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 un predicted situations !!!

My intention through this blog post is to bring your attention to the Microsoft’s works towards the enhancement on Lagged Database Copies in Exchange 2013 when compared to earlier verions

 

Lagged copy enhancements include integration with Safety Net (Similar feature like Transport Dumpster in Exchange 2010) Activating a lagged database copy becomes significantly easier as it uses SafetyNet.

 

For example consider a lagged copy that has a 2-day replay lag. In that case, you would configure Safety Net for a period of 2 days. If you encounter a situation in which you need to use your lagged copy, simply do:

• Mount the lagged copy: This will trigger an automatic request to SafetyNet to redeliver the last two days of mail.

• You get the last two days mail, minus the data ordinarily lost on a lossy failover

 

Lagged copies can now care for themselves by invoking automatic log replay to play down the log files in certain scenarios:

• When a low disk space threshold is reached

• When the lagged copy has physical corruption and needs to be page patched

• When there are fewer than three available healthy copies (active or passive) for more than 24 hours

• Lagged copy play down behavior is disabled by default, and can be enabled by running the following command.

Set-DatabaseAvailabilityGroup <DAGName> -ReplayLagManagerEnabled $true

• After being enabled, play down occurs when there are fewer than 3 copies. You can change the default value of 3, by modifying the following registry value :

HKLM\Software\Microsoft\ExchangeServer\v15\Replay\Parameters\ReplayLagManagerNumAvailableCopies

• To enable play down for low disk space thresholds:

HKLM\Software\Microsoft\ExchangeServer\v15\Replay\Parameters\ReplayLagPlayDownPercentDiskFreeSpace

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