand#y Data#ase operating in N6 D!4! '6SS %ode Detected
pri%ary data#ase alive) shutdo"n pri%ary first) shutdo"n a#orted 8.
Gow ill the /=OG process to simulate a failure on the standby
database server as follows- K ps (ef W#rep smonL2GD2+ oracle
7FE> 8 F /ep8E R FF-FF-FF oraLsmonL2GD2+ K ill (9 7FE> 8. The
Oracle instance will be terminated in the standby database after
the ill command. Gow try modifyin# the primary database by
insertin# data into a table as shown in the followin# ,uery- ORACLE
DATAGUARD ADMINISTRATION 42/45P 2G/"3T 2GTO 73.3"G2OG/ V+5$"/
)8F:,%T"/T%*D 8 row created. /45P &O==2TD 8. The commit
statement will wait and not be e'ecuted. +t this sta#e, the primary
database will not accept any chan#e because of the =a'imum
1rotection mode%s characteristic. Then the instance will be
terminated by 5G63 as shown in the followin# alert lo# lines-
Destination '6G,!R=20/,D/S4,- is 1NSYN=R6N2\/D 'GWRJ !ll stand#y
destinations have failed
UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU W!RN2NGJ !ll
stand#y data#ase destinations have failed W!RN2NGJ 2nstance
shutdo"n re. /etthe 5OGL+3&72V"LD"/TLn
initiali!ationparameterontheprimarydatabasetosupportthe re,uired
protection mode-M&onnect to the primary databaseN/45P connect
sysFchange,on,installStestd#,v%linuLKGidevelop%entGinfo as
sysd#aM&onfi#ure lo# transport services to support the desired
protection modeN/45P alter syste% set
log,archive,dest,-3+service3testd#,v%linuL@Gidevelop%entGinfo 'GWR
SYN !FF2RM+./45P alter syste% set
log,archive,dest,state,-3ena#le.?. The primary database will need
to be closed and then placed in the =O$GT sta#e./45P shutdo"n
i%%ediate/45P startup %ountE. On the primary database, chan#e the
protection mode and open the database-.or =a'imum 1rotection
mode-/45P alter data#ase set stand#y data#ase to %aLi%iMe
protection./45P alter data#ase open..or =a'imum +vailability
mode-/45P alter data#ase set stand#y data#ase to %aLi%iMe
availa#ility./45P alter data#ase open.7. 4uery the data dictionary
on the primary database to verify the new protection mode-/45P
S/'/4 PR64/426N,M6D/) PR64/426N,'/0/') D!4!5!S/,R6'/ FR6M
0*D!4!5!S/.13OT"&T2OGL=OD"
13OT"&T2OGL5"V"5D+T+0+/"L3O5"(((((((((((((((((((
((((((((((((((((((( ((((((((((((((=+T2=$= 13OT"&T2OG=+T2=$=
13OT"&T2OG132=+3.2D"V"5O1="GT.2G.Oo urrent Protection
Mode-=a'imum protection usin# 5G63 /. Down#rade the primary
database to the desired protection mode-M&onnect to the primary
databaseN/45P connect
sysFchange,on,installStestd#,v%linuLKGidevelop%entGinfo as
sysd#aM.or =a'imum 1erformance modeN/45P alter data#ase set stand#y
data#ase to %aLi%iMe perfor%ance.M.or =a'imum +vailability
modeNORACLE DATAGUARD ADMINISTRATION 61/45P alter data#ase set
stand#y data#ase to %aLi%iMe availa#ility.?. 0oth of the hi#her
protection modes )=a'imum +vailability and =a'imum 1rotection*
re,uirethe use of standby redo lo#s on the destination standby
database. 2t you were to down#radefrom =a'imum 1rotection mode to
=a'imum +vailability mode, you would need to eep thestandby redo
lo#s that e'ist for the standby database. 2f down#radin# to
=a'imum1erformance mode, you can drop the standby redo lo#s from
the standby unless you are #oin#tobe confi#urin#
lo#transportservicestouse 5G63..orthise'ample, 2am
#oin#tobedown#radin# to ma'imum performance mode and usin# +3&7
for lo# transport services andcan therefore drop any standby redo
lo#s from the standby database )and the primary if
theye'ist*-M&onnect to the standby databaseN/45P connect
sysFchange,on,installStestd#,v%linuL@Gidevelop%entGinfo as
sysd#aMDrop any standby redo lo#s from the standby databaseN/45P
alter data#ase drop stand#y logfile group @./45P alter data#ase
drop stand#y logfile group D./45P alter data#ase drop stand#y
logfile group 9./45P alter data#ase drop stand#y logfile group B.E.
2f standby redo lo#s e'ist on the primary and you are down#radin#
to =a'imum 1erformance mode, they can be removed-M&onnect to
the primary databaseN/45P connect
sysFchange,on,installStestd#,v%linuLKGidevelop%entGinfo as
sysd#aMDrop any standby redo lo#s from the primary databaseN/45P
alter data#ase drop stand#y logfile group @./45P alter data#ase
drop stand#y logfile group D./45P alter data#ase drop stand#y
logfile group 9./45P alter data#ase drop stand#y logfile group B.7.
1ut the standby database in mana#ed recovery mode and open the
primary database with the new protection mode enabled-M&onnect
to the standby databaseN/45P connect
sysFchange,on,installStestd#,v%linuL@Gidevelop%entGinfo as
sysd#a/45P alter data#ase recover %anaged stand#y data#ase
disconnect fro% session.M&onnect to the primary databaseN/45P
connect sysFchange,on,installStestd#,v%linuLKGidevelop%entGinfo as
sysd#a/45P alter data#ase open.ORACLE DATAGUARD ADMINISTRATION 62A.
4uery the data dictionary on the primary database to verify the new
protection mode-/45P select protection,%ode) protection,level)
data#ase,role fro%
v*data#ase.13OT"&T2OGL=OD"13OT"&T2OGL5"V"5
D+T+0+/"L3O5"(((((((((((((((((((( ((((((((((((((((((((
((((((((((((((=+T2=$= 1"3.O3=+G&"=+T2=$= 1"3.O3=+G&"132=+3
bit to a 3: bitdatabase which is #reat if we want to mi#rate to a
different O;/ of 3:;E> bit architecture, also ourrollbac option
is very easy simply switchbac if it did not wor.-G8FailoverJ
.ailoverisaunplannedevent,
thisiswherethe"O3wasneverwrittenbytheprimary database, the standby
database process what redo it has then waits, data loss now
dependson the protection mode in affect . =a'imum 1erformance (
possible chance of data loss =a'imum +vailability ( possible chance
of data loss =a'imum 1rotection ( no data losswe have the option to
manually failover or mae the whole process automatic, manual #ives
the D0+ma'imum control over the whole process obliviously the the
len#th time of the outa#e depends on#ettin# the D0+ out of bed and
failin# over. Otherwise Oracle Data Guard .ast(/tart .ailover
featurecan automatically detect a problem and failover
automatically for us. The failover process should taebetween 8? to
:? seconds.ORACLE DATAGUARD ADMINISTRATION 63Which Role 4ransition
6peration Should 2 1se?6henfaced with the decisionon
whichroletransition isbest forthe #ivensituation, weneed
toalwayschooseonethatbestreducesdowntimeandhastheleastpotential
fordataloss. +lsotoconsider is how the chan#e will affect any other
standby database in the confi#uration. 6e shouldconsider the
followin# when main# the decision on which operation to use- 6hat
is the current state of the primary database 9ust before the
transitionR 2s it availableR 6hat is the state of the selected
standby database to be used in the role transition at the timeof
transitionR 2s the standby database confi#ured as a physical or
lo#ical standby databaseRThefollowin#decisiontreecanbeusedtoassist
whenmain#thiscritical decisionastowhichoperation to perform-ORACLE
DATAGUARD ADMINISTRATION 64One ey point to consider is that if it
would be faster to repair the primary database )from failure or
asimple planned hardware;software up#rade*, the most efficient
method would be to perform the tassand then to brin# up the primary
database as ,uicly as possible and not perform any type of
roletransition. This method can impose less ris to the system and
does not re,uire any client software tobe re(confi#ured.+nother
consideration involves a Data Guard confi#uration which includes a
lo#ical standby database.+ switchover operation can be performed
usin# either a physical or lo#ical standby database. Taenote,
however,of thefollowin#issuesyoumayrunintore#ardin#physical
andlo#ical standbyconfi#urations. 2f the confi#uration includes a
primary, a physical standby, and a lo#ical standby, and aswitchover
is performed on the lo#ical standby, the physical standby will no
lon#er be a part of theconfi#uration and must be rebuilt. 2n the
same scenario, if a switchover operation is performed on
thephysical standby, the lo#ical standby remains in the Data Guard
confi#uration and does not need to berebuilt. Obviously, a physical
standby is a better option to be a switchover candidate than a
lo#icalstandby when multiple standby types e'ist in a #iven
confi#uration.7ence finally we come to conclusion that the order to
setup Data Guard is the followin# - The primary database is up and
runnin# &reate a standby database /etup the redo transport
rules &reate the /35 files "'ecute one of the followin#/45P
alter database set standby to ma'imum performanceD;;)default* /45P
alter database set standby to ma'imum availabilityDORACLE DATAGUARD
ADMINISTRATION 65/45P alter database set standby to ma'imum
protectionDOracle Data Guard confi#uration consists of one database
which functions in the primary role and oneor more databases which
function in the standby role. The database in the standby role can
be a1hysical /tandbyor a 5o#ical /tandby.1hysical /tandby@ 2s an
identical copy of the primary database, with on dis database
structures thatare identical to primary database on a bloc(for(bloc
basis. 1hysical /tandbyis ept synchroni!ed withprimary usin# 3"DO
+115FE9 to 7>F9:*. .or
e'ample,OD+T+;proddbL:;archivelo#;:FFALF9L87;threadL8Lse,L7>FE9.99?.EE?E3FAE8:.
3e#ister all these archived lo#s with the standby database.alter
database re#ister lo#file
%OD+T+;proddbL:;archivelo#;:FFALF9L87;threadL8Lse,L7>FE9.99?.EE?E3FAE8%Dalter
database re#ister lo#file
%OD+T+;proddbL:;archivelo#;:FFALF9L87;threadL8Lse,L7>F7F.99A.EE?E38>F?%Dalter
database re#ister lo#file
%OD+T+;proddbL:;archivelo#;:FFALF9L87;threadL8Lse,L7>F78.79:.EE?E337??%Dalter
database re#ister lo#file
%OD+T+;proddbL:;archivelo#;:FFALF9L87;threadL8Lse,L7>F7:.:E3.EE?E33783%DRR..R.and
so on till the last one.3. Gow chec the view v7archived5log and
finally should see the lo#s bein# applied. The status of=31 should
chan#e from +3&72V"L5OGLG+1 to +115
