Applications Server

Exchange Server 2010 Mailbox Services Configuration (part 2) - Database Maintenance

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10/17/2010 5:15:04 PM

3. Database Maintenance

Database maintenance is divided into two parts: store mailbox maintenance and ESE database maintenance. Store maintenance includes performing cleanup within the database; ESE database maintenance includes online database scanning (database checksum), defragmentation, and compaction.

3.1. Database Cleanup

Database cleanup involves identifying newly disconnected mailboxes and evaluating the deleted item retention period on deleted messages and mailboxes and purging any items that have expired. Database cleanup is now the only task that is, by default, configured to run during the online maintenance window. Although online maintenance no longer consumes as many resources as it did previously, it is recommended to schedule the maintenance window after the backup window. This will allow backups to capture data before they are purged from the database. Configuring the online maintenance schedule is shown in Figure 1.

Figure 1. Configuring the database online maintenance schedule

3.2. Online Database Scanning

The integrity of data within the database is of the utmost importance—if there is a physical page corruption, it is important to know about it so that something can be done. Often this type of corruption is due to a problem with the underlying storage. In Exchange Server 2007 RTM and earlier versions, each page was scanned during backups using the streaming API. With the deprecation of the streaming backup API in Exchange Server 2007 and the removal of it in Exchange Server 2010, another facility to ensure data integrity was needed. In Exchange Server 2007 SP1 an option was introduced to run a database checksum during the scheduled online maintenance window.

In Exchange Server 2010, the default setting allows the online database scanning to run in the background continuously. Alternatively, the online database scan process can be set to run only during the schedule online maintenance window; however, because of the time it takes to complete, this is only recommended for databases smaller than 1 terabyte (TB). The best practice is to leave the default setting and allow the scanning to run continuously.

3.3. Defragmentation and Compaction

Defragmentation and compaction happen at run time and are balanced to provide data contiguity rather than to optimize space. Table 1 summarizes each type of online maintenance activity and how they have been improved in Exchange Server 2010.

Table 1. Comparing Online Database Maintenance Operations
CleanupPerformed during online defragmentation, which occurs during online maintenanceESE performs cleanup at run time when a store hard delete occurs. This happens during dumpster cleanup during the online maintenance.
Database checksumWhen configured, half of OLD maintenance window reserved for sequential scan (Checksum), manual throttle. Active DB copy only.Two options (both active and passive copies):
  1. Run DB Checksum in the background 24 x 7 (default).

  2. Run DB Checksum during online maintenance.

Maintain contiguityN/A (compaction activities during online defragmentation prevent data contiguity).Database is analyzed at run time and is defragmented in the background.
Space compactionDatabase is compacted and whitespace is reclaimed during online defragmentation.Database is compacted and space reclaimed at run time.

3.4. Offline Maintenance

One of the most frequently asked questions continues to be, "How often should you do an offline defragmentation of a database?" The trigger of this question in previous versions of Exchange Server was often Event ID 1221, which detailed the amount of free space available within each database after a full online database defragmentation pass was completed. Many messaging administrators would see this free space as an indicator of inefficiency and want to reduce the size of the database to improve performance and reduce the backup size. Because defragmentation and compaction are now continuous processes in Exchange Server 2010, there is no longer a point in the process when the free space within the database base is published to the event log. There is a way, however, to get this information using the Exchange Management Shell. For example, the cmdlet to find the free space in Dallas-EX01-MB04 would look like this:

Get-MailboxDatabase Dallas-EX01-MB01 -Status | Format-List AvailableNewMailboxSpace

Note: When using the Get-MailboxDatabase cmdlet, the –Status switch must be specified to be able to view the available space, whether online maintenance is running, whether the database is mounted, or whether a backup is in process. If theStatus switch is not specified, the information returned by the cmdlet is retrieved from Active Directory Domain Services. When the –Status switch is specified, the additional information is retrieved directly from the Mailbox server.

As with previous versions of Exchange, even with a significant amount of free space available in the database, more efficient ways are available for reclaiming this space. Using database tools such as ESEUtil.exe requires that the database be taken offline and that enough space is available on disk to complete the maintenance. Although the performance of ESEUtil has improved, it still requires extended downtime for all mailboxes hosted within that database. A much more reasonable approach would be to create a new blank database and move all of the mailboxes from the bloated databases into the newly created database. For example, the command to move all mailboxes from Dallas-EX01-MB02 into Dallas-EX01-MB22 would look like this:

Get-Mailbox -Database Dallas-EX01-MB02 | New-MoveRequest -Local -TargetDatabase Dallas-

If a physical corruption is found in the database during the online database scanning (database checksum), this same process can be used to move all mailboxes to a newly created database and delete the problematic database. This eliminates the need to take the database offline to repair it, which caused extended downtime for the affected mailboxes.
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