Why Did Microsoft Pick “16” As The Maximum Number Of Servers In A DAG?…

ByRajith Enchiparambil

I was asked this question today. Why did Microsoft pick “16” as the maximum number of servers that you can have in a DAG? Having 16 servers in a DAG means that you can have a maximum of 16 copies of a database (one active and 15 passive). Isn’t it a bit too much to…

I was asked this question today. Why did Microsoft pick “16” as the maximum number of servers that you can have in a DAG? Having 16 servers in a DAG means that you can have a maximum of 16 copies of a database (one active and 15 passive). Isn’t it a bit too much to have 15 passive copies of a database? How can you justify this figure?

These were the questions that bugged an exchange admin that I met. To make it clear, the Exchange team didn’t go for a “magic” number for the maximum number of servers in a DAG. The limit is imposed on the DAG by the underlying Windows failover clustering, on which DAG is based.

The Windows failover cluster can only support a maximum of 16 nodes. This limitation kind of flows into DAG. So, though a DAG can only have a maximum of 16 servers, the limit actually comes from the Windows failover cluster.

There you go, the story behind the magic numberWinking smile

Rajith Enchiparambil

Cloud Architect & Blogger with interests in Microsoft 365, Azure, AWS & PowerShell. I am active on Experts Exchange & TechNet forums and I am a technical author for SearchExchange. Follow me on Twitter, LinkedIn, or Facebook for the latest updates. For consultancy opportunities, drop me a line.

7 Comments

  1. Baldeo Rai says:

    Hi Rajith,

    Thanks for sharing this information. How many database can be added in single DAG ? Is there any limitation ?

    Reply
    1. Rajith Jose Enchiparambil says:

      Hi Baldeo,

      You can have a maximum of 100 databases (active and passive combined) on a server if you have an Enterprise license and 5 if you have Standard edition of Exchange.

      Thanks

      Reply
  2. Yusuf Ahmad says:

    In Windows Server 2012, maximum cluster node supported is 64, then why Exchange 2013 still support only 16 nodes ?

    Reply
    1. Rajith Jose Enchiparambil says:

      HI Yusuf,

      It is an Exchange limitation, not a Windows one. Why would you need to have a DAG with more than 16 nodes when the max database size is 2TB?

      Reply

  3. Thanks for clarification on it, prior i was thinking it designated for DAG only 16 copy while it imposed by windows clustering features :-)

    Reply

  4. Hi

    Is it possible to have multiple DAG’s in single Exchange Organization?

    Best regards
    Muralee

    Reply
    1. Rajith Jose Enchiparambil says:

      Yes Muralee, you can have multiple DAGs

      Reply

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