Points To Note While Creating A Database Availability Group (DAG)…

I have written an article detailing the steps for creating a Database Availability Group (DAG). As more and more companies are thinking about moving to Exchange 2010 with high availability, few points have to be noted before creating the first DAG. You can use both EMC and shell to create a DAG. The DAG name…

I have written an article detailing the steps for creating a Database Availability Group (DAG). As more and more companies are thinking about moving to Exchange 2010 with high availability, few points have to be noted before creating the first DAG.

  • You can use both EMC and shell to create a DAG.
  • The DAG name can have a maximum of 15 characters.
  • The name of the DAG isn’t used by end users or administrators. It is only used by the system for internal communication and to secure the DAG.
  • All DAG members should have the same operating system, either 2008 SP2 or 2008 R2.
  • The operating system should be enterprise or datacenter edition, while Exchange 2010 can be either standard or enterprise.
  • You can install other roles like CAS & HUB on DAG nodes.
  • You have to use an external load balancer to load balance CAS & HUB installed on DAG nodes.
  • If you don’t specify a witness server, the system will attempt to automatically select a hub transport server that doesn’t have the mailbox server role installed to be used as the witness server.
  • The witness server cannot be a member of the DAG.
  • After you create a DAG, it will be configured to use DHCP. If you don’t want the DAG to use DHCP, you can use the Set-DatabaseAvailabilityGroup cmdlet to configure an IP address for the DAG.
  • You can configure more than one IP address for the DAG if the DAG is spread across different sites.
  • You don’t have to fill in all the details while creating a DAG in the EMC. Four combinations of options are possible.
  1. You can specify only a name for the DAG and leave the witness server and witness directory check boxes cleared. Then, the wizard will search for a hub transport server that doesn’t have the mailbox server role installed. It will automatically create the default directory and share on that hub server and use that server as the witness server.
  2. You can specify a name for the DAG, the witness server that you want to use and the directory you want created and shared on the witness server.
  3. You can specify a name for the DAG and the witness server that you want to use and leave the witness directory check box cleared. Then, the wizard will create the default directory on the specified witness server.
  4. You can specify a name for the DAG, leave the witness server check box cleared and specify the directory you want to be created and shared on the witness server. Then, the wizard will search for a hub server that doesn’t have the mailbox role installed and will automatically create the specified directory on that server, share the directory and use that hub server as the witness server.

6 Comments

  1. Rajith Jose Enchiparambil says:

    My pleasure, ECL.

  2. Anonymous says:

    Thanks for the advice Rajith

    ECL

  3. Rajith Jose Enchiparambil says:

    Hi,

    Since you have an even number of MBX servers in DAG, FSW will be used and it is good to have FSW in the DC which has most of your production/online servers. You can configure an alternate witness server in DC2. That means configuring another hub in DC2 as an alternate one.

  4. Anonymous says:

    Hi Rajith,

    Thanks for the quick response. So lets says I have 2 Data Centres with the DAG spread accross both (DC1 is primary and DC2 is secondary).

    Each DC has 5 Mailbox servers (the 5 in DC2 will hold the passive DB's) so thats 10 in total for the DAG and the FSW will be hosted in DC1.

    What is the recomendation would you advise in this recommendation? when you mean have a alternate witness server do you mean move the FSW to another Hub?

    Thanks

    ECL

  5. Rajith Jose Enchiparambil says:

    Hi ECl,

    You can have an alternate witness server setup. When the DAG has even number of mailbox servers, it uses the FSW as a voter. Taking it down won't affect DAG to go down, as long as all mailbox servers are in the same AD site.

    If differs in different design situations.

  6. Anonymous says:

    Hi Rajith, great post. Is there any way to setup resilience / ha for the file share witness? E.g if I am taking the hub that is hosting the fsw down for maintenance or it has a hardware fault? Or is my only option is to recreate it as and when I need to?

    Thanks

    Ecl

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