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How do database copies enable Exchange server redundancy?

When email databases fail, important messages are lost and other issues arise. Database availability groups can help prevent these problems.

Every email traversing the Exchange platform is saved in a database on an Exchange 2013 or 2016 server. While a...

database structure provides an excellent means of organizing and retaining email content for the business, it also presents a potential point of failure. If the email database is compromised -- perhaps due to a storage failure -- the business can lose email messages, potentially opening the business to compliance or other regulatory violations.

DAGs are basically groups of mailbox servers that host duplicate databases and offer automated database recovery for failed servers or databases.

Microsoft realized early on that email databases must provide resilience to serve an enterprise, and Exchange has long supported resilience features such as single copy clustering, clustered continuous replication, local continuous replication and standby continuous replication.

However, starting with Exchange Server 2010, Microsoft employed the concept of database availability groups (DAGs) for email database resilience and server redundancy. DAGs are basically groups of mailbox servers that host duplicate databases and offer automated database recovery for failed servers or databases. DAGs effectively separate databases from the email server's functionality, so any server within a DAG can host a copy of the mailbox database from the other mailbox servers within the DAG. This way, the email server group can work as a resilient cluster to deal with issues like database corruption, disk or server hardware faults and network problems.

There are some basic rules for mailbox database copies. For example, Exchange 2013 can support up to 16 mailbox database copies across the DAG. Exchange 2016 can support more copies -- Standard Edition can host up to five database copies per host, and Exchange 2016 Enterprise Edition can host up to 100 database copies per host. Those database copies can be replicated to other mailbox servers within the same DAG and Active Directory domain to ensure server redundancy, but not to servers outside the DAG or to older Exchange Server versions.

In general, copies can only be created on mailbox servers that do not host the active database copy. The mailbox servers hosting the backup copies must be accessible over the network with round-trip latency less than 500 milliseconds. Distant mailbox servers, or servers subjected to network bottlenecks, may not be appropriate candidates for backup database copies.

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