IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 139)

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System Architecture 2-45

Managing Shared-Memory Resources

The blobtimestamp onthe disk page changes each time theblob dataon the

page is overwritten. The blob timestamp stored with the forward pointer

changeseachtimeanewblobreplacestheoldblob.Forexample,whenablob

in a data row is updated, the new blob is stored on disk, and the forward

pointerstoredwiththeblobdescriptorisrevisedtopointtothenewlocation.

The blob timestamp in the data row is updated and synchronized with the

blobtimestamponthenewblob’sdiskpage.Theblobtimestamp onthenow-

obsolete disk page is no longer synchronized. (An illustration of blobspace

blob storage shows this in Figure 2-39 on page 2-150.)

Because retrieving a blob can involve large amounts of data, it might be

impossible to retrieve the blob data simultaneously with the rest of the row

data. Coordination is needed for blob reads that OnLine user processes may

perform at the Dirty Read or Committed read level of isolation. Therefore,

each read compares the twomembersof theblobtimestamp pairasa test for

logicalconsistencyofdata.Ifthetwotimestampsinthepairdiffer,thisincon-

sistency is reported as a part of consistency checking. (Refer to page 4-6 for

further information about consistency-checking errors and corrective

actions.)Theerrorindicateseitherthatthepageshavebeencorruptedorthat

the blob forward pointer read by the OnLine user process is no longer valid.

To understand how a forward pointer stored with a blob descriptor or with

the previous segment of blob data may become invalid, consider this

example.AprogramusingDirtyReadisolationis abletoreadrowsthathave

beendeletedprovidedthedeletionhasnotyetbeencommitted.Assumethat

one OnLine server process is deleting a blob from a data row. During the

delete process, another OnLine server process operating with a Dirty Read

isolation level reads the same row, searching for the blob descriptor infor-

mation. In the meantime, the ﬁrst transaction completes, the blob is deleted,

the space is freed, and a third process starts to write new blob data in the

newly freed space where the ﬁrst blob used to exist. Eventually, when the

second OnLine server process starts to read the blob data at the location

where the ﬁrst blob had been stored, the process compares the value of the

timestampreceivedfromtheblobdescriptorwiththevalueofthetimestamp

that precedes the blob data. The timestamps will not match. The blob

timestampontheblobpagewill begreaterthanthetimestamp intheforward

pointer,indicatingto the server processthat the forwardpointerinformation

it has is obsolete.

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