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RE: (((Re))): db_file_mutliblock_read_count and physical IO

From: Bobak, Mark <Mark.Bobak_at_il.proquest.com>
Date: Wed, 18 Aug 2004 18:01:30 -0400
Message-ID: <4C9B6FDA0B06FE4DAF5918BBF0AD82CF09EFEB1B@bosmail00.bos.il.pqe>


That's correct. Each db file scattered read wait event represents a = readv() system call. It's essentially Oracle saying "Hmmmm...I need = this and it's not in the buffer cache. O/S, please get it for me." For = the duration of the readv() call, the server process waits on a db file = scattered read event. As Cary mentioned previously, the O/S may find = the data in a variety of places: a filesystem buffer cache (if you're = not running raw or directio), or in a disk subsystem cache (EMC, NetApp, = etc, most have large caches), or in a disk controller cache, or possibly = in the disk's local cache, or, ultimately, on the physical disk platter = itself.

-Mark

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of ryan gaffuri Sent: Wednesday, August 18, 2004 5:43 PM To: oracle-l_at_freelists.org
Subject: (((Re))): db_file_mutliblock_read_count and physical IO

so how do i determine by looking at trace files whether changing the db_file_multiblock_read_count makes a difference? I am assuming I should = see
a reduction in the sheer number of 'db file scattered reads' in my trace file correct?

this is more for general knowledge. I am not trying to tune by strictly focusing on physical IO.
----- Original Message -----=20
From: "Wolfgang Breitling" <breitliw_at_centrexcc.com> To: <oracle-l_at_freelists.org>
Sent: Wednesday, August 18, 2004 12:39 PM Subject: Re: db_file_mutliblock_read_count and physical IO

> tkprof summarizes and for "disk IO" it counts the number of blocks =
read,
> not the number of physical I/O. So if you have 1 scattered read that =
read
> 128 blocks, or 16 scattered read that each read 8 blocks, tkprof will
> report 128 "disk IO"
> That is one of the (many) reasons Cary advocates to look at the raw



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Received on Wed Aug 18 2004 - 16:57:43 CDT

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