Re: Dataguard / Archive Logs

David,
Is the SAN replicating synchronously or asynchronously? Data Guard needs less bandwidth than storage based replication. Most of the times SAN's are configured to replicate asynchrously. Just think of the amount of data that needs to be sent over when storage is replicated: Redo writes (all members), archive copies (maybe also redundant?), the writes to data files, and the updates of the controlfile. Mind that storage based replication is often disk block based, or even disktrack-based! You can imagine what amount of data needs to go through the pipe for that. Compare that with just sending the redo entries with Data Guard. The standby site will take care of applying them, writing to data files and performing the archive job locally.

Furthermore, Data Guard gives you the feature applying the redo at a delayed basis. This gives you the opportunity to recover from 'human' errors, e.g. dropping a table. When the delay is 4 hours, and you discover the problem within that time, you can make the standby read-only and copy the missing table to the primary. Of course Flashback offers this option also, but at the cost of extra I/O on the primary. The delay comes at the cost of longer failover time, becouse the redo of the 4 hours delay need to be applied at that time.

SAN replication wil just replicate any error on the primary to the standby. With Data Guard you can put the standby in read-only mode (e.g. for reporting purposes). In the meantime it will continue receiving the redo, your transactions are still safe. Again, at the cost of somewhat longer failover time bevause of extra redo waiting to be applied. Some of my DG customers decided to have two standby's for some reasons: The delay is enabled at only one site (most of the times the remote standby), the other (the 'local' standby in the same building) is as actual as possible.

FInally, SAN replication pretty much requires the same (expensive) iron on both ends. Data Guard can have a highly sophistacated SAN at one end, and some cheap system with cheaper disks at the other end. Of course that might hurt performance and availability in a failed over situation, but having two cheap standby's can be better than just one expensive one.

It's getting a little bit a chapter of a book in stead of a post to elaborate on all that, but maybe this may help you to determine the right configuration for your situation.

Best regards,

Carel-Jan Engel

===
If you think education is expensive, try ignorance. (Derek Bok) ===

On Wed, 2005-06-01 at 20:29, David Sharples wrote:

> I think you are right, im not convicned it will work though - doesnt
> seem right to me
>
> On 6/1/05, Dimitre Radoulov <cichomitiko_at_gmail.com> wrote:
> > do you really need DataGuard with
> > this configuration?
> >
> > You just can stop the primary and start the other one ... or you need the
> > secondary open in read only(and you could open it anyway without DataGuard)?

--
http://www.freelists.org/webpage/oracle-l