Step 3: Identify frequently used and rarely used data blocks. Cache frequently used blocks and discard rarely used blocks.
If you have a low buffer hit ratio and you cannot increase the size of the database block buffers, you can still gain some performance advantage by tuning the block buffers and efficiently caching the data block that will provide maximum benefits. Ideally, we should cache data blocks that are either frequently used in SQL statements, or data blocks used by performance sensitive SQL statements (A SQL statement whose performance is critical to the system performance). An ad-hoc query that scans a large table can significantly degrade overall database performance. A SQL on a large table may flush out frequently used data blocks from the buffer cache to store data blocks from the large table. During the peak time, ad-hoc queries that select data from large tables or from tables that are rarely used should be avoided. If we cannot avoid such queries, we can limit the impact on the buffer cache by using RECYCLE buffer pool.
A DBA can create multiple buffer pools in the SGA to store data blocks efficiently. For example, we can use RECYCLE pool to cache data blocks that are rarely used in the application. Typically, this will be a small area in the SGA to store data blocks for current SQL statement / transaction that we do not intend to hold in the memory after the transaction is completed. Similarly, we can use KEEP pool to cache data blocks that are frequently used by the application. Typically, this will be big enough to store data blocks that we want to always keep in memory. By storing data blocks in KEEP and RECYCLE pools you can store frequently used data blocks separately from the rarely used data blocks, and control which data blocks are flushed from the buffer cache. Using RECYCLE pool, we can also prevent a large table scan from flushing frequently used data blocks. You can create the RECYCLE and KEEP pools by specifying the following init.ora parameters:
DB_KEEP_CACHE_SIZE = <size of KEEP pool>
DB_RECYCLE_CACHE_SIZE = < size of RECYCLE pool>
When you use the above parameters, the total memory allocated to the block buffers is the sum of DB_KEEP_CACHE_SIZE, DB_RECYCLE_CACHE_SIZE, and DB_CACHE_SIZE.
Step 4: Assign tables to KEEP / RECYCLE pool. Identify buffer hit ratio for KEEP, RECYCLE, and DEFAULT pool. Adjust the initialization parameters for optimum performance.
By default, data blocks are cached in the DEFAULT pool. The DBA must configure the table to use the KEEP or the RECYCLE pool by specifying BUFFER_POOL keyword in the CREATE TABLE or the ALTER TABLE statement. For example, you can assign a table to the recycle pool by using the following ALTER TABLE SQL statement.
ALTER TABLE <TABLE NAME> STORAGE (BUFFER_POOL RECYCLE)
The DBA can take help from application designers in identifying tables that should use KEEP or RECYCLE pool. You can also query X$BH to examine the current block buffer usage by database objects (You must log in as SYS to query X$BH). The following query returns a list of tables that are rarely used and can be assigned to the RECYCLE pool.
Col object_name format a30
Col object_type format a20
SELECT o.owner, object_name, object_type, COUNT(1) buffers
FROM SYS.x$bh, dba_objects o
WHERE (tch = 1 OR (tch = 0 AND lru_flag < 8))
AND obj = o. object_id
AND o.owner not in ('SYSTEM','SYS')
GROUP BY o.owner, object_name, object_type
ORDER BY buffers;
The following query will return a list of tables that are frequently used by SQL statements and can be assigned to the KEEP pool.
SELECT o.owner, object_name, object_type, COUNT(1) buffers
FROM SYS.x$bh, dba_objects o
WHERE tch > 10
AND lru_flag = 8
AND obj = o.object_id
AND o.owner not in ('SYSTEM','SYS')
GROUP BY o.owner, object_name, object_type
ORDER BY buffers;
Once you have setup the database to use KEEP and RECYCLE pools, you can monitor the buffer hit ratio by querying V$BUFFER_POOL_STATISTICS and V$DB_CACHE_ADVICE to adjust the buffer pool initialization parameters.
Step 5: Identify the amount of memory needed to maintain required performance.
Oracle 9i maintains block buffer advisory information in V$DB_CACHE_ADVICE. This view contains simulated physical reads for a range of buffer cache sizes. The DBA can query this view to estimate buffer cache requirement for the database. The cache advisory can be activated by setting DB_CACHE_ADIVE initialization parameter.
DB_CACHE_ADVICE = ON
There is a minor overhead associated with cache advisory collection. Hence, it is not advisable to collect these statistics in production databases until there is a need to tune the buffer cache. The DBA can turn on DB_CACHE_ADVISE dynamically for the duration of sample workload period and collect advisory statistics.
Conclusion
Using this methodical approach, a DBA can easily identify the problem areas, and tune the database block buffers. The DBA can create the following buffer pool to efficiently cache data blocks in SGA:
- KEEP: Cache tables that are very critical for system performance. Typically, lookup tables are very good candidates for the KEEP pool. The DBA should create the KEEP pool large enough to maintain 99% buffer hit ratio on this pool.
- RECYCLE: Cache tables that are not critical for system performance. Typically, a table containing historical information that is either rarely queried or used by batch process is a good candidate for the RECYCLE pool. The DBA should create the RECYCLE pool large enough to finish the current transaction.
- DEFAULT: Cache tables that do not belong to either KEEP or RECYCLE pool.
The DBA can setup OEM jobs, Oracle statspack, or custom monitoring scripts to monitor your production database block buffer efficiency, and to identify and tune the problem area. |
