Unmasking Buffer Busy Waits in Oracle: A Deep Dive with v$session_wait

Purpose

"Buffer busy waits" are a common performance bottleneck in Oracle databases, occurring when multiple sessions try to access the same data block concurrently. Identifying the specific blocks and understanding the reasons behind these waits is crucial for effective troubleshooting and optimization. Pinpointing the Contention with v$session_wait The v$session_wait view provides real-time information about sessions waiting for specific events. To identify the file, block, and reason code associated with "buffer busy waits," use the following query:

Sample SQL Command

1select	p1 "File #"
2,	p2 "Block #"
3,	p3 "Reason Code"
4from	v$session_wait
5where	event = 'buffer busy waits'
6/

Code Breakdown

• select p1 "File #", p2 "Block #", p3 "Reason Code": This selects the relevant columns from v$session_wait:
  • p1: Represents the file number of the data file containing the contended block.
  • p2: Represents the block number within the data file.
  • p3: Represents the reason code for the "buffer busy waits" event (prior to Oracle 10g). In Oracle 10g and later, this represents the block class.
• from v$session_wait: This specifies the v$session_wait view as the source of the data.
• where event = 'buffer busy waits': This filters the results to include only sessions waiting for the "buffer busy waits" event.

Key Points and Insights

• v$session_wait: This dynamic performance view provides real-time information about sessions waiting for various events, including resource contention.
• "Buffer busy waits": These waits indicate contention for data blocks in the buffer cache.
• Reason codes (pre-10g) / Block class (10g and later): These values provide further information about the type of block involved in the wait.

Explanations

• Buffer cache: A memory area where Oracle stores copies of frequently accessed data blocks from data files.
• Contention: Occurs when multiple processes try to access the same resource simultaneously, leading to delays.

Further Analysis and Troubleshooting

• Identify the object: Use the file and block number to identify the specific segment (table, index, etc.) experiencing contention.
• Analyze SQL statements: Examine the SQL statements causing the contention. Look for inefficient queries, missing indexes, or excessive full table scans.
• Consider optimization techniques: Explore options like index optimization, table partitioning, or application code changes to reduce contention and improve performance.

By utilizing the v$session_wait view and understanding the information it provides, you can effectively diagnose and resolve "buffer busy waits" in your Oracle database, leading to improved performance and reduced contention.

References

• Oracle Documentation: v$session_wait https://docs.oracle.com/en/database/oracle/oracle-database/21/refrn/V-SESSION_WAIT.html