Understanding the Oracle Direct Path Read Temp Wait Event
Dean Richards
Senior DBA
Confio Software
8/27/2013 1
About Direct Path Read Temp
• Closely related to direct path write temp • Direct path read temp waits occur when a session
reads buffers from disk directly into the PGA – If I/O subsystem doesn’t support asynchronous I/O
• Each wait corresponds to a physical read request
– If I/O subsystem supports asynchronous I/O: • The process overlaps read requests with processing the
blocks already in the PGA • When the process attempts to access a block in the PGA that
hasn’t been read from disk, it issues a wait call and updates the statistics
• The number of waits is not always the same as the number of read requests
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About Direct Path Write Temp
• Direct path write temp (and direct path write) waits occur when a session writes buffers directly from the PGA
• The process waits on this event for the write call to complete • Operations that perform direct path writes include:
– When a sort goes to disk – Parallel DML operations – Direct-path INSERTs – Parallel create table as select – Some LOB operations
• If I/O subsystem supports asynchronous I/O – The number of waits is not always the same as the number of write
calls – The session will wait if it has processed the buffers in the PGA and
can’t continue work until and I/O request complete
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Diagnosing Direct Path Read Temp
• Check the v$session_wait view: SELECT p1 “file#”, p2 “block#”, p3 “class#”
FROM v$session_wait
WHERE event = ‘direct path read temp’;
where
– p1 represents the file_ID for the read call
– p2 represents the start block_ID for the read call
– p3 is the number of blocks in the read call
• Check file_ID to see if it is for temp: 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
;
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Understand Causes of Direct Path Read Temp
• High disk sorts – When sorts are too large to fit in memory, they get sent to disk
• Parallel slaves – When parallel slaves are used for scanning data or parallel DML are used to
create and populate objects
• Direct path loads – When the direct path API is used to pass data to the load engine in the server
• Server process ahead of I/O – If the server process is processing buffers faster than the I/O system can return
THEM, this can indicate an overloaded I/O system
• Data warehouse sorts – These sorts may always go to disk, which can lead to high waits on direct path
read temp and/or direct path write temp
• Hash area size – For query plans that call for a hash join, excessive I/O could result from having
HASH_AREA_SIZE too small
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Tuning for Direct Path Read Temp
• Direct path read temp waits often occur:
– When the PGA can’t support the size of the sorts
• Direct path read, direct path write temp, direct path write waits can occur:
– Parallel operations, direct path inserts or overloaded I/O
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Check for High Disk Sorts
• To check for high disk sorts: 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$statname c
where c.name=’sorts (disk)’
and a.snap_time>sysdate-7
and c.statistic#=b.statistic#
and b.snap_id=a.snap_id
order by a.instance_number,a.snap_time
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If High Disk Sorts Confirmed…
• If high disk sorts are confirmed, use pga_aggregate_target: – Total work area cannot exceed 200 MB RAM
• This is because of the default setting for the hidden parameter_pga_max_size.
– No RAM sort may use more than 5% of pga_aggregate_target or _pga_max_size, whichever is smaller.
– No task may use more than 200 megabytes for sorting or hash joins.
• Note that Oracle does not recommend changing hidden parameters, but the result can be beneficial for certain environments.
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Tune the Sorts
• Find the cause of the sorts:
– Find the session experiencing the waits; look at the SQL statement currently being run.
• Query V$TEMPSEG_USAGE to find the SQL statement generating the sort
• Query V$SESSTAT for the session to determine the size of the sort
• Is it possible to reduce the sorting by tuning the SQL statement?
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If Full Table Scans Are the Cause, Consider Disk Striping
• If tables are defined with a high degree of parallelism, the optimizer leans towards using full table scans with parallel slaves:
– Locate the object being loaded
– Consider using disk striping or Automatic Storage Management (ASM) which can stripe for you
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Optimize for Parallel DML
• Check the I/O distribution across the disks
• Make sure I/O is configured for the parallelism being used. V
• erify that the parameter DISK_ASYNCH_IO is set to TRUE.
8/27/2013 11
Learn More About Oracle Performance Tuning
• Go to confio.com/logicalread • Free performance tuning tips for Oracle,
including:
– Tips on tuning for specific wait types and response time analysis
– Indexing strategies that can improve performance
– Much more, from database professionals, industry experts and Oracle ACEs
8/27/2013 12
Finally, Confio Ignite Can Help, Too
• Confio Ignite can help:
– Identify queries with large amounts of waits and many other database issues
– Install in minutes
– Find issues in 5 clicks or less
Download Ignite trial at
www.confio.com/freetrial
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