Re: San-Based replication VS DataGuard replication

From: DA Morgan <>
Date: Thu, 02 Oct 2008 15:40:00 -0700
Message-ID: <>

macdba321 wrote:

> On Sep 27, 1:40 am, DA Morgan <> wrote:

>> macdba321wrote:
>>> 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.)
>>> I am researching Pros & Cons of using DataGuard to keep Site2 ready
>>> for disaster time, VS letting the SAN manage keeping the two sites in
>>> sync.
>>> What are your opinions on these 2 methods?
>>> Thank you!!
>>> -mac
>> There is a place in an Oracle shop for both SAN based replication and
>> Data Guard but the SAN based replication will NEVER better Data Guard
>> if the point is to have a DR site: Here are just a couple of the reasons.
>> 1. Data Guard checks what it replicates for internal logic. A SAN will
>> happily replicate corrupt blocks.
>> 2. Data Guard can guarantee zero data loss even when the data has not
>> been written to a data file. You can not make SAN replication
>> synchronous.
>> 3. If SAN replication fails your primary will happily keep right on
>> running transactions. Data Guard can be configured to bring things
>> to a halt until the issue is resolved.
>> 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.
>> --
>> Daniel A. Morgan
>> Oracle Ace Director & Instructor
>> University of Washington
>> (replace x with u to respond)
>> Puget Sound Oracle Users
> Thank you all for the great input.
> In the last post, can you elaborate on #2? Regarding how dataguard can
> guarantee zero data loss if it is configured for maximum protection.
> (Essentially, the final transaction would not go through if the
> primary database can not get the confirmation back.) However, can you
> elaborate on your statement:  "You can not make SAN replication
> synchronous." ??
> Thanks again!

With synchronous replication DataGuard will not commit locally until it has a guarantee that at least one remote copy of the shipped log data has been successfully received. Not necessarily applied but at least received.

SANs are configured to host multiple applications from different vendors simultaneously. They have no way of understanding Oracle traffic versus any other traffic.

Daniel Morgan
Oracle Ace Director Received on Thu Oct 02 2008 - 17:40:00 CDT

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