[20180316]理解db file parallel read等待事件.txt

[20180316]理解db file parallel read等待事件.txt

--//一直对db file parallel read等待事件不理解,因为在实际系统中很少遇到这样的等待事件.

SCOTT@book> @ &r/ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SCOTT@test01p> @ ev_name 'db file parallel read'
EVENT#   EVENT_ID NAME                  PARAMETER1 PARAMETER2 PARAMETER3 WAIT_CLASS_ID WAIT_CLASS# WAIT_CLASS
------ ---------- --------------------- ---------- ---------- ---------- ------------- ----------- --------------------
   152  834992820 db file parallel read files      blocks     requests      1740759767           8 User I/O

--//参数1,2,3对应的files,blocks,requests.非常的不明确.

https://docs.oracle.com/cd/E11882_01/server.112/e40402/waitevents003.htm#REFRN00531

db file parallel read

This happens during recovery. It can also happen during buffer prefetching, as an optimization (rather than performing
multiple single-block reads). Database blocks that need to be changed as part of recovery are read in parallel from the
database.

Wait Time: Wait until all of the I/Os are completed
------------------------------------------------------------------------------------------------
Parameter     Description
------------------------------------------------------------------------------------------------
files         This indicates the number of files to which the session is reading
blocks        This indicates the total number of blocks to be read
requests      This indicates the total number of I/O requests, which will be the same as blocks
------------------------------------------------------------------------------------------------

--//这里实际上参数P1是files值读文件的数量.而不是文件号.
--//P2,读取的数据块数量.注意读取的块可以不连续.
--//P3.requests .理论应该等于P2的数值.

--//按照文档介绍发生在This happens during recovery. It can also happen during buffer prefetching, as an optimization
--//(rather than performing multiple single-block reads).

--//如何测试呢?一般awr报表很难出现这个等待事件.留待下个星期测试.12c的TABLE ACCESS BY INDEX ROWID BATCHED特性会出现这个等待事件.
--//回家测试看看.