How Buffer Cache Works - Proligence

How Buffer Cache Works Arup Nanda

Longtime Oracle DBA

Oracle Instance

Source: Oracle Database Documentation Concepts Guide

@ArupNanda

How Buffer Cache Works

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Buffer Operation 1

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Buffer Cache

SELECT … FROM EMP WHERE …

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Buffer Operation 1

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1 Data Block

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Buffer Cache

SELECT … FROM EMP WHERE …


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Buffer Insertion 10

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Buffer Header 1

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Buffer Cache

Shared Pool

X$BH V$BH

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buffhan.sql

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Buffer Header Management 10

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Linked List

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When a new buffer comes in, only the pointers are updated

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Hash Chain

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Linked List X$BH NXT_HASH PRV_HASH

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Test for Buffer Header select ltrim(addr,'0') buffer_address, ltrim(nxt_hash,'0') next_buffer, ltrim(prv_hash,'0') prev_buffer, case when nxt_hash = prv_hash then 'Unlinked' else 'Linked' end as linked from x$bh where hladdr = '000007FF3C8B1568' bh1.sql

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Where did I Park My Car?

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Data Block Address • Get the relative file# and block# select col1, dbms_rowid.rowid_relative_fno(rowid) rfile#, dbms_rowid.rowid_block_number(rowid) block# from cbctest; fblk.sql

• Get the DBA select dbms_utility.make_data_block_address (file#,block#) from dual;

get_dba.sql

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Chain Placement

Get the DBA

Find out the HC

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Data Block

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CBC Latch Latch

H.C.

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Catch Buffer Chain (CBC) Latch

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Latches and Hash Chains Hash Chain Latch

HC1

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No. of hash buckets = init.ora parameter _db_block_hash_buckets No. of latches = _db_block_hash_latches

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Identifying Buffer Latches • Demo – – – – – – –

Find out the rows and blocks – qsales.sql Find out the data object id – dobjid.sql Find out the data block address – dba1.sql Find out the child latch address – hladdr1.sql Find out the partition name – extents1.sql Find out the objects protected by a latch – latchobjs.sql Find out the total buffers per latch – clatchcount.sql

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Latches Latch

Process 1

Process 2

Process 1 and 2 will try to get the “latch”, a area in memory that does not have any required data. Whoever gets the latch now gets to access the memory area exclusively When done, the process releases the latch

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Linked List X$BH NXT_HASH PRV_HASH

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Linking/Unlinking Chain 1 Latch

Link

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Chain 2 Latch

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Unlink

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Cache Buffer Chain Latch _db_block_hash_latches @ArupNanda


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Summary • • • • • • • • •

Buffers are placeholders in memory Empty when the instance comes up Server process brings a data block from database to occupy a buffer There is no “database” of which block is in which buffer. A process has to scan the buffers to find what it needs The buffers are spread over several hash chains to help in searching A data block address is fixed for the block DBA determines which chain the block may be found A latch prevents multiple processes from walking the chain

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Thank You! Blog: arup.blogspot.com Tweeter: @ArupNanda Facebook.com/ArupKNanda Google Plus: +ArupNanda @ArupNanda


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