8/8/2016

Exchange 2016 DAG

Md Shaifullah Mozide Palash OVIWIN

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Table of Contents What Is Database Availability Group? .......................................................................................................... 2

DAG witness server and witness directory ................................................................................................... 2

DAG membership .......................................................................................................................................... 2

Automatic database mount dial ................................................................................................................... 3

Quorum Models ............................................................................................................................................ 3

Exchange Server 2016 DAG Configuration .................................................................................................... 4

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What Is Database Availability Group? A database availability group (DAG) is a set of up to 16 Microsoft Exchange Server 2016 Mailbox servers that provides automatic, database-level recovery from a database, server, or network failure. DAGs use continuous replication and a subset of Windows failover clustering technologies to provide high availability and site resilience. Mailbox servers in a DAG monitor each other for failures. When a Mailbox server is added to a DAG, it works with the other servers in the DAG to provide automatic, database-level recovery from database failures. When you create a DAG, it's initially empty. When you add the first server to a DAG, a failover cluster is automatically created for the DAG. In addition, the infrastructure that monitors the servers for network or server failures is initiated. The failover cluster heartbeat mechanism and cluster database are then used to track and manage information about the DAG that can change quickly, such as database mount status, replication status, and last mounted location.

DAG witness server and witness directory When creating a DAG, you need to specify a name for the DAG no longer than 15 characters that's unique within the Active Directory forest. In addition, each DAG is configured with a witness server and witness directory. The witness server and its directory are used only when there's an even number of members in the DAG and then only for quorum purposes. You don't need to create the witness directory in advance. Exchange automatically creates and secures the directory for you on the witness server. The directory shouldn't be used for any purpose other than for the DAG witness server. The requirements for the witness server are as follows:

The witness server can't be a member of the DAG.

The witness server must be in the same Active Directory forest as the DAG.

The witness server must be running Windows Server 2012, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003 R2, or Windows Server 2003.

A single server can serve as a witness for multiple DAGs. However, each DAG requires its own witness directory.

Regardless of what server is used as the witness server, if the Windows Firewall is enabled on the intended witness server, you must enable the Windows Firewall exception for File and Printer Sharing.

DAG membership When the first Mailbox server is added to a DAG, the following occurs:

The Windows failover clustering component is installed, if it isn't already installed.

A failover cluster is created using the name of the DAG. This failover cluster is used exclusively by the DAG, and the cluster must be dedicated to the DAG. Use of the cluster for any other purpose isn't supported.

A CNO is created in the default computers container.

The name and IP address of the DAG is registered as a Host (A) record in Domain Name System (DNS).

The server is added to the DAG object in Active Directory.

The cluster database is updated with information on the databases mounted on the added server.

When the second and subsequent servers are added to the DAG, the following occurs:

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The server is joined to the Windows failover cluster for the DAG.

The quorum model is automatically adjusted: A Node Majority quorum model is used for DAGs with an odd number of members. A Node and File Share Majority quorum model is used for DAGs with an even number of

members.

The witness directory and share are automatically created by Exchange when needed.

The server is added to the DAG object in Active Directory.

The cluster database is updated with information about mounted databases.

Automatic database mount dial The AutoDatabaseMountDial parameter specifies the automatic database mount behavior after

a database failover. You can use the Set-MailboxServer cmdlet to configure the

AutoDatabaseMountDial parameter with any of the following values:

BestAvailability If you specify this value, the database automatically mounts immediately after a failover if the copy queue length is less than or equal to 12. The copy queue length is the number of logs recognized by the passive copy that needs to be replicated. If the copy queue length is more than 12, the database doesn't automatically mount. When the copy queue length is less than or equal to 12, Exchange attempts to replicate the remaining logs to the passive copy and mounts the database.

GoodAvailability If you specify this value, the database automatically mounts immediately after a failover if the copy queue length is less than or equal to six. The copy queue length is the number of logs recognized by the passive copy that needs to be replicated. If the copy queue length is more than six, the database doesn't automatically mount. When the copy queue length is less than or equal to six, Exchange attempts to replicate the remaining logs to the passive copy and mounts the database.

Lossless If you specify this value, the database doesn't automatically mount until all logs generated on the active copy have been copied to the passive copy. This setting also causes the Active Manager best copy selection algorithm to sort potential candidates for activation based on the database copy's activation preference value and not its copy queue length.

The default value is GoodAvailability. If you specify either BestAvailability or GoodAvailability, and all the logs from the active copy can't be copied to the passive copy being activated, you may lose some mailbox data. However, the Safety Net feature (which is enabled by default) helps protect against most data loss by resubmitting messages that are in the Safety Net queue.

Quorum Models Exchange Server DAGs make use of an underlying Windows Failover Cluster. You don’t need to create, configure, or even touch the Windows Failover Cluster using cluster management tools, except in specific maintenance scenarios that are clearly documented. When you add members to a DAG the failover clustering components are automatically installed and configured for you. Quorum is the voting process that the cluster uses to determine whether the DAG should remain online or go offline. If the DAG goes offline all of the databases in the DAG are dismounted and inaccessible to end users, causing an outage.

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There are two quorum models:

Node Majority – when the DAG has an odd number of members the file share witness is not required for the quorum voting process, because the DAG members can determine a “majority” themselves. For example, if one DAG member fails, 2/3 DAG members are still online (a majority) and the DAG can remain online. If two DAG members fail, 1/3 DAG members are still online, which may result in quorum being lost and the DAG going offline.

Node and File Share Majority – when the DAG has an even number of members the file share witness is included in the quorum voting process to ensure that a “majority” can be determined. For example, in a two-member DAG if one member fails, 1/2 members are still online (not a majority), but you would expect the DAG to be able to withstand a single node failure. The file share witness is used as the tie-breaker, meaning 2/3 “votes” are still available, and the DAG can stay online. Similarly with a four-member DAG, if two members failed, with the file share witness there are still 3/5 “votes” online, so the DAG can stay online.

Exchange Server 2016 DAG Configuration

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The replication interfaces are also not configured with a default gateway. In the case where replication interfaces for the same replication network are on separate IP subnets, static routes are configured. However in this example that is not required.

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