Common Wait Events---db file scattered read

db file scattered read

The db file scattered read event is posted by the session when it issues an I/O request to read multiple data blocks. The blocks read from the datafiles are 

scattered into the buffer cache. These blocks need not remain contiguous in the buffer cache. The event typically occurs during full table scans or index 

fast full scans. The initialization parameter,DB_FILE_MULTIBLOCK_READ_COUNT determines the maximum number of data blocks to read.

当session发出一个I/O请求去读取多个数据块时就会记录一个db file scattered read 事件，这些块从数据文件中分散读到buffer cache中，这些块被读到buffer cache中依然不

需要连续，这个事件 一般发生在全表扫描或全索引快速扫描时，初始参数DB_FILE_MULTIBLOCK_READ决定了一次要读取的最大数据块数。

Waiting on datafile I/O completion is normal in any Oracle database. The presence of this wait event
does not necessarily indicate a performance problem. However, if the time spent waiting for
multiblock reads is significant compared to other waits, you must investigate the reason for it.
Wait Parameters

在数据库中等待I/O完成都是正常的等待，在性能问题中是不必太观注的事件，然而，如果相比其他的等待在多块读上花费了大量的时间则必须找清楚原因。

来看一下这个事件各个参数的含义是什么：

SQL> SELECT * FROM V$EVENT_NAME e WHERE e.NAME LIKE 'db file scattered read%';     EVENT#   EVENT_ID NAME                     PARAMETER1    PARAMETER2    PARAMETER3   WAIT_CLASS_ID WAIT_CLASS#  WAIT_CLASS---------- ---------- -----------------------  ------------  -----------   ------------ ------------- -----------  -----------       116  506183215 db file scattered read   file#         block#        blocks       1740759767              8  User I/O

•  P1 File number to read the blocks from       ：要读取数据的文件号
•  P2 Starting block number to begin reading ：要读取的起始数据块号
•  P3 Number of blocks to read                           ：要读取多少个块