Exchange lagged database copy

I have done some readings on Microsoft website about Lagged database copy, but still not clear.
I want to know what s the purpose of it. I know the lagging is applied only on the passive database copy not on the active and the lagging delays the replay of the logs for a specified time.

However I believe if something happen to active copy, the passive will become the active and the logs will be replayed...anyway

in Some articles, they say that the lagging comes in handy when the active database is corrupted, and when the corrupted data is copied to the passive copy, it will not replayed until it reach the specified time in Lagging...if we realize that there is corruption within that time frame, then we can just initiate the replay of the logs generated before the corruption happened, and delete the logs generated after the corruption happened. .....Too complicated...

I could be wrong...I need Exchange expert to shade some lights on this topic.

Thanks
jskfanAsked:
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Simon Butler (Sembee)ConsultantCommented:
The articles you have read about database corruption being a reason for using a lagged copy are wrong.
Database corruption cannot replicate across a DAG, because the replication happens via the logs, not the actual database file.

The only reason I can see for having a lagged copy is to allow an error by a user or admin to be caught.
For example an admin is doing a mailbox export and gets it wrong and deletes a lot of content. A user deletes a lot of content and there are no backups etc.

However, I have never deployed a lagged copy of the Exchange databases. Most clients and implementations want a live copy, so if the active server fails all of the content is located on the passive server and nothing can be lost.
With deleted item retention on the database, the reason for having the lagged copy pretty much goes away.

The only other time I can imagine having a lagged copy would be to delay the replication because the link is so slow or heavily utilised. Put a lag of 8-10 hours on the database and move content replication will happen out of business hours. Pretty poor reason though, and not something I have ever done or even suggested to a client.

Simon.
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jskfanAuthor Commented:
Do not you think if the Active Database gets corrupted at certain point in time , it will still replicate the corrupted logs to the passive database copy.

 Then if they get replayed right away on the passive copy , then both Active and passive copy will be corrupt
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Simon Butler (Sembee)ConsultantCommented:
If the active database gets corrupted then that corruption isn't going to replicate because the second copy is created using the transaction logs.
In effect, database corruption isn't an issue, because if you get a corrupt database then simply drop the copy that is corrupt and reseed. Corruption cannot move between the database copies.

Simon.
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jskfanAuthor Commented:
Thank you!
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