DBA Tips Archive for Oracle
Tuning the Redo Log Buffer
by Jeff Hunter, Sr. Database Administrator
All changes (insert, update or delete) made to an Oracle data block will be contained in the Oracle redo log buffer. The redo log buffer gives the Oracle database the ability to recover from a media failure.
If the redo buffers are not large enough, the Oracle log-writer process waits for space to become available. This wait time becomes wait time for the end user. The size of the redo log buffer is defined in the init.ora file using the 'LOG_BUFFER' parameter. The script below can be used to monitor if the log-writer process is waiting for redo log buffers. The time being reported is in 1/100's of a second
SELECT SUM(value) "Redo Buffer Waits" FROM v$sysstat WHERE name = 'redo log space wait time';
LGWR writes redo entries from the redo log buffer to a redo log file. Once LGWR copies the entries to the redo log file the user process can over write these entries. The statistic 'redo log space requests' reflects the number of times a user process waits for space in the redo log buffer. Use the script below to monitor the number of times a user process had to wait for space within the redo log buffer.
SELECT SUM(value) "Redo Buffer Waits" FROM v$sysstat WHERE name = 'redo log space requests';
Here are some tips in sizing the redo logs:
Finally, the following script can be used to view various statistics on your redo log buffer and files.
COLUMN value FORMAT 999,999,999,990 COLUMN name FORMAT a31 SELECT a.name,b.value FROM v$statname a,v$sysstat b WHERE a.statistic# = b.statistic# AND a.name like '%redo%'; NAME VALUE ------------------------------- ---------------- redo synch writes 4,940,014 redo synch time 4,299,767 redo entries 203,212,938 redo size 47,844,123,778 redo entries linearized 0 redo buffer allocation retries 2,822 redo small copies 1,853,067 redo wastage 1,051,015,930 redo writer latching time 10,898 redo writes 5,382,902 redo blocks written 98,714,473 redo write time 5,216,053 redo log space requests 601 redo log space wait time 25,567 redo log switch interrupts 0 redo ordering marks 0
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