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Re: Data Mirroring on two data centers -- How to use ASM ?

From: Tanel Põder <>
Date: Sun, 21 May 2006 13:20:27 +0800
Message-ID: <0ace01c67c96$47061f40$6401a8c0@porgand>


One benefit of using storage level replication is that it doesn't require much DBA work to do it.

But if your primary database crashes because of a block corruption in a data dictionary table, then your DR site will have exactly the same problem in it, doesn't matter how far away it is. With applying archive/redologs to DR database you will less likely have this issue (e.g. corruptions of primary database blocks due software failures or disk controller glitches won't be tranferred over to DR).

And if we'd have a corruption somewhere in redo stream, then it would highly likely be detected immediately when applying the redo to standby.

We would have two options in such case:
1) redo in source db is ok - then just reget the log and apply again 2) redo in source db is corrupt as well - issue an immediate checkpoint in source db and alarm DBA

Also, storage level replication has less flexibility than log apply one..


> Hello Tanel
> You might be right about the amount of data that goes over the line but we
> replicate our mainframe storage to remote site and have almost nil impact
> on performance. We are a bank with over 100 branches, so there is a lot of
> activity when the branches are open.
> If we are talking about DR scenario, and using fiber to connect the sites,
> I would still go with SAN replication.
> Adar Yechiel
> Rechovot, Israel
> Tanel Põder wrote:
>> One more reason to use data guard instead of storage/LVM level
>> replication in high-activity OLTP environments is that redolog entry
>> based shipping is much more more fine grained than storage block level
>> replication.
>> I once asked one EMC admin, they told that the minimum block size for
>> SRDF is 32k. So if you update one row and commit, you'd need to ship few
>> hundred bytes to standby, while with SRDF you'd need to transfer 32
>> kilobytes over the fibre when the block is written to disk /plus/ you
>> need to continuously transfer redolog writes before the datafile blocks
>> are sent to remote.
>> If your management definitely requires SAN based replication, then you
>> could just keep your archivelogs on a replicated volume/storage and do
>> frequent log switches to keep the lag small in case of primary failure.
>> Tanel.
>> ----- Original Message -----
>> *From:* Carel-Jan Engel <>
>> *To:* <>
>> *Cc:* <>
>> *Sent:* Saturday, May 20, 2006 3:06 AM
>> *Subject:* Re: Data Mirroring on two data centers -- How to use ASM ?
>> Hi Madhu,
>> I'm wondering your primary 'requirement' of mirroring data across
>> TWO data centers.
>> IMHO, mirroring between data centers is a solution, or if you
>> like, tool. Whatever, it isn't a requirement.
>> Requirements could be something like:
>> - After a server failure, the database should be available again
>> within 30 minutes
>> - After a server failure, no more than 5 minutes worth of
>> transactions may be lost
>> - After a database corruption, the database should be available
>> again within 6 hours
>> - After a database corruption, no more than 30 minutes of
>> transactions may be lost
>> - Restoring of the database to any point in time between now and
>> now - 6 days must be possible
> --

Received on Sun May 21 2006 - 00:20:27 CDT

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