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Exchange Server uses a transactional database that generates logs equal in size to data committed to the Exchange Information store. When mailboxes are moved, regardless of the source, the target database will be where the database logs are created. If the total mailboxes' size is greater than the available free space where the logs will be created, careful disk space planning is essential. Unfortunately, many Exchange administrators only figure this out when a large batch of mailbox moves fails when the log file drive fills up.
Suppose you want to move 1 TB of mailbox data from Exchange 2007 to Exchange 2013, for example. If the target Exchange 2013 server is configured with four Mailbox Databases where all Databases are stored on the E: drive that is 1.5 TB in size and there's also an L: drive configured for all of the database logs at 500 GB, there are three options to prevent the control log drive size.
Option 1. One of the most common methods is to configure circular logging on the target databases. This will allow the Information Store service to automatically overwrite database logs on the file system. This isn't a desirable option in the long term as circular logging can't be used to recover data in the event that something goes wrong with the target database during the move. If admins choose this option, be prepared to disable circular logging and take a full backup of the target databases immediately after the move is complete.
Option 2. Move no more than 400 GB (80% of the available free space) worth of mailboxes at a time. Then perform a backup of the target database after each batch. Choose a backup option (full or incremental) that will truncate the database logs on the target server and repeat this process until all mailboxes are moved.
The downside to this option is that this could take longer to complete. But the upside is that the logs can be used in a disaster recovery situation where something goes wrong with the target database.
Option 3. Increase drive space or use a temporary drive for the database logs on the target server during the move.
While there may be variations of the above options, I cannot emphasize enough the importance of an active backup strategy on the target servers. Once mailboxes are moved, backups that exist from the source server can't be used to recover the data to the new database. This is especially true if you're moving mailboxes from one version of Exchange to another version of Exchange. Restores will fail if the database being restored isn't the same version that was backed up. But don't discard old backups -- they can still be used to recover data, but the process of doing so is a bit more complex.
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. He's 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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