Answers to the 10 most Common Questions on the Exchange DAG.

Friday, June 11, 2010 1:54
Posted in category Exchange & OCS

Working the Exchange booth during TechEd 2010 in New Orleans, I got lot of questions regarding the Database Availability Group, or DAG, in Exchange Server 2010. Here’s the 10 most predominant questions (and of course, the answers to them);

Q1. Can I use the DAG with Exchange Server 2010 Standard Edition?

A1. Yes, you can use the DAG with Exchange Server 2010 Standard Edition. Yet, with Exchange Server 2010 Standard Edition you are limited to 5 databases at most per mailbox server. With Exchange Server 2010 Enterprise Edition you can have anywhere from 1 to 100 databases per mailbox server.

Q2. How many mailbox servers can I have in a single DAG?

A2. You can have up to 16 mailbox servers in a single DAG. This limit is not so much imposed by the Exchange product, but it is a limitation of the Windows clustering technology that is behind Exchange’s DAG.

Q3. Can I have more then 1 DAG in a single Active Directory Site?

A3. Yes you can. Although you can have only 1 Client Access Server (CAS) Array per AD-site, you can have multiple DAGs in a single AD-Site.

Q4. How large can a single database in a DAG be?

A4. Whether you are using a DAG or not, a single mailbox database in Exchange Server 2010 (Standard or Enterprise) can hold up to 16TB of data. Depending on how you will or will not backup the data in these databases will impact how large you want a single database to become. Restoring 16TB of data, if required, takes a long time. Even from a fast disk array…

Q5. Can I run a DAG on Windows Server 2008 (R2) Standard Edition?

A5. No. Since Exchange Server 2010 uses Windows Clustering for running the DAG, you need a Windows version that supports clustering. Hence, Windows Server 2008 (R2) Standard Edition cannot be used. You will have to use either Windows Server 2008 (R2) Enterprise Edition or Datacenter Edition.

Q6. Can a DAG span multiple IP Subnets?

A6. Yes. Since Exchange Server 2010 uses Windows Clustering for running the DAG, and Windows Clustering now supports stretching the cluster over multiple IP-subnets, you can span a DAG over multiple IP-subnets.

Q7. Can I have a DAG server ‘in the cloud’?

A7. No you cannot have a DAG server ‘in the cloud’ using Microsoft Exchange Online.

Q8. Do I need an odd number of mailbox servers in a DAG?

A8. In order for a cluster to make decisions about whether a database is ‘up’ or not, we need an odd number of servers making that decision. By default, each member of the DAG has one vote. If you have an even number of nodes, and the first two nodes were separated of the other two nodes, there is no way of telling if the passive copy of the data on one of those two nodes should be activated. Therefore, another voter is required. In stead of having to put another mailbox server in the DAG, you can use a File Share Witness to break the tie. This ‘server’ does not have to be part of DAG. Hence, no, you do not need an odd number of mailbox servers in a DAG.

Q9. Will failover in a DAG always be automatic?

A9.Depending on the design of your DAG, failover may occur automatically or not. So, it depends.If you have four servers in your DAG, of which two reside in Data Center A and two reside in Data Center B, and you have a File Share Witness in Data Center A, databases will not fail over automatically to Data Center B mailbox servers in case you loose Data Center A completely because there can never be a majority of voters in Data Center B in this scenario. You will have to plan for a manual (or scripted) failover in this scenario.

Q10. Do all mailbox servers in the DAG have passive copies of all active databases?

A10. No. You can have 10 mailbox servers in a single DAG and set it up in such a way that there are never more then (for example) 3 copies of each active database at any given time. So you can, for example, have 30 active databases distributed over the 10 nodes in you DAG. Each node can than have 3 active databases. You can now configure each node to have, again, for example, 6 passive copies only.

Remember that in a DAG you can failover a single database, and you do not always have to failover an entire server!

Hope this helps!

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