Re: San-Based replication VS DataGuard replication
From: DA Morgan <damorgan_at_psoug.org>
Date: Fri, 03 Oct 2008 13:20:06 -0700
Message-ID: <1223065203.324576@bubbleator.drizzle.com>
>
> 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?
Date: Fri, 03 Oct 2008 13:20:06 -0700
Message-ID: <1223065203.324576@bubbleator.drizzle.com>
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 damorgan_at_x.washington.edu (replace x with u to respond) Puget Sound Oracle Users Group www.psoug.orgReceived on Fri Oct 03 2008 - 15:20:06 CDT