[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/ver1PORT_STRING VERSION BANNER x86_6 ...
[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
--//參數1,2,3對應的files,blocks,requests.非常的不明確.
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
------------------------------------------------------------------------------------------------
--//這裡實際上參數P1是files值讀文件的數量.而不是文件號.
--//P2,讀取的數據塊數量.註意讀取的塊可以不連續.
--//P3.requests .理論應該等於P2的數值.
--//按照文檔介紹發生在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.
