Datacenter Activation Coordination (DAC) Mode In Exchange 2010 DAG…

MS Exchange

Datacenter Activation Coordination (DAC) Mode is a property of a DAG which can be turned on or off. DAC mode is disabled by default and should only be enabled for DAGs with three or more DAG members that have been deployed in a multi-datacenter configuration. DAC mode shouldn’t be enabled for

  • 2 member DAGs where each member is in a different AD site
  • 2-16 member DAGs where all members are in the same AD site

In case you try to turn on DAC where it is not supported, an error will be returned.

DAC Error

Note: In the expected Exchange 2010 SP1, DAC mode will be extended to support two-member DAGs that have each member in a separate datacenter. In addition, DAC mode will be extended to support DAGs that have all members deployed in a single Active Directory site, including AD sites that have been extended to multiple locations. So in SP1, you can now use DAC mode for all DAGs with two or more members.

DAC mode is configured to avoid a “split brain syndrome”. I will explain with an example. Let’s say we have a four member DAG, with two servers in each datacenter. The primary datacenter hosts the witness server and hence will always be in quorum. Now, let’s say that a power outage occurs in the primary datacenter and the exchange admin activates the secondary datacenter with an alternate file share witness.

When the power is restored in the primary site, servers come online quicker than the WAN links. Hence, when the two DAG members and witness server comes online, it has a quorum (majority) and will try to activate the databases. This will cause a “split brain syndrome” where both datacenters think that they are hosting the active databases.

DAC mode was introduced to avoid this situation. When DAC mode is enabled and the DAG members come back online, they will leverage a protocol called Datacenter Activation Coordination Protocol (DACP) before trying to mount the databases.  The DACP is used to determine the current state of the DAG and whether Active Manager should try to mount the databases or not.

Now for the technical bit as to how DAC works!

Active Manager stores a bit in memory (either a 0 or  1) that tells the DAG whether it’s allowed to mount local databases that are assigned as active on the server. When a DAG is running in DAC mode, each time Active Manager starts up, the bit is set to 0, which means that it isn’t allowed to mount databases. When in DAC mode, the server must try to communicate with all other members of the DAG that it knows to get another DAG member to give it an answer as to whether it can mount local databases that are assigned as active to it. The answer comes in the form of the bit setting for other Active Managers in the DAG. If another server responds that it’s bit is set to 1, it means that servers are allowed to mount databases. Hence, the server starting up sets its bit to 1 and mounts its databases.

Let’s find how DAC helps in “split brain” scenario in our example. When power is restored to the primary datacenter, the servers come online before WAN links and all of the DAG members in the primary datacenter will have a DACP bit value of 0. Hence, none of the servers in the primary datacenter will mount databases as they cannot communicate with a DAG member that has a DACP bit value of 1.

DAC mode can be turned on by running the shell command below.

Set-DatabaseAvailabilityGroup –identity “dagname” –DatacenterActivationMode DagOnly

Turn DAC On

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  1. Hi Rajith,
    I have seen lot of articles in our website and it is very useful for us ..thank you rajith.

    What is Split Brain and how it is working in DAC , because i can not understand these things .let me know if you have any idea…..

    Reply
  2. Hi Rajith,

    These could be a silly question but need to know for my beter understanding, So Please help me for below my doubts:

    1. ”it has a quorum (majority) and will try to activate the databas”, does this mean, Even though database/s in secondry site active, it will also attempt to make the databases in primary site online after power restored??. And that is where ”Split brain syndrome” situation arise.

    2. When you say restore DAG with an alternate witness server, deosnt it mean we restore DAG in Secondary site and configure new FSW?? Or FSW will already be there and we will just restore DAG. Please clarify.

    Reply
  3. Hi Rajith,

    These could be a silly question but need to know for my beter understanding, So Please help me for below my doubts:

    1. ”it has a quorum (majority) and will try to activate the databas”, does this mean, Event to database/s in secondry site active, it will alos attempt to make them online in primary site??. And that is where ”Split brain syndrome” situation arise.

    2. When you say restore DAG with an alternate witness server, deosnt it mean we restore DAG in Secondary site and configure new FSW?? Or FSW will already be there and we will just restore DAG. Please clarify.

    Reply
  4. Hello,
    Need help in DR Switchover {DAG across two data centers}
    Setup: Two AD sites [Production and DR] DR site is passive, no user connectivity. It is only use in disaster scenario when Primary site is down.
    ProductionSite
    1 HT & CAS servers [FSW on Hub 1] 1 HT & CAS server
    DAG: 1 mailbox servers 1 MBX (Part of DAG)
    AD Servers AD Server

    Requirement: 1. Production site should be up and running even if the WAN link is down between production and DR.
    2. When Production site is down, DR manual switchover should not take more time and when the production site is up, it should easily switch back to primary site.
    Any pointer to this would be appreciated.
    DAC Mode Off
    Alternate FSW is not configured
    Exchange 2010 SP1

    Reply
    • Hi Devendra,

      You need DAC mode to ON.
      If SR site fails, everything continues to work as witness and MBX1 is online.

      If prod fails, you need to restore DAG with an alternate witness server. You can pre-configure the alternate witness server or not. Not a problem.

      Reply
  5. I can't seem to get this to work, I recieve the error requires 2 sites. It's a 4 member dag with all except 1 in the same site (nltest confirms different sites). Only 1 database which is healthy on all nodes. Any ideas?

    Reply

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Disable Windows Copilot Using Intune

Windows Copilot is Microsoft’s take on making life easier for Windows users using the power of AI. This article explains how to disable the feature using Intune, if your organization is not ready yet to walk into the AI world.

Disable Windows Copilot Using Intune

We need to create a Configuration Profile for Windows devices in the Intune portal to disable Windows Copilot. Below are the steps that we need to create the profile.

Launch the Intune Portal and login as a Global Admin or Intune Admin.

Navigate to Devices -> Windows -> Configuration Profiles.

Windows Configuration Profile Intune

Click on Create -> New Policy.

Select Windows 10 & later as the platform and Settings Catalog as the profile type & click on the Create button.

Settings Catalog Intune CoPilot

Give the policy a meaningful name & description and click Next.

Policy Name Disable CoPilot

Within the configuration settings, click on the Add Settings option.

Add Settings Disable CoPilot

Search for ‘copilot’, Windows AI will come up as the category. Click on Windows AI and the Turn off Copilot in Windows (User) setting will come up. Check the box and click Next.

Turn off CoPilot Setting Intune
Turn off CoPilot Setting Intune Summary 1

Specify scope tags if required and click Next.

Select tags CoPilot Intune

Select who this policy should apply to in the Assignments section. I have selected to add all users. If you want to test the setting, you can create a test group and select that group here.

Similarly, you can also exclude certain group from disabling AI (say IT team) if required.

Add all users disable copilot intune

A summary of selected settings will be displayed. Click on the Create button to setup the policy to disable Windows Copilot.

Create configuration policy disable CoPilot Intune 1

Wait for the replication to complete in the cloud backend and login to your machine. Your chatty Copilot should now be disabled.

Disable Windows Copilot On Windows 11 Pro

Follow the steps below to disable Copilot on a personal Windows 11 Pro machine (say your own laptop).

Search for ‘group’ in Windows 11 and click on Edit Group Policy option.

Group Policy Windows 11 Disable Copilot

Navigate to User Configuration -> Administrative Templates -> Windows Components -> Windows Copilot.

Windows Copilot GPO setting

Double click on Turn off Windows Copilot setting on the right pane.

Select Enabled and click OK.

Turn off copilot gpo Windows 11 Pro 1

Close the Group Policy Editor. This will disable Windows Copilot on a Windows 11 Pro machine.

Summary

We have learned to disable Windows Copilot using Intune and Group Policy on Windows 11 machines.

Please let me know if you have any questions in the comments section.

Promote Windows Server 2025 To Domain Controller

Domain controllers are the backbone of any Active Directory domains in the Microsoft world. Any Windows server can be promoted to be a domain controller. In this article, we will go through the steps of promoting a Windows 2025 Server to be a domain controller.

Windows Server 2025

The latest version of the server operating system has been named Windows Server 2025. You can start with a 2025 Server & create an AD domain or you can promote a member server that is already a part of a domain.

The Windows Server 2025 needs to be installed on a machine before it can be promoted to be a domain controller.

Promote Windows Server 2025 To Domain Controller

If you have been working with Windows servers long enough, everything starts with the Server Manager app. Promoting a server to a domain controller is no different.

Launch ‘Server Manager’ & click on Add roles and features.

Windows 2025 Server Manager

You land on the summary page that explains what is required to run this wizard successfully. Click Next.

Windows Server 2025 Add Remove Roles

Select Role-based or Feature-based installation and click Next.

Windows Server 2025 Role Based Install

Select the server that needs to be promoted and click Next.

Windows Server 2025 Destination Server

Select Active Directory Domain Services (second option) and click on Add Features.

Windows Server 2025 AD Domain Services

Go with the default options for features that need to be installed.

Windows Server 2025 AD Domain Services Features

A summary of AD DS pops up next, click next to continue.

Windows Server 2025 AD DS

Select Restart the server automatically if required and click Install.

Windows Server 2025 AD Restart Server

You get to keep an eye on the progress of the installation.

Windows Server 2025 AD Install Progress

Once the role has been installed, you will find an exclamation mark on the top right corner of the Server Manager. Click on that and select Promote this server to be a domain controller.

Windows Server 2025 AD Install Continue

You get an error straight away (which you have never seen before) – Error determining whether the target server is already a domain controller. Role change is in progress or this computer needs a restart.

Windows Server 2025 Domain Controller Setup Error

We never needed to restart the server after installing the role in the DC promotion process. Given that it is an insider build of Server 2025, I am hoping that this will get fixed before the public release.

Restart the server, launch Server Manager and click on the Promote this server to be a domain controller option again.

Windows Server 2025 AD Install Continue 1

I am setting up a brand new AD forest and hence I select the third option (Add a new forest) and enter my root domain name.

Windows Server 2025 Add a forest

Next window brings the option to set your forest & domain functional level and the DSRM password. In the insider build, it shows what looks like a variable (the Windows server version on which the you are working).

Windows Server 2025 Forest Functional Level

You can leave the default options in the DNS options wizard and click next.

Windows Server 2025 DNS Options

Enter the netbios name of the domain in the next window and click next.

Windows Server 2025 Netbios Domain Name

You can stick with the default paths for the AD database, log & sysvol folder or pick a location of your choice.

Windows Server 2025 AD Paths

Review the selections that you have made so far and click next.

Windows Server 2025 Options Review

Wait for the green check mark on the prerequisites page and click next.

Windows Server 2025 Pre reqs Check

Click Install in the final window & wait for the magic to happen. Once the machine gets restarted (which it will do automatically), you will have a brand new domain controller based on Windows Server 2025.

Windows Server 2025 AD Snap In

Summary

Promoting a Server 2025 to a domain controller follows pretty much the same steps as previous operating systems. The Insider build has few errors that needs to be fixed, but hey, it is an insider build!

Please let me know if you have any questions in the comments section.

Install Windows Server 2025 – Full Guide

Microsoft has released an insider preview of it’s next server operating system named Windows Server 2025. We will have a look at the installation steps involved in setting up a 2025 server.

Windows Server 2025

Microsoft has gone with the same look and feel of Windows 11 operating system in it’s current server operating system – Windows Server 2025. As the product is in insider preview, there might be slight changes before it hits the public shelves.

It is refreshing to see a ‘modern’ feel in the installation process of a server operating system. Gone are the days where the installation of a consumer based OS felt much better compared to it’s server counterpart.

Installing Windows Server 2025

Let’s take a look at the steps involved in setting up a Windows Server 2025 machine. First step is to download the ISO from the Windows Insider portal.

Next step is to boot the virtual / physical machine from the ISO which will kick off the installation of Server 2025.

First option to select is the language settings. Pick the one based which relates to you and click next.

Windows Server 2025 language settings

Select the keyboard settings in the next screen and click next.

Windows Server 2025 keyboard

You get the option to select whether you want to Install Windows Server or Repair the installation. The bottom left corner also has the option to go to the previous version of setup.

Windows Server 2025 setup option

You are asked to enter the product key, which is available in the Windows Insider portal.

Windows Server 2025 product key

Next option to choose is the type of image you want to install – Windows 2025 core or full blown desktop experience.

Windows Server 2025 Desktop

You need to agree to the licensing terms to move forward in the next step.

Windows Server 2025 Agreement License

Select the partition on which the server OS should be installed and click next. You also have the option to slice the partitions the way you see fit in the same screen.

Windows Server 2025 Disk Partitions

The Ready to Install window comes up, click the install button.

Windows Server 2025 Install 1

Installation of Server 2025 is underway and you get to see the progress.

Windows Server 2025 Install Progress

Once the installation is complete, you need to enter an administrator password of your choice to finalize the setup.

Windows Server 2025 Password

And there you go! You see a Windows 11 login screen staring at you ;-)

Windows Server 2025 Login Screen

After logging in, you get to set the options around sending diagnostic data to Microsoft, which I always set as ‘required only’.

Windows Server 2025 Diagnostic Data

The Windows Server 2025 desktop looks similar, doesn’t it? ;-)

Windows Server 2025 Desktop Feel scaled

Now that the server is up and running, you can promote it to be a domain controller.

Summary

The Windows Server 2025 has the same look and feel as a Windows 11 operating system. The installation options also provide that modern ‘feel’ and makes it a bit soothing to the eyes!

Please let me know if you have any questions in the comments section.