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Automatic tuning of db_file_multiblock_read_count

Oracle Database Tips by Donald BurlesonJune 9, 2015

IMPORTANT:  In Oracle 10gr2 and beyond, it's critical to un-set the db_file_multiblock_read_count parameter and remove it completely from your pfile or spfile.  If not, your optimizer may choose unnecessary large table full-table scans.

Also see these important notes on db_file_noncontig_mblock_read_count.

Oracle notes that the cost of reading the blocks from disk into the buffer cache can be amortized by reading the blocks in large I/O operations.

The db_file_multiblock_read_count parameter controls the number of blocks that are pre-fetched into the buffer cache if a cache miss is encountered for a particular block.

The value of db_file_multiblock_read_count can have a significant impact on the overall database performance and it is not easy for the administrator to determine its most appropriate value.

  • Oracle Database 10g Release 2 automatically selects the appropriate value for this parameter depending on the operating system optimal I/O size and the size of the buffer cache.
     
  • Before 10g R2, DBA's used the db_file_multiblock_read_count initialization parameter to tell Oracle how many block to retrieve in the single I/O operation.
     
  • Before Oracle10g R2, the permitted values for db_file_multiblock_read_count were platform-dependent. The most common settings ranged from 4 to 64 blocks per single multi-block I/O execution.

     

Automatically tuned db_file_multiblock_read_count

Now in 10gr2 and beyond, Oracle introduces "automatically tuned db_file_multiblock_read_count", an exciting new features that uses external disk workload statistics (via dbms_stats.gather_system_stats) to optimizer the setting for db_file_multiblock_read_count.  The Oracle Performance Tuning documentation notes:

""In release 10.2, the optimizer uses the value of mbrc when performing full table scans (FTS).

The value of db_file_multiblock_read_count is set to the maximum allowed by the operating system by default. However, the optimizer uses mbrc=8 for costing.

The "real" mbrc is actually somewhere in between since serial multiblock read requests are processed by the buffer cache and split in two or more requests if some blocks are already pinned in the buffer cache, or when the segment size is smaller than the read size.

The db_file_multiblock_read_count value gathered as part of workload statistics is thus useful for FTS estimation."
 

As a review, ?multi-block? reads are commonly db file scattered read operations, where the disk read-write head locates itself under the proper cylinder, and sits there, as the disks rotation feeds the data blocks into the Oracle SGA (or PGA if using parallel full-scans).

TA suboptimal setting for db_file_multiblock_read_count can cause the optimizer to smile to favorably upon full-scan access, causing some DBA to wrongly lower their setting for optimizer_index_cost_adj and set a higher value for optimizer_index_caching, both used as alternatives to dbms_stats.gather_system_stats in cases where the optimizer ?wrongly? chooses full scan access over index access.

In 10gr2 and beyond, the db_file_multiblock_read_count is not used to estimate the average number of blocks read and a separate metric for the estimated number of actual block reads.  Instead, the optimizer computes two new values, one for optimizer costing and another for the number of I/O requests.

  •  _db_file_optimizer_read_count:  The average block reads for optimizer costing

  • _db_file_exec_read_count:  The actual number of block to read in real I/O operations

There are several hidden parameters relating to file I/O in multiblock reads:

_db_file_exec_read_count          
_db_file_noncontig_mblock_read_count        
_db_file_optimizer_read_count              
_sort_multiblock_read_count
_index_prefetch_factor
db_file_noncontig_mblock_read_count

In sum, it's important to ?unset? db_file_multiblock_read_count (by removing it from the spfile), so that the optimizer can independently compute these two estimates for multiblock I/O.

*********************************************

The combination of your db_block_size and the setting for db_file_multiblock_read_count determine the "real" read sizes for an Oracle database, and prior to Oracle 10g release 2, setting the db_file_multiblock_read_count was tricky. 

According to Oracle, this is the formula for setting db_file_multiblock_read_count:

                                 max I/O chunk size
db_file_multiblock_read_count  = -------------------
                                 db_block_size

When you implement multiple blocksizes you should set your db_block_size based on the size of the tablespace where your large-object full-scans will be occurring. Remember, the parameter db_file_multiblock_read_count is only applicable for tables/indexes that are full scanned.

See my related notes on db_file_multiblock_read_count:


 

 

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