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Don Burleson Blog 







Oracle: flush the data buffer cache

Oracle Database Tips by Donald BurlesonJune 19, 2015

Question:  How can I flush the buffer cache in Oracle?  I'm doing testing and I don't want to have to bounce the database.  I'm on Oracle 10g release 2.

Answer:  When performing performance tests, it's important to replicate the real-world production environment as closely as possible, especially with regard to disk reads since they are a major time-consuming event.  Flushing the data buffer cache is a great testing tool, and save you from having to bounce (stop and re-start) your database instance between test runs.  We DO NOT recommend flushing your data buffer cache on a production system!

The opposite of flushing the data buffers is to use the KEEP pool to eliminate disk I/O, and do the performance test without disk reads.

In Oracle 9i and beyond you can flush the buffer cache with this command:

alter session set events ?immediate trace name flush_cache?;

In Oracle 10g and beyond, this command will flush the buffer cache:

alter system flush buffer_cache;

Note:  Flushing the data buffer cache imposes a serious performance overhead, especially on RAC databases.  Using the flush buffer cache was intended only for test system.

In Oracle you can also flush the shared pool, if needed.  See flushing the shared pool

In the past, we had a facility to flush the shared pool. The FLUSH SHARED POOL clause of ALTER SYSTEM lets you clear all data from the shared pool in the SGA (system global area). This is a useful feature to clear existing data and re-load fresh data. Now, with 10g, it becomes possible for users to flush the cache buffers also.

Before 10g, Oracle used to internally flush the buffer cache blocks as needed. The FLUSH SHAREDPOOL clause is useful if you need to measure the performance of rewritten queries or a suite of queries from identical starting points. Use the following statement to flush the buffer cache.


System altered.

However, note that this clause is intended for use only on a test database. It is not advisable to use this clause on a production database, because subsequent queries will have no hits, only misses.

The following example shows the effect of flush buffer_cache. There are 50,000 rows in the table ?POLICYREC?.

SQL> update POLICYREC set sum_assured = sum_assured + 15;

50000 rows updated.

SQL> commit;

Commit complete.

SQL> select o.OBJECT_TYPE, substr(o.OBJECT_NAME,1,10)
  objname , b.objd , b.status, count(b.objd) from v$bh b,
  dba_objects o where b.objd = o.data_object_id and
  o.owner = 'NYUSER' group by o.object_type,
  o.object_name,b.objd, b.status ;


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

TABLE       TEST1             43058 free         6

TABLE       POLICYREC         43061 cr          47

TABLE       POLICYREC         43061 free       238

TABLE       POLICYREC         43061 xcur       376


SQL> alter system flush buffer_cache;


System altered.


SQL> select o.OBJECT_TYPE, substr(o.OBJECT_NAME,1,10)

 objname , b.objd , b.status, count(b.objd) from  v$bh b,

 dba_objects o where b.objd = o.data_object_id and

 o.owner = 'NYUSER' group by o.object_type,

 o.object_name,b.objd, b.status ;




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

TABLE           TEST1      43058 free         6

TABLE       POLICYREC      43061 free       660




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