Rocstor Rocpro 850 - HFS Plus or HFS

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43ROCSTOR‐ROCPRO850usermanual
HFSPlusorHFS+
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ThisisafilesystemdevelopedbyAppleInc.toreplacetheirHierarchicalFileSystem(HFS)as
theprimaryfilesystemusedinMacintoshcomputers(orothersystemsrunningMacOS).Itis
alsooneoftheformatsusedbytheiPoddigitalmusicplayer.HFSPlusisalsoreferredtoasMac
OSExtended(or,erroneously,“HFSExtended”),whereitspredecessor,HFSisalsoreferredto
asMacOSStandard(or,erroneously,as“HFSStandard”).Duringdevelopment,Applereferred
tothisfile
systemwiththecodenameSequoia.
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HFSPlusisanimprovedversionofHFS,supportingmuchlargerfiles(blockaddressesare32‐bit
lengthinsteadof16‐bit)andusingUnicode(insteadofMacOSRomanoranyofseveralother
charactersets)fornamingtheitems(files,folders).Nameswere
normalizedtoaformvery
nearlythesameasNFD(therearesomeminordifferencesderivedfromthefactthattheHFS
PlusformatwasfinalizedbeforeUnicodehadstandardizedtheNFDformat).HFSPluspermits
filenamesupto255UTF‐16charactersinlength,andn‐forkedfilessimilartoNTFS,though
almostnosoftwaretakesadvantageofforksotherthanthedataforkandresourcefork.HFS
Plusalsousesafull32‐bitallocationmappingtable,ratherthanHFS’s16bits.Thiswasaserious
limitationofHFS,meaningthatnodiskcouldsupportmorethan65,536allocationblocksunder
HFS.Whendisksweresmall,thiswasoflittleconsequence,butaslarger‐capacitydrives
becameavailable,itmeantthatthesmallestamountofspacethatanyfilecouldoccupy(a
singleallocationblock)becameexcessivelylarge,wastingsignificantamountsofspace.For
example,ona1GBdisk,theallocationblocksizeunderHFSis16KB,soevena1bytefilewould
takeup16KBofdiskspace.
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HFSPlusvolumesaredividedintosectors(calledlogicalblocksinHFS),thatareusually512
bytesinsize.Thesesectorsarethengroupedtogetherintoallocationblockswhichcancontain
oneormoresectors.Thenumberofallocationblocksdependsonthetotalsizeofthevolume.
HFSPlususesalargervaluetoaddressallocationblocksthanHFS,32bitsratherthan16bits.
Thismeansitcanaccess4,294,967,296(=2
32
)allocationblocksratherthanthe65,536(=2
16
)
allocationblocksavailabletoHFS.
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