1.OverviewTheconnectionfromtheserverthroughtheHBAtothestoragecontrollerisreferredasapath.Whenmultiplepathsexiststoastoragedevice(LUN)onastoragesubsystem,itisreferredasmultipathconnectivity.Itisaenterpriselevelstoragecapability.Mainpurposeofmultipathconnectivityistoprovideredundantaccesstothestoragedevices,i.etohaveaccesstothestoragedevicewhenoneormoreofthecomponentsinapathfail.Anotheradvantageofmultipathingistheincreasedthroughputbywayofloadbalancing.Note:Multipathingprotectsagainstthefailureofpath(s)andnotthefailureofaspecificstoragedevice.CommonexampleofmultipathisaSANconnectedstoragedevice.UsuallyoneormorefibrechannelHBAsfromthehostwillbeconnectedtothefabricswitchandthestoragecontrollerswillbeconnectedtothesameswitch.Asimpleexampleofmultipathcouldbe:2HBAsconnectedtoaswitchtowhichthestoragecontrollersareconnected.InthiscasethestoragecontrollercanbeaccessedfromeitheroftheHBAsandhencewehavemultipathconnectivity.Inthefollowingdiagrameachhosthas2HBAsandeachstoragehas2controllers.Withthegivenconfigurationsetupeachhostwillhave4pathstoeachoftheLUNsinthestorage.InLinux,aSCSIdeviceisconfiguredforaLUNseenoneachpath.i.e,ifaLUNhas4paths,thenonewillseefourSCSIdevicesgettingconfiguredforthesamedevice.DoingI/OtoaLUNinasuchanenvironmentisunmanageableapplications/administratorsdonotknowwhichSCSIdevicetouseallapplicationsconsistentlyusingthesamedeviceincaseofapathfailure,knowledgetoretrytheI/OonadifferentpathalwaysusingthestoragedevicespecificpreferredpathspreadingI/Obetweenmultiplevalidpaths1.1.DeviceMapperDevicemapperisablocksubsystemthatprovideslayeringmechanismforblockdevices.Onecanwriteadevicemappertoprovideaspecificfunctionalityontopofablockdevice.Currentlythefollowingfunctionallayersareavailable:concatenationmirrorstripingencryptionflakydelaymultipathMultipledevicemappermodulescanbestackedtogetthecombinedfunctionality.ClickHYPERLINK"http://sourceware.org/dm/"hereformoreinformationondevicemapper.1.2.DeviceMapperMultipathingObjectofthisdocumentisto