A test to understand what is primary and secondary LRU (Oracle Cuug)

LRU is an important linked list in buffer cache, its function I no longer detailed, already has a lot of relevant information. This is mainly discussed with you the main LRU, auxiliary LRU role.

Let's look at a test first.

Step 1: Environment introduction

Let's look at the size of the buffer cache:

Sql> Show SGA

Total System Global area 1073741824 bytes

Fixed Size 1284344 bytes

Variable Size 960497416 bytes

Database buffers 104857600 bytes

Redo buffers 7102464 bytes

Buffer Cache Size 100M.

Take a look at the test table size:

Sql> Set Linesize 1000

Sql> Col segment_name for A30

Sql> sql> Select segment_name,bytes/1024/1024 from dba_segments where segment_name= ' a3_70m ';

Segment_name bytes/1024/1024

------------------------------ ---------------

a3_70m 80

Sql> sql> Select segment_name,bytes/1024/1024 from dba_segments where segment_name= ' a4_70m ';

Segment_name bytes/1024/1024

------------------------------ ---------------

a4_70m 80

Two test tables, a3_70m, a4_70m, each size 80M. Originally wanted to build their two size to 70M, so the name with the suffix _70m, but built a little bit, however, this has no

That will not affect our test results.

Step 2: Refresh buffer cache pool, observe the length of LRU chain

Sql> alter system flush Buffer_cache;

Sql> Select Cnum_set,cnum_repl,anum_repl,cnum_write, anum_write from X$kcbwds;

Cnum_set Cnum_repl Anum_repl cnum_write anum_write

---------- ---------- ---------- ---------- ----------

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

6238 6238 6219 0 0

6237 6237 6224 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

Cnum_set Cnum_repl Anum_repl cnum_write anum_write

---------- ---------- ---------- ---------- ----------

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

Rows selected.

This view returns 16 rows, but only two rows have data. The other lines are 0, what's the reason?

Check the select name from V$latch_children where name = ' Cache buffers LRU chain '; You know, I have a total of 16 cache buffers LRU

Chain Latch, but only two were used.