Re: San-Based replication VS DataGuard replication

From: DA Morgan <>
Date: Fri, 03 Oct 2008 13:20:06 -0700
Message-ID: <>

Madison Pruet wrote:
> DA Morgan wrote:

>> macdba321 wrote:
>>> Group,
>>>  I have a database at Site1 stored on a SAN, and a disaster-recovery
>>> site2 with identical hardware. They are connected by high-speed fiber.
>>> (Both SANs are enterprise-class with full journaling capabilities in
>>> case the connection were ever severed.)
>> 4. Data Guard, interestingly enough, is more efficient. What is being
>>    replicated is the transactions themselves not operating system
>>    blocks so are shipping less data.

> This does not make sense. SAN based replication is done only when a
> physical write occurs. Since DG is pushing the logs to the secondary to
> achieve replication, it is replicating for any change in the page.
> Unless Oracle is flushing every page to disk as it is updated, then the
> impact to performance for a SAN based solution should be much more
> efficient than pushing the logs to the secondary.
> Also consider the case with hot pages, such as index pages. DG will be
> forced to send each update to the page to the secondaries while SAN
> based replication will only replicate the page as it is flushed to disk.
> The only logical way that DG could be more efficient would be if the
> Oracle database flushes every dirty page to disk as it is updated. I can
> see the logs being flushed immediately, but the data and index pages????
> Is that the case?

You are assuming all Data Guard activities involve log file shipping: They do not. Synchronous Data Guard, would never function if a commit required waiting for a log file switch. I should have been clearer.

Daniel A. Morgan
Oracle Ace Director & Instructor
University of Washington (replace x with u to respond)
Puget Sound Oracle Users Group
Received on Fri Oct 03 2008 - 15:20:06 CDT

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