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Can I thwart Exchange mailbox recovery by generating logs?

I have limited drive space on my Exchange Server but need to restore large mailboxes. Can I prevent mailbox restores from a recovery database by circular logging?

Exchange mailbox recovery adds new items to the information store where the recovered users' mailboxes are homed. This generates logs that take up space equal to the amount of mail that you restore.

In a database availability group (DAG) configuration, the logs replicate to all servers with a copy of its mailbox's database, therefore consuming space here as well.

Try to preserve the logs. With limited space, perform a backup after moving the mailboxes to truncate the logs. This means that if the mailboxes are large, each Exchange mailbox must be recovered one at a time. If your recovery database is on the same drive as your production database or logs, find an alternate storage location for it.

If the Exchange mailbox recovery is time sensitive, consider enabling circular logging on the target database(s). You need to know which databases users are on. Enable circular logging on each database to which you will restore data.

The Set-MailboxDatabase DBName -CircularLoggingEnabled $True cmdlet enables circular logging. Dismount and then remount the database(s) for the change to take effect for standalone databases (JET circular logging). If you have multiple database copies in a DAG, then this will enable the continuous replication circular logging. In a DAG, you do not need to dismount and mount the database -- the change takes place dynamically.

Circular logging does not disable transaction logging, but it will prevent the logs from growing until they overwrite older log files and fill up your hard drive. This is also common when migrating a large number of mailboxes to new servers and/or databases.

Long term, look at ways of regaining or adding free space to the drives. For optimal file system performance, production drives should not exceed 80% capacity. When extending storage, consider including additional space to recover your largest mailbox database.

The downside is that all data committed to the database, not just the recovered Exchange mailboxes, uses circular logging. This setting creates risk for your mailboxes. You will not be able to replay your logs in the event of a system failure. Circular logging may also disrupt backup procedures if you use incremental or differential backups that depend on transaction logs. If not carefully addressed, it could render your ongoing backups unusable for recovery. Disable circular logging if you no longer need it -- after the restore is complete.

About the author: 
Richard Luckett is a consultant and instructor specializing in messaging and unified communications. He's been a certified professional with Microsoft since 1996 and has 20 years of experience in the public and private sectors. Luckett is a Microsoft Certified Trainer with more than 15 years of training experience with the Microsoft product line and received the Exchange MVP award in 2006, 2007 and 2008. He's also an expert in deploying and integrating Exchange Server and Lync Server. He leads the Microsoft training and consulting practice at LITSG.

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