Thursday, November 18, 2010

wait events

db file scattered read: full table scan

db file sequential read
In most cases, this event means that a foreground process reads a single block (because it reads a block from an index or because it reads a block by rowid).

log file sync:

The log file sync Oracle metric indicates the process is waiting for LGWR to finish flushing the log buffer to disk. This occurs when a user commits a transaction (a transaction is not considered committed until all of the redo to recover the transaction has been successfully written to disk).

Log Buffer Space

Log Buffer Space wait event occurs when server processes write data into the log buffer faster than the LGWR process can write it out. The LGWR process begins writing entries to the online redo log file if any of the following conditions are true:

* The log buffer reaches the _log_io_size threshold. By default, this parameter is set to one third of the log buffer size.
* A server process performing a COMMIT o ROLLBACK posts to the LGWR process.
* The DBWR process posts to the LGWR process before it begins writing.

As the LGWR process writes entries to disk, user processes can reuse the space in the log buffer for new entries. If the Log Buffer is too small, user processes wait for “Log Buffer Space� until the LGWR flushes the redo information in memory to disk.

Free buffer waits:

The Free Buffer Waits Oracle metric wait event indicates that a server process was unable to find a free buffer and has posted the database writer to make free buffers by writing out dirty buffers. A dirty buffer is a buffer whose contents have been modified. Dirty buffers are freed for reuse when DBWR has written the blocks to disk.

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