Re: San-Based replication VS DataGuard replication

From: joel garry <>
Date: Fri, 3 Oct 2008 11:40:28 -0700 (PDT)
Message-ID: <>

On Oct 3, 7:24 am, 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.

You got it wrong. DG is only replicating the logs, while the db writer can do its thing at any time later (even never, in some cases - while several things can, and do, signal the writer to write, there are cases where Oracle doesn't even bother, google delayed block cleanout). The SAN would have to replicate the logs _and_ the db writer writes as they happen, that's way more to do over the critical network resource. The logs are then applied on the other end in a continuous recovery. Yes, there is a trade-off between network and local bandwidth. Which is cheaper? How do you define efficient? Doing less stuff in the critical path usually leads to better performance.

> 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.

No, read the Oracle Concepts manual, available online at DG doesn't know jack about hot pages, doesn't care. The redo logs are the secret. The Achilles' heel, for that matter. You need to understand recovery to understand how this works with DG.

> 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 REALLY need to learn the architecture. The database writer flushes dirty blocks to disk at its leisure. It's the redo buffers that are critical for being flushed to the logs. Since the data being changed can be a lot less than a block, that's a lot less data to deal with.


-- is bogus.
Received on Fri Oct 03 2008 - 13:40:28 CDT

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