Database Availability Group

DAG (Database Availability Group):-

In Exchange Server 2010, we don’t have the concept of LCR, CCR, SCC and SCR. In actual LCR and SCC concepts has been removed and the CCR and SCR concepts are combined and have evolved into more unified High Availability Solution knows as Database Availability Group.

A database availability group (DAG) is a high availability (HA) and data recovery feature of Exchange Server 2010.

A database availability group, which can consist of up to 16 Exchange mailbox servers, automates recovery at the database-level after a database, server or network failure.

With DAG, We achieve redundancy at the server level, database level and the data resiliency. Primary component of DAG is called Active manager, Instead of the exchange cluster resource DLL and the associated cluster services used in the previous version of exchange, exchange 2010 use AM to manage switch over and failovers.  Active Manager runs on all mailbox servers that are part of DAG member.

  • Remember In exchange 2007 CCR, Cluster continues replication you can have one active one passive database configuration..
  • DAG gives you 16 mailbox servers thus you get 16 copies of each database on each server.

Few questions and answers about Database Availability Group,

How many nodes can I have in a DAG?

Anywhere from 1 to 16 mailbox servers can be included in a DAG.

Does DAG use SMB replication like CCR and SCR did?

No, DAG uses one TCP socket per database for replication.

Which ports does DAG use for replication?

DAG uses a single port for replication and it is port 64327. This is configurable by administrators if needed.

Can you leverage Storage Groups with DAG?

Storage groups have been removed in Exchange 2010 in order to leverage database level failover.

Can I put a public folder database in a DAG?

No, in order to maintain PF database availability it is recommended to setup a public folder replica.

How do I failover to another datacenter? Can I do this after setup or does it have to be performed from initial setup?

SCR used to be the method for datacenter resiliency in Exchange 2007 and it has been replaced with DAG in Exchange 2010.  The nice part about a DAG is you can add additional datacenter sites for failover at a later point post DAG setup.

How many NICs do I need in a DAG server? Does it all have to be on one subnet?

2 NICs per server minimum are needed for a DAG server node. DAG also supports multiple subnets (multi-datacenter locations).

Do I have to run a static IP for my DAG

No, DAG defaults to a DHCP based IP and can be used with either a static or dynamic IP.

What OS can I run and what OS version for a DAG node is required?

Windows Server 2008 or Windows Server 2008 R2 Enterprise or Datacenter Edition is required.

Where can I put my file share witness?

It has to be in the same AD forest as the DAG and cannot reside on a DAG member.  It is recommended to be placed on the hub transport in order to be administered by Exchange admins.

Do I have to pre-create my file share witness like in Exchange 2007?

No, Exchange 2010 will auto-create the FSW share with correct Exchange permissions.

Can I encrypt or compress DAG over the wire?

Yes to both. You would leverage the Set-DatabaseAvailabilityGroup cmdlet to enable either feature.

Any new DAG features coming in SP1 I should know about?

There are some enhancements which are slated (subject to change) to be included with DAG SP1 including:

Block mode replication

DAG server maintenance mode option

DAG database re-distribution

Better cross-datacenter DAG experience for Outlook users (read – fewer Outlook restarts needed)

Better DAG reporting

DAC mode available for one site now

Re-seeds can use spare storage

Other minor DAG additions and tweaks

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