Explanation/Reference:
Explanation:
File Share Witness
The file share witness is used to establish a majority node set. This is done by create a share on a server that gets a little file place into it automatically.
The server hosting the cluster resource (which in the DAG I think is the Primary Activation Manager server) keeps an open file lock on this file.
The other servers see this open file lock and interpret this as meaning another cluster node is online, healthy, and available.
A file share witness is used when the DAG contains an even number of servers within it.
When you initially create the DAG you must specify the server and file location that will act as the file share witness regardless of how many servers are in the DAG (0 to start) to ensure that if you do add an even number of DAG members the FSW will be properly used.
Database Availability Group
A database availability group (DAG) is a set of up to 16 Microsoft Exchange Server 2013 Mailbox servers that provide automatic database-level recovery from a database, server, or network failure. 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, server, and network failures.
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, and a directory object is created in Active Directory that represents the DAG. The directory object is used to store relevant information about the DAG, such as server membership information. 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.
Witness server and witness directory The witness server is a server outside the DAG that acts as a quorum voter when the DAG contains an even number of members. The witness directory is a directory created and shared on the witness server for use by the system in maintaining a quorum.
Lagged copy of a mailbox database
A Lagged Mailbox Database Copy is a mailbox database copy configured with a replay lag time value greater than 0.
A lagged database copy is one that is not updated by replaying transactions as they become available.
Instead, the transaction logs are kept for a certain period and are then replayed.
The lagged database copy is therefore maintained at a certain remove to the active database and the other non-lagged database copies. If you are planning to have more than two passive database copies of a database, think about a lagged copy also as an additional protection against unpredicted situations Lagged copies aren't considered highly available copies. Instead, they are designed for disaster recovery purposes, to protect against store logical corruption.
The greater the replay lag time set, the longer the database recovery process. Depending on the number of log files that need to replayed during recovery, and the speed at which your hardware can replay them, it may take several hours or more to recover a database.
References:
http://jaworskiblog.com/2011/05/17/exchange-2010-design-principles-for-high-availability-and-site- resiliency/