DAG is a group of about 16+ mailbox servers that host a set of databases. Exchange server 2010 support up to 100 databases per server. The Mailbox server, which is a member of the DAG is able to host copies of active and passive database.
Fig: Exchange Server 2010 DAG
In this fig, the Mailbox Server 1 host the active copy of Database 1 and the remaining Database 2 and 3 are Passive copy of mailbox database. The active Database 1 has its data or information stored as a replicated copy in database 2 and 3, whatever modifications made in Database 1, the same will reflect in Database 2 and 3.
But when Database 1 got corrupted or server down then it became Passive and switch over to another DAG member i.e.; Database 2 which stores its copy has become Active. And replication of files between Database 2 (Active) and Database 3 (Passive) begins.
This describes that Mailbox Server provides the ability to deploy services and enhance the availability of data from Mailbox server. Mailbox server provides the ability of data failure recovery through replication system. Whenever the active database gets corrupted due to any reason, it switches over to another database which belongs to DAG (Database Availability Group) member and become active.
In Database Availability Group, witness server is the additional server which is not the part of DAG but plays a vital role that it reserve over failure by continuing updating in every 30 seconds to the DAG.
- Witness server should not be the member of a DAG.
- The witness server must be in active directory.
Occurrence of Mailbox Database Replication
Data Replication is done in two ways:
- File Replication
- Block mode Replication
In File Replication system, the transaction logs are created and gets closed after reaching 1 MB size and then replicate in each member of a DAG that stores in mailbox server. File Replication is done in Exchange server 2010RTM (initial release version).
Block mode Replication
In exchange server 2010 sp1, the file replication makes the database copies to sync with each other. Once the mailbox database sync to each other they switch over to ‘block mode replication’. It is a continuing mode of replication, where the chances of failure decrease. In Block mode Replication each database replication is transmitted to the log buffer and further transfer to the active and passive copies of the database.