vsamWord格式.docx
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vsamWord格式.docx
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Attheendofthischapter,youwillbeableto:
Definedatasets.
Describetheorganizationofdatasetsand,
DescribehowMVSstoresandretrievesdatasets.
Section1
ThedatamanagementserviceofMVSstoresrelateddataasunitscalleddatasets.Adataset,inthemainframeenvironment,isverysimilartoafileinaDOSoranUNIXenvironment.Withinadataset,thedataisorganizedintosmallerunitscalledrecords.Everydatasetisidentifiedbyauniquename.
ProgramscalledAccessMethodServices(访问方法服务)orAMSmanagethestorageandretrievalofrecords.Theefficiencywithwhichdataisaccessedfromadatasetisdirectlyrelatedtothewaytheyareorganized.(从数据集访问数据的效率与数据的组织方式有关)
数据集最多允许8个字符,并且以”.”区分,”.”被计算在数据集名的长度内。
数据集名的第一个字符必须是字母或者@#$,最后一个字符不可以是”.”
Section2
TheAMSandthereforetheorganizationofdatasets,isclassifiedintothefollowingtwocategories:
数据集分类:
Non-VirtualStorageAccessMethods(Non-VSAM)
VSAM
Non-VSAMorganizationswereoriginallydevelopedinthemid-60s.VSAMorganizationswereintroducedintheearly1970sinanefforttoovercometheshortcomingsofNon-VSAMorganizations.However,asVSAMdidnotachievethisobjectivecompletely,bothNon-VSAMandVSAMorganizationscoexisttoday.Asthenamesindicate,themaindifferencebetweentheseaccessmethodsisthatVSAMutilitiesresideinthevirtualstorageandare,therefore,moreefficient.TheNon-VSAMutilitiesresideonsecondarystoragedevicesandhavetobetransferredintothevirtualstoragebeforetheycanbeused.(非虚拟存储访问方法在使用前必须调入虚拟存储器)
Section3
Non-VSAMdatasetscanbestoredandaccessedinfivedifferentways.Theyare
存储和访问非虚拟存储访问方法数据集的五种方法:
PhysicalSequentialorPS,物理连续
PartitionedDataSetsorPDS,分割数据集
IndexedSequential,连续索引Thisorganizationisobsoletenow
DirectDataSets直接数据集Thisorganizationisobsoletenow
GenerationDataGroupsorGDG阶段数据组
Let'
stakeacloserlookatthesedatasets.
Section4(PS)物理连续
TheQueuedSequentialAccessMethod(连续队列访问方法)orQSAMutilitycontrolstheorganizationofPSdatasets.
PSdatasetsarestoreandaccessedbytheQSAMutility
Therecordsarewrittenandreadsequentially记录被顺序访问
Inthisorganization,therecordsofthedatasetarewrittenandreadsequentially.Thatis,toaccessthe50threcord,thefirst49recordshavetoberead.Inthisorganization,newrecordsareaddedattheendofthedataset.数据必须顺序存取,新增的数据放在数据集的最后
PSdatasetscanbestoredonDirectAccessStorageDevicesDASDortapes.物理连续数据集存储在DASD或是磁带
Thesedatasetsareusuallyusedtostoreuserdata.通常用来存储用户数据
Section5(PDS)
PDS(PartitionedDataSets)arestoredandaccessedbythePartitionedAccessMethodorPAMutility.
APDSispartitionedintooneormorememberseachofwhichcanbeprocessedasanindependentPSdataset.分割数据集被分割为一个或多个部分,每个部分作为相互独立的物理连续数据集(PS)处理Thenumberofmemberstobecreatedhastobespecifiedbytheuseratthetimeofcreationofthedataset.Eachmemberisidentifiedbyauniquenamethatstartswithanalphabetandcanhaveupto8characters.ThesenamesaremaintainedinadirectorythatisstoredatthebeginningofthePDS.每个部分都有唯一的以字母开头的最多8个字符的名字,这些名字存储在PDS的开头的一个目录里ThemembersofthePDScanbeaccessedindividuallyortheentirePDScanbeprocessedasasingleunit.PDSPDS的每个部分可以被独立访问或者整个PDS可以作为一个单一单元被处理
Themembersaregroupedintodirectoryblockstoenablebetteraccess.Eachdirectoryblockcanhaveamaximumoffivemembers.PDS的各个部分被集中到一个目录的区域以便于更好的访问,每个目录最多拥有5个成员
Themembersofablockcanbeaddedordeletedasandwhenrequired.However,itshouldbenotedthatwhenamemberisdeleted,onlythecorrespondingentryisremovedfromthedirectoryandthememberisnotphysicallydeleted.当目录里的成员被删除的时候,只是相应的入口被删除,成员在物理上没有被删掉Therefore,thedatasethastobereorganizedtoreclaimtheunusedspace.因此数据集必须重新组织以收回没有被使用的空间ThiscanbedoneusingutilitiessuchasPDS,REORG,IEBCOPYorCOMPRESS.
PartitioneddatasetsaremostlyusedtostoreapplicationprogramsandJCLs.TheycanonlybestoredonDASD.分割数据集通常用来存储应用程序和JCL,他们只能存储在DASD上
PDSDirectorystructure
Themembernamesaresortedinascendingorderinthedirectory.Thesizeofthedirectoryisfixedatthetimeofallocationandcannotbechangedexceptthatthefileiscopiedtoanotherfilewithadifferentsizeddirectory.
APDSisactuallytwodatasetinone.Thefirstdatasetisdirectory,consistsof256bytefixedlengthrecordswhichcontaininformationabouteachmember.
Thedirectorycontains
(i)Nameofthemember
(ii)arelativelocat
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