0316理解db file parallel read等待事件

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

--//一直對db file parallel read等待事件不理解,因為在實際系統中很少遇到這樣的等待事件.

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

--//引數1,2,3對應的files,blocks,requests.非常的不明確.

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
------------------------------------------------------------------------------------------------

--//這裡實際上引數P1是files值讀檔案的數量.而不是檔案號.
--//P2,讀取的資料塊數量.注意讀取的塊可以不連續.
--//P3.requests .理論應該等於P2的數值.

--//按照文件介紹發生在This happens during recovery. It can also happen during buffer prefetching, as an optimization
--//(rather than performing multiple single-block reads).

--//如何測試呢?一般awr報表很難出現這個等待事件.留待下個星期測試.12c的TABLE ACCESS BY INDEX ROWID BATCHED特性會出現這個等待事件.
--//回家測試看看.