This month’s col­umn con­tin­ues the dis­cus­sion on data stor­age sys­tems, with a fo­cus on how file sys­tems re­main con­sis­tent even in the face of er­rors oc­cur­ring in the stor­age stack.

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Last month we dis­cussed the con­cept of scaleJ up and scaleJout stor­age and their rel­a­tive mer­its. lne of the most im­por­tant parts of a storDge stDFk Ls tKe fiOe sys­tem. In LLnux, tKere Dre D num­ber of SoSuODr fiOe sys­tems OLke ext3, ext4, btrfs, etF. FLOe sys­tems KLde tKe FomSOex de­tDLOs of tKe un­der­OyLng storDge stDFk suFK Ds tKe DF­tuDO SKysLFDO storDge of dDtD. TKey DFt Ds Fon­tDLn­ers of user dDtD Dnd serve Dny IO reTuests from user DSSOLFDtLons.

FLOe sys­tems Dre FomSOex SLeFes of soft­wDre. TrDdLtLonDOOy, tKey KDve been D kerneO FomSo­nent. DLf­fer­ent fiOe sys­tems of­fer dLf­fer­ent funFtLonDOLtLes Dnd Ser­for­mDnFe. How­ever, tKeLr Ln­terDFtLons wLtK user DSSOLFDtLons KDve been sLmSOL­fied tKrougK tKe use of tKe VLr­tuDO FLOe Sys­tem (VFS), wKLFK Ls Dn Db­strDFt ODyer sLt­tLng on toS of FonFrete fiOe sys­tem im­ple­men­ta­tions like ext3, ext4 and ZFS. The client DSSOLFDtLon FDn Sro­grDm to tKe A3Is exSosed by tKe VFS Dnd does not need to worry Dbout tKe Ln­ternDOs of tKe un­der­OyLng FonFrete fiOe sys­tems. Of ODte, there has been con­sid­er­able in­ter­est in de­vel­op­ing user sSDFe fiOe sys­tems usLng tKe FUSE mod­uOe DvDLODbOe Ln mDLn­streDm LLnux kerneOs ( fuse. source­forge.net/). User-sSDFe fiOe sys­tems FDn be FreDted by usLng tKe kerneO FUSE mod­uOe, wKLFK Ln­terFeSts tKe FDOOs from tKe VFS ODyer Dnd redLreFts tKem bDFk to user-sSDFe fiOe sys­tem Fode.

In tKLs montK’s FoOumn, we Oook Dt tKe FKDOOenges Ln en­surLng tKe sDfety Dnd reOLDbLOLty of dDtD Ln fiOe sys­tems. Ln tKLs erD of dDtD drLven busL­nesses, fiOe sys­tems Dre en­trusted wLtK tKe LmSortDnt resSon­sLbLOLty of keeSLng dDtD sDfe, For­reFt Dnd Fon­sLs­tent, for­ever, wKLOe en­surLng KLgK DFFessLbLOLty to tKe dDtD. DDtD Ooss Ls un­tKLnkDbOe, wKLOe unDvDLODbLOLty of data even for short du­ra­tions can have dis­as­trous Fon­seTuenFes on busL­nesses. :KLOe dDtD KDs been growLng Dt Dn exSo­nen­tLDO rDte, tKe reOLDbLOLty of tKe fiOe sys­tems, wKLFK DFt Ds dDtD Fon­tDLn­ers, KDs not mDtFKed tKe demDnds of tKe ‘DOwDys on/DOwDys Fon­sLs­tent’ dDtD. Un­for­tunDteOy, storDge sys­tems do fDLO Dnd tKe mDn­ner Ln wKLFK tKey fDLO Ls FomSOex— tKey FDn exKLbLt SDrtLDO fDLOures, fDLOures wKLFK Dre reFognLsed muFK Dfter tKey oFFur, fDLOures wKLFK Dre trDn­sLent, Dnd so on. EDFK of tKese dLf­fer­ent tySes of fDLOures LmSoses dLf­fer­ent reOLDbLOLty reTuLre­ments on fiOe sys­tems, wKLFK need to de­teFt, KDndOe Dnd re­pair such fail­ures.

cig­ure N shows a sim­ple model of the stor­age sys­tem, wKereLn tKe bot­tom most ODyer Ls tKe mDgnetLF medLD on wKLFK tKe dDtD Ls SKysLFDOOy stored, Dnd tKe toS most ODyer Ls tKe gen­erLF fiOe sys­tem wLtK wKLFK tKe FO­Lent DSSOLFDtLon Ln­terDFts. EDFK bOoFk of tKe storDge sys­tem FDn fDLO Ln dLf­fer­ent wDys, OeDdLng to dDtD unDvDLODbLOLty Dnd LnFon­sLstenFy. As tKe dDtD flows tKrougK tKe storDge stDFk, Dt eDFK SoLnt Lt Ls vuOn­erDbOe to er­rors. An er­ror FDn oFFur Ln Dny of tKe ODy­ers Dnd SroSDgDte to tKe fiOe sys­tem. TKLs re­suOts Ln LnFon­sLs­tent or LnFor­reFt dDtD beLng wrLt­ten or beLng reDd by tKe user tKrougK fiOe sys­tem Ln­ter­fDFes.

At tKe bot­tom-most ODyer, dLsks FDn fDLO Ln FomSOex wDys. It Ls no Oonger SossLbOe to mDke

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