A note on clustering in Exchange 2010
I just had a conversation about how many nodes can fail in a three node DAG and since there was a misconception present I figured I should point out a section of the “Planning For High Availability and Site Resilience” article on TechNet.
From “Witness Server Requirements”
DAGs with an odd number of members do not use a witness server. All DAGs with an even number of members will use a witness server. The witness server can be any computer running Windows Server.
What does this mean? You need to have node majority within the DAG; if you have a three server DAG and two fail you only have one left and therefore don’t have majority. If you want to sustain two failures in a DAG then you need to design your DAG with four nodes and use a witness server.
When you add/remove a server from the DAG (note that a node failing does not count) the addition or removal of a witness server happens automatically. You can either specify a directory (The witness server cannot be a member of the DAG) or by automatically selecting a 2010 Hub Transport server in the site that does not have the Mailbox role installed.
Questions are welcome in the comments, a link to the TechNet article is below.
Planning for High Availability and Site Resilience: Exchange 2010 Help
Categories: Exchange Server
Tags: Exchange 2010, Exchange Server
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