IBM 000-8697 Manual de usuario descargar pdf (Pagina 155)

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System Architecture 2-61

How a User Process Acquires a Buffer

Step 1: Identify the Data

OnLine user processes request a speciﬁc data row by rowid. (Refer to

page 2-123foradeﬁnitionofrowid.)OnLinetranslatesthelogicalrowidinto

a physical page location. The user process searches for this page.

Step 2: Determine Lock-Access Level

Next OnLine determines the level of lock access required by the requesting

userprocess:share,update,orexclusive.(Refertopage 2-38 for further infor-

mation about buffer locks.)

Step 3: Locate the Page in Memory

The user process ﬁrst attempts to locate the requested page in shared

memory.Todothis,ittriestoacquirealatchonthehashtableassociatedwith

the buffer table. Ifthe processcan acquire the latch,it searches the hash table

tosee if anentrymatches the requestedpage. If itﬁndsan entry forthepage,

it releases the latch on the hash table and tries to acquire the latch on the

buffer header entry in the buffer table.

With accessto thebufferheader, the requesting processadds itsuser process

IDtotheuser listbitmapforthebufferandincrementstheshared-usercount

by 1.

The user process tests the current lock-access level of the buffer.

If the levels are compatible, the requesting user process gains access to the

buffer. Ifthe current lock-access levelis incompatible, therequestingprocess

puts itself on the user wait list of the buffer. The buffer state, unmodiﬁed or

modiﬁed, is irrelevant; even unmodiﬁed buffers can be locked.

For further information about the entry stored in the buffer table, refer to

page 2-48.

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IBM 000-8697 Manual de usuario Pagina 155

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