远程数据复制和恢复保证数据统一.pdf

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DATAREPLICATIONANDRECOVERYOVERANY
DISTANCEENSURING100%DATACONSISTENCY
NetworkAppliance,Inc.
December2006|WP-7007-1206
TABLEOFCONTENTS
1Introduction..................................................................................3
2AShortHistoryofDisasterRecoveryApproaches..............4
Tape-BasedBackup.................................................................4
SynchronousReplication..........................................................5
AsynchronousReplication........................................................6
3TheChallengesofAsynchronousReplication......................6
NoMechanism..........................................................................6
Serialization...............................................................................6
SingleDeviceTimeStamping..................................................7
GroupStamping........................................................................7
MultideviceTimeStamping......................................................7
4TDPS:TheOptimalReplicationArchitecture........................8
5Summary......................................................................................9
2DataReplicationandRecoveryoverAnyDistanceEnsuring100%DataConsistency
1INTRODUCTION
Allenterpriseapplicationsthathandledata,suchasfilesystemsanddatabases,
arewrittensothattheycanreliablyrecoverfromfailuresthatcausetheapplication
,andtheirrecoveryisbased
onthefactthatanychangestotheirdataarewritteninaverywell-definedorder

environment,anychangestothereplicaorcopyattheremotesitemustbe
appliedinexactlythesameorderasdefinedbytheapplicationattheprimarysite.
Ifthisorderispreservedattheremotesite,thencrashtoleranceismaintained,
,thendata
consistencyiscompromisedattheremotesiteandareliablerecoverycannot
occur.
Synchronousreplicationisawellunderstoodandwidelydeployedtechnologyfor

preservestheprimarysitewriteorderingbyperformingallwritesinlockstepatall
,itmustbe
appliedinalllocationsbeforeanacknowledgementisreturned,allowingthe
,primarysiteapplicationwriteorderingis
preservedandcrashtoleranceismaintainedinthedatareplicaattheremotesite,
,therearelimitationstosynchronous
replicationinthreeareas:performance,distance,anduseinIP-basednetworks.
Synchronousreplicationintroduceslatencywhilewaitingforthecompletionofthe
(forexample,whenitisperformedacross
serversthatresideinthesameroom),thelatencytocompleteallwritescanbe

affectstheperformanceoftheapplicationenvironmentbeingreplicated,slowingit

replicationsourceandtarget,latencyisfurtherincreasedbytheamountoftime
requiredtosendthewritetotheremotesiteandreceiveanacknowledgementof
,thelatencybecomessogreatthatsynchronous

limitedtheuseofsynchronousreplicationtoconfigurationswherethesitesare
typicallynomorethan30to50milesapart.
TheuseofIP-basednetworksisalsoproblematicinsynchronousreplication
,andthelatencies
causedbyIProutinganderrorrecoverymechanismsgenerallyintroducesomuch
additionallatencythatsynchronousreplicationcannotbeusedinthese

onlyacrossdedicated,proprietary,andveryexpensivenetworks.
Theconceptofasynchronousreplicationwasintroducedtomeettheneedfor
longerdistancereplicationconfigurationsthatcaneffectivelydealwithlatencyand

thatasynchronousreplicationoffersthesamedataconsistencyandreliabilityover
longdistancesthatsynchronousreplicationdoesovershortdistances;however,
preservingprimarysitewriteorderingbecomesasignificantchallengein
,forany
asynchronousreplicationsolutionbeingevaluated,howtheissueofdata

orderingtobemaintainedattheremotesite,theoperationalimpactofthe

overlookedduringevaluation,andcancausesignificantimpactonoperationsand
recoverabilityasenvironmentsscalefrompilottoproduction.
3DataReplicationandRecoveryoverAnyDistanceEnsuring100%DataConsistency
Thiswhitepaperdiscussesthestorageindustry
sattemptstoaddressthedistance
issuefordisasterrecoverypurposes,evaluatestheapproachestopreserving
primarysitewriteorderinginasynchronousreplicationsolutions,andpointsoutthe
keyissuestounderstandwhenconsideringlongerdistancedisasterrecovery
configurations.
2ASHORTHISTORYOFDISASTER
RECOVERYAPPROACHES
TAPE-BASEDBACKUP
Disasterrecoveryhasbeenaconcernsincetheearliestdaysofbusiness

DataflowsBackup
acrossaserversbusinessoperationstoberecoveredintheeventofacatastrophicdisasterwasa
,thisneedwasmetthroughtheuseof
tothedatatotapeenterprisebackupsoftwarethatallowedcopiesofoperationaldatatobeperiodically
backupcreatedonmediathatcouldeasilybetransportedtooff-
serverstheeventofacatastrophicfailurethatdestroyedservers,storagesubsystems,orin
somecasesevenentiresites,recentoperationaldatacouldberetrievedfromthe
.off-
.LAN/WANbackupoperationshashistoricallybeenmagnetictape.
Whentheresponsetimeofbusinessoperationswasmeasuredinhoursordays,
tape-basedbackupdidagoodjobofmeetingrecoveryrequirementsinacost-
,moreand
moredataneededtobewrittentocreatethesecopies,whichtookmoretime.
ClientsIncreasingglobalizationdrovetheindustrytoward7x24operations,cuttingdownthe
amountoftimethatcomputerscouldbeofflinetocreatethesecopies(ortape-based
backups).Becauseofpoliticalandnaturaleventsthathaveledtowidespread
disruptionofbusinessoperations,theabilitytoreliablyrecoverhasgrown

pointedoutthefollowingweaknessesofthetape-basedbackupsoftwareapproach.
Backupmaster•Backupwindow:Applicationsareshutdownsothatatransaction-
consistentcopyofthedatacanbebackedup,creatingawindowof


backupwindowshavebeenamanagementissueforatleastthelast10
years.
•Dataloss:Backupscreatedacopyofthedataatapointintime(thepoint
Offsitestorageatwhichtheapplicationwasshutdowntotakethebackup).Becauseof
shrinkingbackupwindowsandtheincreasingtimeittooktobackan
Figure1)Tape-basedbackups(copies)areapplicationup(duetolargercapacity),backupswerenottakenvery
,withhigherspeedbackuptechnologies,backups

tapewasrequired,thebestthatcouldbeexpectedwasrecoveryfromthe

backupwereusuallylost.
•Mediaintegrityissues:Asdatawascopiedtotape,itwasconvertedfrom

,tapeswere
removedandsenttooff-sitelocationssothattheywouldnotbeaffectedby
catastrophicdisasters,
companiesoftenstoredthousandsortensofthousandsoftapesatthese

,theGartner
Groupstatedthat,onaverage,oneinfourbackuptapeshadunrecoverable

4DataReplicationandRecoveryoverAnyDistanceEnsuring100%DataConsistency
StorageGroupinApril2004,61%ofrespondentsidentifiedmediafailure
,ofcourse,
isthatacompanycouldnotknowthatafilewasunrecoverableuntil
recoverywasattempted,atwhichpointitwastoolate.
•Restoreperformance:Mostenterprisebackupsoftwareisbasedon

metricintheearlierera,whendisasterswererare,andarchitecturesand
featuresweredevelopedthattradedoffrestoreperformancetogetbetter

important,manybackupvendorsarelimitedbyarchitecturaldecisionsthey
madeinthepastthatareverydifficulttochange.
SYNCHRONOUSREPLICATION
Itsoonbecameapparentinthelarge,verycomputer-intensivebusinessoperations
thatbackupcouldnotadequatelymeettheneedforbusinessoperationrecovery.
PRIMARYSITEREMOTESITEThesecompaniesbegantolookatanalternativetechnologycalledsynchronous
replication,whichcreatedamirrorcopyofdatainaseparatephysicallocation,

copieswerereferredtoasthesource(theoriginalcopy)andthetarget(thereplica).
Thereplicationwassynchronousinthesensethatbothcopieswereupdatedwith
everywritesothattheysharedthesamedatastateatalltimesandprimarysite

industries,andwasalmosta$1Bbusinessin2003.
Synchronousreplicationaddressedthebackupwindow,dataloss,mediaintegrity,
,transparentlytothe
SourcedataTargetorreplicaprimaryapplication,
continuousandthesourceandtargetwerekeptinperfectlockstepatalltimes,


Exactcopiesofthedataareformatattacheddirectlytoaserveratthesecondarysite,sotherewerenotape
keptinsyncattwositesmediaintegrity,formatconversion,

Disk-basedsynchronousasingle-stepprocessfromdiskbecauseadumpfromtapetodiskwasnolonger
replicationsolvesbackuprequired,
window,dataloss,anddiskwasamuchfastermediumthantapethatalsosupportedrandomaccess,
dataintegrityissuesrestoreswerefasterstill.

Introducesnewproblems:Synchronousreplicationwasnotapanacea,however,andintroducedthreenew
–Vendorlock-,allthepopularimplementationsarestorage-based,locking
–
directlytothesecondproblem,
–Distancelimitations
arerequiredtopurchaseaminimumoftwoenterprisestoragearrays(oneforeach
Figure2)Synchronousreplicationaddressedsite),theentrypricepointforevenasmallreplicationconfigurationisontheorderof
theissueswithtape-basedbackupbut$500Kto$1M,,synchronous

toanonlinetargetintroducedlatency,andthefartherawaythetarget,themore
,

impacttoaminimum,companiesgenerallylimitedsynchronousreplication
vconfigurationstodistancesofnomorethan50miles.
5DataReplicationandRecoveryoverAnyDistanceEnsuring100%DataConsistency
ASYNCHRONOUSREPLICATION
Toenablelongerdistanceconfigurationsthatcouldensuresurvivabilityeveninthe
PRIMARYSITEREMOTESITEworstdisasters,
decoupledthewritetothesecondarysitefromthewritetotheprimarysite,allowing
theprimaryapplicationtofuncti