This alert indicates that a replication issue may affect the mailbox database copies in a particular Exchange Server 2010 database availability group (DAG).

Microsoft Exchange Server 2010 uses continuous replication to create and maintain database copies. To maintain a synchronized copy of a mailbox database, transaction log files from the active mailbox server are replayed into the passive database of another server in the DAG. This provides high availability and resiliency in the Exchange environment.

The information store is responsible for replaying the log file for the passive copy of the database that is hosted on a server. When a replica instance of a copied database is started, the Replay service contacts the information store and creates an ESE instance for the database to be mounted. This mount is for log replay only. The Replay service Log Replayer component then contacts the store over the RPC STORE ADMIN Interface (ExRpcAdmin.LogReplayRequest) to pass the log files that have to be replayed by the store.

If the volume that hosts the passive database copy runs out of disk space, transaction logs can no longer be played to the database. In this scenario, the passive database copy status changes to Failed, and the database copy will no longer be synchronized with the active mailbox database.

A warning (Yellow) alert is generated when the Windows Reliability and Performance Monitor LogicalDiskSpace\% Free Space object has a value that is less than 50 percent. When the object's value is less than 35 percent, a severe problem (Red) alert is generated.

To resolve this error, do one or more of the following:

• Review the Application log and System log on your Exchange 2010 servers for related events. For example, events that occur immediately before and after this event may provide more information about the root cause of this error.
  
  
• Review Operations Manager for detailed information about the cause of this problem. For more information, see the "Introduction" section in this article.
  
  
• Check the following performance counter by using Windows Reliability and Performance Monitor:
  
  
  • Object: LogicalDiskSpace\% Free Space
    
    
• Examine the network topology between any connected storage that may be used for the log file location to determine whether the Log Replayer can write the changes to the passive database quickly enough.
  
  
• Examine the System log to determine whether any issues are affecting the disk subsystem.
  
  
• Examine the CPU usage of the Store.exe process to determine whether an issue is affecting the information store on the passive Mailbox server.
  
  
• Move the transaction log storage location to a faster disk subsystem. RAID arrays are not required for use together with Exchange 2010.
  
  
• Move the transaction log storage location to a larger volume.
  
  
• Examine the memory and CPU usage on the passive mailbox server to determine whether a memory bottleneck is causing additional disk usage.
  
  
• Determine whether the LogTruncationExtendedPreservation registry entry has been set to a high value. This registry entry forces the log truncator to preserve additional transaction log files. The value must be a positive integer. It specifies the number of logs that will be retained, even though the logs are not required. The default value is 10. The registry entry is located in the following registry subkey:
  
  HKLM\SOFTWARE\Microsoft\ExchangeServer\v14\Replay\Parameters
  
  
• Examine the current replication status for each replica database. To do this, use the Get-MailboxDatabaseCopyStatus cmdlet. This cmdlet returns information about all copies of a particular database, information about a specific copy of a database on a specific server, or information about all database copies on a server.
  
  

For more information, see the following topics in Exchange 2010 Help:

• Understanding Mailbox Database Copies
  
  
• Understanding Database Availability Groups
  
  

If you are not already doing so, consider running the Exchange tools created to help you analyze and troubleshoot your Exchange environment. These tools can help make sure that your configuration aligns with Microsoft best practices. They can also help you identify and resolve performance issues, improve mail flow, and better manage disaster recovery scenarios. To run these tools, go to the Toolbox node of the Exchange Management Console. To learn more about these tools, see Managing Tools in the Toolbox.