Frammentazione

Reasons

ThelinklayerhasthefeatureofmaximumtransmissionunitMTU,whichlimitsthemaximumlengthofdataframes,anddifferentnetworktypeshaveanupperlimit.TheMTUofEthernetis1500,youcanusethenetstat-icommandtocheckthisvalue.IfthereisadatapackettobetransmittedattheIPlayerandthelengthofthedatapacketexceedstheMTU,theIPlayerwillfragmentthedatapacketsothatthelengthofeachfragmentislessthanorequaltotheMTU.WeassumethataUDPdatapacketistobetransmitted,theMTUofEthernetis1500bytes,thegeneralIPheaderis20bytes,theUDPheaderis8bytes,andthepayloadpartofthedataisreservedfor1500-20-8=1472bytes.Ifthedatapartislargerthan1472bytes,fragmentationwilloccur.

Detailedintroduction

Shardingisatypeofdatabasepartitioning,whichdivideslargedatabasesintosmaller,fasterandeasiertomanageparts.Thesepartsarecalleddatafragments..Thewordfragmentmeansasmallpartofthewhole.

JasonTeesaid:"Inshort,ashardingdatabaseneedstodividethedatabaseintomultiplesmalldatabasesthathavenocommonground,andtheycanbespreadacrossmultipleservers."

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Technicallyspeaking,shardingissynonymouswithhorizontalpartitioning.Inpractice,thistermisoftenusedtorefertoalldatabasepartitionsthatmakealargedatabaseeasiertomanage.

Thecoreconceptofshardingisbasedonanidea:thesizeofthedatabaseandthenumberoftransactionsperunittimeonthedatabaseincreaselinearly,andtheresponsetimeofqueryingthedatabaseincreasesexponentially.

Inaddition,thecostofcreatingandmaintainingalargedatabaseinoneplacewillincreaseexponentially,becausethedatabasewillrequirehigh-endcomputers.Conversely,datafragmentscanbedistributedtoalargenumberofmuchcheapercommercialservers.Intermsofhardwareandsoftwarerequirements,datafragmentationisrelativelyunrestricted.

Insomecases,databaseshardingcanbedonesimply.Splittingtheuserdatabasebygeographiclocationisacommonexample.Usersontheeastcoastareassignedtooneserver,andusersonthewestcoastareassignedtoanotherserver.Assumingthatnouserhasmultiplegeographiclocations,thistypeofpartitioniseasytomaintainandcreaterules.

Butdatashardingcanbeamorecomplicatedprocessinsomecases.Forexample,ifadatabaseholdsverylittlestructureddata,shardingmaybeverycomplicated,andtheresultingfragmentationmaybedifficulttomaintain.

Fragmentation

Fragmentationprocess

ForeachIPdatagramsentbythesender,itsidentificationfieldcontainsavalue.Thisvalueiscopiedtoeachfragmentwhenthedatagramisfragmented.Theflagfieldusesonebittoindicate"moreslices".Exceptforthelastslice,everyotherslicethatcomposesthedatagrammustsetthisbitto1.Thefragmentoffsetfieldreferstothepositionofthefragmentoffsetfromthebeginningoftheoriginaldatagram.Inaddition,whenthedatagramisfragmented,thetotallengthofeachfragmentshouldbechangedtothelengthofthefragment.

Finally,thereisabitintheflagfieldcalledthe"non-fragmented"bit.Ifthisbitissetto1,IPwillnotfragmentthedatagram.Instead,thedatagramisdiscardedandanICMPerrormessageissenttotheinitiator.

AftertheIPdatagramisfragmented,eachfragmentbecomesapacketwithitsownIPheader,andisindependentofotherpacketswhenselectingaroute.Inthisway,thesepiecesofdatagrammaybeoutofsequencewhentheyarriveatthedestination,butthereisenoughinformationintheIPheaderforthereceivingendtoassemblethesepiecesofdatagramcorrectly.

Technicalinformation

TheIPheadercontainstheinformationneededforfragmentationandreorganization:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

|Identification|R|DF|MF|FragmentOffset|

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

||||

Identification:TheidentificationfieldoftheIPdatapacketsentbythesenderisasinglevalue,whichiscopiedtoeachsliceduringfragmentation.

R:Keepunused.

DF:Don'tFragment,"Donotfragment"bit.Ifthisbitissetto1,theIPlayerwillnotfragmentthedatagram.

MF:MoreFragment,"morefragments",exceptforthelastfragment,thisbitmustbesetto1foreveryotherfragmentthatcomposesthedatagram.

FragmentOffset:Thepositionofthefragmentoffsetfromthebeginningoftheoriginaldatapacket.Thenumberofbytesoftheoffsetisthisvaluemultipliedby8.

Inaddition,whenthedatagramisfragmented,thetotallengthofeachfragmentshouldbechangedtothelengthofthefragment.

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