(OWI学习笔记索引)

Log file switch(archiving needed)

当LGWR要切换到下一个redo log group时发现arch进程尚未将该日志组写道archived log  file destination,此时就会出现该等待事件。

Log file switch(checkpoint incomplete)

LGWR试图切换日志文件时由于checkpoint尚未完成导致的等待。这一事件一般是在redo log files过小的情况下出现。

Log file switch completion

进程等待log file switch to complete.

Log file sync

用户完成一个事物后commit或者rollback都会触发LGWR将redo log写入online redo log。用户进程等待LGWR进程完成写入redo log的IO过程就会产生该等待事件。

如果一个用户遇到log file sync等待事件，通过v$session_wait查看发现它一直在等待同一个buffer#(P1)，那么v$session_wait表中该Session对应的log file sync等待事件的SEQ#会不断增长（否则的话timeout可能存在问题），此时的瓶颈就在LGWR进程，就需要查找堵塞LGWR进程的原因了。

调优LGWR的关键是磁盘IO，例如，redo logs不应该放在RAID5的磁盘阵列上。另外，将分散的事务合并成大事务也有助于减少该等待事件。

参数：P1-The number of the buffer in the log buffer that needs to be synchronized

SQL*Net message from client

Session在等待client的输入。

参数：P1-Prior to Oracle8i release, the value in this parameter was not of much use. Since Oracle8i, the P1RAW column contains an ASCII value to show what type of network driver is in use by the client connections; for example, bequeath, and TCP.

P2-The number of bytes received by the session from the client—generally one, even though the received packet will contain more than 1 byte.

SQL*Net message to client

This wait event is posted by the session when it is sending a message to the client. The client process may be too busy to accept the delivery of the message, causing the server session to wait, or the network latency delays may be causing the message delivery to take longer.

参数：P1-Prior to Oracle8i Database, the value in this parameter was not of much use. Since Oracle8i, the P1RAW column contains an ASCII value to show what type of network driver is in use by the client connections, for example, bequeath and TCP.

P2-Number of bytes sent to client. This is generally one even though the sent packet will contain more than 1 byte.