Oracle DBA

In Oracle databases, understanding and tuning wait events is crucial for database performance. One set of wait events that often demands attention, particularly in data-intensive operations, is the direct path read temp and direct path write temp events. These events relate closely to how data is read and written between disk and memory, specifically the Program Global Area (PGA). This blog explores these events, their causes, and tuning strategies to optimize performance.

Direct Path Read Temp Wait Event

When an Oracle session reads buffers from disk directly into the PGA, the session waits on a direct path read temp event. This is closely tied to the general direct path read wait event, but with a temporary buffer.

• Asynchronous I/O: If the I/O subsystem supports asynchronous I/O, read requests overlap with the processing of blocks already in the PGA, allowing concurrent processing. If asynchronous I/O isn’t supported, each wait corresponds directly to a physical read request, creating more serial dependencies on disk reads.
• Waits vs. Read Requests: When the process accesses a block in the PGA that hasn’t been fully read from disk, it issues a wait call, increasing the event’s wait statistics. However, because of asynchronous I/O, the number of waits may not match the number of read requests.

To analyze this wait event, you can use the v$session_wait view to get detailed information on the waiting session:

SELECT p1 "file#", p2 "block#", p3 "class#"
FROM v$session_wait
WHERE event = 'direct path read temp';

Here, p1 is the file ID, p2 is the starting block ID, and p3 is the number of blocks being read.

Direct Path Write Temp Wait Event

Similarly, direct path write temp and direct path write events occur when a process writes buffers directly from the PGA. Common scenarios for direct path writes include:

• Disk sorts for large sorting operations
• Parallel Data Manipulation Language (DML) operations
• Direct-path inserts and parallel table creation
• Some Large Object (LOB) operations

When writing from the PGA, if the process completes all buffer processing and can’t continue until an I/O request finishes, it waits on the direct path write temp event.

Diagnosing Direct Path Read Temp Waits

To troubleshoot direct path read temp waits, you can identify details on the blocks being accessed using the v$session_wait view as shown above. To narrow down the segments affected, you may further query the dba_extents view:

SELECT relative_fno, owner, segment_name, segment_type
FROM dba_extents
WHERE file_id = &file
AND &block BETWEEN block_id AND block_id + &blocks - 1;

The file ID and block information help identify if the wait event is related to a temporary segment, and can provide insights into any specific tables or segments causing the issue.

Causes of Direct Path Read Temp Wait Events

Several factors can lead to high direct path read and write temp waits:

1. High Disk Sorts: If sorts are too large to fit in memory, they are written to disk, resulting in direct path read temp waits.
2. Parallel Processes: Parallel slaves used for scanning data or performing parallel DML operations may increase read/write waits.
3. Direct Path Loads: The direct path API loads data directly into the PGA and can increase the frequency of write waits.
4. I/O System Lag: When the server process works faster than the I/O subsystem can return data, it may indicate an overloaded I/O subsystem.
5. Data Warehouses: Due to their data volumes, warehouses often experience high disk usage, leading to waits on direct path read/write events.
6. Hash Area Size: For hash joins, inadequate HASH_AREA_SIZE may lead to increased disk I/O, causing waits.

Tuning Direct Path Read Temp Waits

• Optimize Disk Sorts: To check disk sort distribution, use a query like:

SELECT a.instance_number, TO_CHAR(a.snap_time, 'DD/Mon/YYYY HH24:MI') meas_date, b.value
FROM stats$snapshot a, stats$sysstat b, v$statname c
WHERE c.name = 'sorts (disk)'
AND a.snap_time > SYSDATE - 7
AND c.statistic# = b.statistic#
AND b.snap_id = a.snap_id
ORDER BY a.instance_number, a.snap_time;

Use PGA_AGGREGATE_TARGET for managing memory used for sorts and hash joins. Note that Oracle limits maximum PGA usage to 200 MB by default to prevent excessive memory use.

SELECT * FROM v$pgastat;

• Tuning SQL Statements: Identify SQL statements causing disk sorts. Use the V$TEMPSEG_USAGE view to find SQL generating sorts, and review session statistics from V$SESSTAT to assess sort size. Optimize SQL queries to reduce sorting needs.
• Avoid Unnecessary Full Table Scans: High degrees of parallelism may cause the optimizer to favor full table scans. For direct path reads, consider using disk striping or Automatic Storage Management (ASM) for better data distribution.
• Parallel DML Tuning: When performing parallel DML, check disk I/O distribution and configure I/O for the level of parallelism used. Ensure DISK_ASYNCH_IO is set to true to enable asynchronous I/O.

Conclusion

Direct path read temp and direct path write temp events often result from insufficient PGA for sorting needs or from intensive parallel operations. To mitigate these waits, optimize PGA settings, review SQL for unnecessary sorts, and ensure efficient I/O distribution. By tuning these elements, you can significantly improve the performance and reduce these wait events in your Oracle database environment.