Oracle-L: RE: I/O contention with external process reading the oracle logs (online redo logs)

From: <johanna.doran_at_sungard.com>
Date: Thu, 06 Jun 2002 06:32:18 -0800
Message-ID: <F001.004761CA.20020606063218@fatcity.com>

NB_ RESENDING in plain text - sorry, Outlook keeps seinding in html no matter what default i set! Hi lists,

I am using Quest Shareplex product for Oracle to Oracle one way replication. I have two systems (source and target) and two environments (dev, demo). On system one, the environments are setup as schemas within one oracle instance (therefore each schema will be a SOURCE in the replication). My other system has each environment set up a separate Orace Instances (therefore each instance will become a TARGET in the replication).

I am trying to configure 2 separate replication streams (ie so that each replication process is SEPARATE from the other - one for DEV and one for DEMO). I will accomplish this by setting up Shareplex to use mulitple processes.

HOWEVER, Quest technical support has told me that this will cause contention. However, I dont see why is would from an os/oracle point of view. Basically Shareplex has a process which reads the online redo logs......... tech support is suggesting that is there a two processes trying to access the same block in the logs that contention can occur. This does not make sense to me. Below is the blurb from techincal support when I questioned their initial repsonse:

The reason you might run into a contention is because multiple captue processes may be reading the same data block in the redo log. Since there is only one process that can access a single block, the other process may have to wait. Contention is a possibilty, and you will need to run some bench marks to find out how much, if any, contention you will have.

I would find it HARD to believe that only ONE process can read a block at a time. If this were true, then OLTP system would FAIL miserably!

Anyone have any ideas/comments regarding the OS and Oracle interaction .... I mean are not the logs at this pointa UNIX file? and can't multiple processes read a single unix file without bringing the whole system to its knees? Also, I am NOT knocking the techincal support, but I believe that the opinion was formulated on an incorrect assumption on the operating system and Oracle. Thoughts/comments?

Thanks in advance.

Hannah

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INET: johanna.doran_at_sungard.com

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