Exchange 2010 - Two-member DAG questions

I've been going over DAG setup for an Exchange 2010 and am a bit confused about how it works.

Looking at the "Database Availability Group Design Examples" at, I find the following two-member DAG to seem easiest and most applicable to my environment.

The diagram is shown as follows:
This raises a couple questions:

1. If databases are "Passive", what does that mean?

2. Since a network load balancer is used, what if somebody whos mailbox is on an "Active" database is directed by the Load Balancer to the other mailbox server where their database is "Passive".

3. What's the availability look like if both DAG members are running Client Access/Mailbox/HUB roles? Would it be wise to add a third member to the DAG and then give it only the Client Access or only Mailbox role and add a delayed replication, whereas the other 2 would be immediate replication?
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AmitIT ArchitectCommented:
Passive means not is use by user. Data is written to active DB and the logs are shipped to Passive copy and Passive copy gets update. It acts as backup copy. In case you need to do some server maintenance or if your primary server goes down. Exchange will switch over to second server and make the passive DB as active. Once you have your problematic server up, you can switch back the db back to active again.

Load balancer is used for CAS role in 2010, not for mbx role. To make CAS Highly Available you need Load balancer. So, if you have two server CAS1 and CAS2. HLB will be configured to use both server. If CAS1 goes down HLB will switch over to CAS2 and server the user.

For your last question multi role is preferred. As that save your extra hardware and software cost. Also reduce admin overhead. I guess you are asking for lagged copies in your last question. Lagged copies means, it is not upto date like passive copy. You can set a lag period of max 14 days. I am not big fan of lagged copies. As it requires lot of admin efforts to recover simple data. Backup is best option.

Let me know, if you have more query.

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Leroy LuffHead of IT & DIgitalCommented:
1. Passive database means it is not the client facing mailbox store at this stage - purely there for redundancy purposes. If the active database has a problem the passive will become the active database.

2.The client access server will automatically know where to direct a client request. It will always point to the active server and not the passive.

3. For a small to medium network I would say 2 servers is enough, each with DAG and CAS roles.
Will SzymkowskiSenior Solution ArchitectCommented:
Answers are below...
1. Databases that are passive are databases that are basically on standby. They do not do anything other than collect and commit then logs to the passive database. At anytime where the active database goes offline the passive database now becomes active.

2. This would only happen if the passive database was set to active, or if the active database goes offline for any reason.

3. It is wise to keep the roles together as it does cut cost for licensing and additional servers. It is also up to your discretion to add more DAG members, this does however provide more redundancy but you need to take into consideration regarding the cost of additional server and exchange licenses and also another big cost is Storage.

You can have up to 16 DAG members in a DAG which provides the most redundancy but DAG members do not have to hold all of the databases for every server, you can mix them up with passive and active.

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garryshapeAuthor Commented:
Ok so the active database doesn't have to be on the Client Access Server, just a mailbox server.
I guess the diagram was confusing me
If active database "DB1" is on server "EX1", and "EX1" gets powered off, then the passive "DB1" on "EX2" will become active?
AmitIT ArchitectCommented:
Yes, DB1 on Ex2 will become active. But remember, if you have even node, then your need File Share Witness server. I suggest you to watch this MS video:
garryshapeAuthor Commented:
So the active databases depicted are split between servers just to alleviate the load on a the servers they are active in?
AmitIT ArchitectCommented:
No, it is not like that, Active DB exist on only one server and passive gets updated to become active.
garryshapeAuthor Commented:
So would this look right?

And MAIL9 and MAIL10 would have to be the same in terms of capacity, hard-drives/partitions?

AmitIT ArchitectCommented:
That's correct.
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