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Re: comments on forcedirectio

From: Stephane Faroult <sfaroult_at_roughsea.com>
Date: Sat, 30 Apr 2005 01:29:23 +0200
Message-ID: <4272C353.4060307@roughsea.com>


geraldine_2_at_comcast.net wrote:

>I've read articles that encourages the use of forcedirectio on redo logs but discourages on oracle datafiles. With my limited understanding on the forcedirectio option, I believe it should be used on Oracle datafiles to eliminate double buffering. The use of forcedirectio should improve database performance.
>Are there any reasons why the option should not be used on Oracle datafiles? Does the use of the option depends on the type of storage array? We have 9.2.0.4 databases on SUN StorEdge T4 and NetApp.
>
>Thanks.
>
>Geraldine
>
>--
>http://www.freelists.org/webpage/oracle-l
>
>
>

Geraldine,

   The reason for buffering in the first place is to return control to the program which issues a 'write' faster, instead of forcing it to wait until the bits are actually recorded on disk. The trouble with redo log files is that to tell you 'committed' Oracle waits until it has actually written on disk (actually, SAN systems cheat but never mind - at least the operating system buffer has been flushed somewhere). With regular I/Os, it writes to a system buffer, then flushes this buffer to guarantee that your change is actually recorded as permanent and stay so even if the system crashes within a millionth of a second. In other words, the writing is synchronous and here of course you may go faster if you write to disk in a single operation. Database files, however, are written asynchronously (by DBWR), so it doesn't matter (and if the system crashes, changes are reconstructed from the log file entries).

HTH

-- 
Regards,

Stephane Faroult

RoughSea Ltd
http://www.roughsea.com


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Received on Fri Apr 29 2005 - 19:33:54 CDT

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