Page zeroing (also called zeroing out or page scrubbing) is a process that is performed at the end of a streaming backup in which the data within the database is overwritten with characters that you have selected for that purpose. This makes the data unrecoverable by conventional means. When an item is deleted from an Exchange server (for example, when users delete messages from their mailboxes) and deleted item retention is disabled, the pages that item was occupying are marked as unused. When page zeroing is enabled, the data that is contained in unused pages is overwritten with the selected overwrite during an online backup. As each database page is backed up, the page is overwritten with the selected characters one time in the database on the hard disk. After the backup is complete, the deleted data exists on the backup copy, but it no longer exists in the database and cannot be recovered by conventional means.
In Exchange 2007 RTM, you could zero out deleted database pages when streaming online backups were performed by setting the Zero Database During Backup registry key. This method worked well. However, with VSS backups and CCR, it is no longer sufficient because VSS backups do not provide a way to zero out deleted pages.
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As a best practice, if you want to enable page zeroing on a database, you should do so when you create the database. If you do not configure page zeroing when you create the database, the first time page zeroing is run against the database, it will significantly impact server performance. The performance impact is considerably less after page zeroing has completed the first pass of the database. You can use throttling to limit the performance impact of the first page zeroing pass
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Page Zeroing with Continuous Replication
In Exchange Server 2003 and Exchange 2007, you have been able to use streaming backups to augment VSS backups when they required page zeroing. With the introduction of CCR and LCR in Exchange 2007, another issue is raised: page zeroing modifies the database without generating corresponding transaction logs. This means that in CCR and LCR organizations, page zeroing activity is not replicated between databases. In Exchange 2007 RTM, to make sure that page zeroing is effective with CCR, you must perform one of the following tasks:
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Run a streaming backup against each copy. This involves moving the CCR clustered CCR Mailbox server between nodes.
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Take the target database offline and use Eseutil to run the eseutil /z command.
However, with increased concerns about security and compliance, these options are no longer satisfactory. In Exchange 2007 SP1, page zeroing is moved to a background process and it now generates logs that can be shipped to replicate page zeroing to database copies in CCR and LCR environments, as well as environments that use standby continuous replication (SCR).
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Enabling page zeroing during online maintenance temporarily results in an increase in log generation. After the feature has been enabled for a while, log generation activity should return to the level it was at prior to enabling page zeroing.
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