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Disadvantages of CCR in Exchange 2007

Posted on 2008-11-02
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Last Modified: 2008-11-08
Hi Experts

At the moment, our Exchange servers (2) are arranged in a 2+1 clustered formation, i.e. active-active-passive.

We are evalutating the different types of DR/HA in Exchange 2007, and are thinking of moving to CCR.

However, I have heard of two drawbacks;

a) CCR only supports two nodes
b) With CCR, the logs are only replicated across from the active to passive once they have been commited.

So, I can see two issues;

a) If we have two Exchange servers, how can we get this onto a two node cluster? Or do we build two clusters, i.e. one active-passive and another active-passive. So really, this is two two-node clusters, as opposed to a four node one?

b) If the active server was to be lost due to corruption and we had to move to the passive, any uncommited transacations would be lost?

Hope someone can clarify
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Question by:bruce_77
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tigermatt earned 250 total points
ID: 22862408

Hey,

You're correct in saying Exchange 2007 CCR only supports two nodes in the cluster, and it is also true that log files are only replicated to the other node in the cluster once the log file is full (5MB) and a new log is started.

Can you please elaborate a little on your first question?

With regards to the second question, if the cluster failed over to the passive node for whatever reason, you would lose any uncommitted transactions in a logfile which hasn't been properly closed. However, this design flaw is intended to have as little impact as possible, because in organizations where CCR is deployed, mail flow would be expected to be high, so perhaps only a few minutes worth of mail would be lost. But yes, I agree it could be a potential problem if, say, the cluster failed over over night.

-tigermatt
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by:gupnit
gupnit earned 250 total points
ID: 22862975
Hi,
Coming to your questions...
  1. If you have 2 Exchange servers, then you want CCR for both, well in that case there will be 2 CCR deployments. I would rather suggest combine both into 1 and then setup a CCR Cluster. Exchange 2007 interms of Sizes it can handle is pretty sturdy.
  2. Agreed, but then thats why In Exchange 2007, Log FIles have been reduced to 1 MB and 1MB is not that bog. Also assuming log was not commited, well in that case we have Transport Dumpster for CCR to exactly take care of this issue.
So you are pretty safe with CCR. here are links to explain what I mentioned above:
Hope this answers your queries
Thanks
Nitin
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by:bruce_77
ID: 22863196
Thanks guys...

Just to ask a question regarding the following comment;

"if the cluster failed over to the passive node for whatever reason, you would lose any uncommitted transactions in a logfile which hasn't been properly closed"

Does this mean that if an email was sent to a user (and received to their Outlook) but Exchange then crashed, and this email was logged as an uncommited log file, then the email would disappear from the user's Outlook once the failover takes place?
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by:bruce_77
ID: 22863208
...and, sorry, one other question..am I correct in thinking that in SCC, when failing over a node from a cluster to the passive node, even uncommited transaction logs can be ported to the passive?
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Expert Comment

by:tigermatt
ID: 22863301

>>Does this mean that if an email was sent to a user (and received to their Outlook) but Exchange then crashed, and this email was logged as an uncommited log file, then the email would disappear from the user's Outlook once the failover takes place?

As far as I am aware, yes. Since the log file containing that email would not have been replicated, it wouldn't have been replayed against the Exchange database and therefore would not be present when the user opens there Outlook connection.

Don't forget the log files log EVERYTHING in and out. It could be a new contact object which is lost, a calendar item/task etc.

Hopefully http://technet.microsoft.com/en-us/magazine/cc137735.aspx will explain some of the log file differences between CCR and SCR. One other key point to note is the failover must be initiated manually - unlike CCR, SCR failover to the passive node is NOT automatic.

-Matt
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by:gupnit
ID: 22863353
Hi,
Transport Dumpster....Like I said...it plays an important role. You would get the mails.
SCR - Yes it is manual and can be copied.
Thanks
Nitin
 
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by:bruce_77
ID: 22863365
Thanks...just one final question ;)...is there any Exchange 2007 DR/HA solution where uncommited log files can be replicated over when the server is failed over?
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Expert Comment

by:gupnit
ID: 22863382
Hi,
How many sites do you have , if one then go for CCR. With CCR , using Transport Dumpster, you will not loose any mails at all. If everything is configured properly.
If multiple sites, then SCR, and Logs will have to be manually copied to passive node.
I need to understand your concern...!
Cheers
Nitin
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