Inside the Oracle data buffer cache
Oracle has an exciting new v$ view called v$bh that shows the contents of the data buffers as well as the number of blocks for each type of segment in the buffer.
Oracle has an exciting new v$ view called v$bh that shows the contents of the data buffers as well as the number of blocks for each type of segment in the buffer.
The v$bh view is especially useful for showing the amount of table and index caching in databases with multiple data buffer pools. For example, researchers have shown that creating a separate index buffer with a very large blocksize can improve Oracle index structures. The v$bh view also provides important information for the proper setting of the optimizer_index_caching parameter, an important cost-based optimizer parameter that tells the CBO how much of the databases index reside in the buffer cache.
Today, many Oracle professionals are segmenting their db_cache_size, using the new db_2k_cache_size, db_keep_cache_size, db_4k_cache_size, db_8k_cache_size, db_16k_cache_size and db_32k_cache_size to create separate data buffers to isolate the caching of data blocks to improve manageability and caching.
Combining the v$bh view with dba_objects and dba_segments provides a block-by-block listing of the data buffer contents and indicates how well the buffers are caching tables and indexes. Of course, this is very important in Oracle9i, since the data buffer sizes can be altered dynamically.
There are several data dictionary tricks required when writing a script for mapping data objects to RAM buffers:
- Duplicate object names - When joining dba_objects to dba_segments, the name, type, and owner are all required to distinguish the object sufficiently.
- Multiple blocksizes - To show objects in the separate instantiated buffers (db_2k_cache_size, etc.), we need to display the block size for the object. We do this by computing the block size from dba_segments, dividing bytes by blocks.
- Partitions - With a standard equi-join, every object partition joins to every segment partition for a particular object. Hence, the following query qualification is required to handle partitioned objects:
and nvl(t1.subobject_name,'*') = nvl(s.partition_name,'*')
- Clusters - Clusters present a challenge when joining the v$bh row with its corresponding database object. Instead of joining the bh.objd to object_id, we need to join into data_object_id.
- Multiple caches - There are situations where a particular block may be cached more than once in the buffer cache. This is a mystifying concept, but it is easily overcome by creating the following in-line view:
(select distinct objd, file#, block# from v$bh where status != 'free')
This is the most important script in this text because it provides a detailed analysis of those objects in the data buffers. This information is critical when considering an alteration to the data buffer sizes.
set pages 999
set lines 92
ttitle 'Contents of Data Buffers'
drop table t1;
create table t1 as
select
o.owner owner,
o.object_name object_name,
o.subobject_name subobject_name,
o.object_type object_type,
count(distinct file# || block#) num_blocks
from
dba_objects o,
v$bh bh
where
o.data_object_id = bh.objd
and
o.owner not in ('SYS','SYSTEM')
and
bh.status != 'free'
group by
o.owner,
o.object_name,
o.subobject_name,
o.object_type
order by
count(distinct file# || block#) desc
;
column c0 heading "Owner" format a12
column c1 heading "Object|Name" format a30
column c2 heading "Object|Type" format a8
column c3 heading "Number of|Blocks in|Buffer|Cache" format 99,999,999
column c4 heading "Percentage|of object|blocks in|Buffer" format 999
column c5 heading "Buffer|Pool" format a7
column c6 heading "Block|Size" format 99,999
select
t1.owner c0,
object_name c1,
case when object_type = 'TABLE PARTITION' then 'TAB PART'
when object_type = 'INDEX PARTITION' then 'IDX PART'
else object_type end c2,
sum(num_blocks) c3,
(sum(num_blocks)/greatest(sum(blocks), .001))*100 c4,
buffer_pool c5,
sum(bytes)/sum(blocks) c6
from
t1,
dba_segments s
where
s.segment_name = t1.object_name
and
s.owner = t1.owner
and
s.segment_type = t1.object_type
and
nvl(s.partition_name,'-') = nvl(t1.subobject_name,'-')
group by
t1.owner,
object_name,
object_type,
buffer_pool
having
sum(num_blocks) > 10
order by
sum(num_blocks) desc
;
A sample listing from this exciting report is shown below. We can see that the report lists the tables and indexes that reside inside the data buffer. This is important information for the Oracle professional who needs to know how many blocks for each object reside in the RAM buffer. To effectively manage the limited RAM resources, the Oracle DBA must be able to know the ramifications of decreasing the size of the data buffer caches.
Here is the report from this script when run against a large Oracle data warehouse.
Contents of Data Buffers
Number of Percentage
Blocks in of object
Object Object Buffer Buffer Buffer Block
Owner Name Type Cache Blocks Pool Size
------------ -------------------------- ----------- ---------- ------- -------
DW01 WORKORDER TAB PART 94,856 6 DEFAULT 8,192
DW01 HOUSE TAB PART 50,674 7 DEFAULT 16,384
ODSA WORKORDER TABLE 28,481 2 DEFAULT 16,384
DW01 SUBSCRIBER TAB PART 23,237 3 DEFAULT 4,096
ODS WORKORDER TABLE 19,926 1 DEFAULT 8,192
DW01 WRKR_ACCT_IDX INDEX 8,525 5 DEFAULT 16,384
DW01 SUSC_SVCC_IDX INDEX 8,453 38 KEEP 32,768
DW02 WRKR_DTEN_IDX IDX PART 6,035 6 KEEP 32,768
DW02 SUSC_SVCC_IDX INDEX 5,485 25 DEFAULT 16,384
DW02 WRKR_LCDT_IDX IDX PART 5,149 5 DEFAULT 16,384
DW01 WORKORDER_CODE TABLE 5,000 0 RECYCLE 32,768
DW01 WRKR_LCDT_IDX IDX PART 4,929 4 KEEP 32,768
DW02 WOSC_SCDE_IDX INDEX 4,479 6 KEEP 32,768
DW01 SBSC_ACCT_IDX INDEX 4,439 8 DEFAULT 32,768
DW02 WRKR_WKTP_IDX IDX PART 3,825 7 KEEP 32,768
DB_AUDIT CUSTOMER_AUDIT TABLE 3,301 99 DEFAULT 4,096
DW01 WRKR_CLSS_IDX IDX PART 2,984 5 KEEP 32,768
DW01 WRKR_AHWO_IDX INDEX 2,838 2 DEFAULT 32,768
DW01 WRKR_DTEN_IDX IDX PART 2,801 5 KEEP 32,768
This is a very important report because we see three object types (tables, indexes, and partitions), and we also see the sub-sets of the DEFAULT pool for KEEP and RECYCLE. Also, note that all indexes are defined in the largest supported block size (db_32k_cache_size), and multiple buffer pools of 4K, 8K, 16K and 32K sizes are defined.
The output of this script is somewhat confusing because of the repeated DEFAULT buffer pool name. This is misleading because the KEEP and RECYCLE buffer pools are sub-sets of db_cache_size and can ONLY accommodate objects with the DEFAULT db_block_size.
Conversely, any block sizes that are NOT the default db_block_size, go into the buffer pool named DEFAULT. As you can see from the output listing, there are really 6 mutually exclusive and independently-sized buffer pools, and four of them are called "DEFAULT."
It is valuable to run this report repeatedly because the Oracle data buffers are dynamic and constantly changing. Running this script frequently allows us to view the blocks entering and leaving the data buffer. We can see the midpoint insertion method in action and the hot and cold regions as they update. Each time a block is re-referenced it moves to the head of the MRU chain on the hot side of the data buffer. Blocks that are accessed less frequently will age-out, first moving into the cold region and eventually being paged-out to make room for new incoming blocks.
This approach is even more important when considering a decrease to a cache size. When you issue an alter system command to decrease the cache size, Oracle will grab pages from the least recently used (LRU) end of the buffer. Depending on the amount of RAM removed, an alter system command will un-cache data blocks that may be needed by upcoming SQL statements.
For more information on SGA internals and data buffer management, see my book, Creating a Self-Tuning Database. If you want details on the new Oracle10g automation features, see Mike Ault's latest book Oracle Database 10g New Features.
