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Database and log files stored on same disk volume

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at http://go.microsoft.com/fwlink/?linkid=34707.]  

Topic Last Modified: 2007-01-05

The Microsoft® Exchange Server Analyzer Tool queries the Active Directory® directory service to determine whether the database files for this mailbox store reside on the same physical disk as the transaction log files for this storage group. Specifically, the Exchange Server Analyzer queries Active Directory to determine the following:

  • Whether the mailbox store has more than 100 mailboxes, determined by counting the values of the homeMDBBL attribute of the mailbox store object (class msExchPrivateMDB). If the mailbox store has fewer than 100 mailboxes, the performance impacts of the file location will be limited.

  • The path of the database files for the mailbox store, determined by reading the value of the msExchEDBFile of the mailbox store object.

  • The path of the transaction log files for the storage group, determined by reading the value of the msExchESEParamLogFilePath of the storage group object (class msExchStorageGroup).

  • Whether that path in msExchESEParamLogFilePath is on the same disk as the path in msExchEDBFile.

Additionally, the Exchange Server Analyzer determines the following:

  • Whether the Exchange Server extensions for Windows-powered NAS are running on the server, determined by querying the Win32_Service Microsoft Windows® Management Instrumentation (WMI) class for the value of the Started attribute for the Exchange Server NAS extensions (wssexchmap). If the Exchange Server extensions for Windows-powered NAS are running, it means that the files under consideration may not be on a local drive and performance considerations may not apply.

  • Whether the files are on a disk volume that has a name longer than three characters, determined by querying the Win32_Volume WMI class. Again, this may mean that the files may not be on a local drive and performance considerations may not apply.

To provide fault tolerance in case a hard disk fails, keep your Exchange transaction log files and database files on separate physical hard disks. When you keep these log files and database files on separate hard disks, you can significantly increase hard disk I/O performance.

noteNote:
  To provide fault tolerance in case a database must be restored, each storage group has its own set of transaction log files. Transaction log files maintain a record of every change to every database in that storage group. Transaction log files are not deleted until a full backup is made of all the databases in a storage group.

The following examples illustrate how to recover data even in extreme circumstances (such as the loss of a hard disk that contains either your database files or your transaction log files) if you have a backup and your transaction log files are on a separate disk.

  • If the hard disk that contains your database files fails, you can replace the failed hard disk, restore the most recent backup of the database files, and then use the current log files that were stored on a separate hard disk to restore the data on your Exchange server and bring it up to date to the time at which the hard disk failed.

  • If the hard disk that contains your transaction logs fails, but the hard disk that contains your database files is functional, and the database store file is gracefully closed, you may be able to repair the failed transaction log disk and remount the database store. If the database store file does not close gracefully, it will remain in an inconsistent state and may only mount after a hard-repair using the eseutil.exe, which may result in some lost data. The other option is to restore the database store file from backup, which will not include new mailbox data created after the time of the backup.

importantImportant:
If you keep your Exchange databases and transaction log files on the same physical hard disk, and that hard disk fails, you lose the data that was created after your last backup.

Additionally, putting database files and transaction log files on separate physical disks increases overall performance. Exchange writes each transaction to the transaction log file first, then to the database file. Additionally, the transaction log files are shared by all the mailbox stores in the storage group. When these files are on the same physical disk, the drive head must move back and forth between the location of the current transaction log file and the locations of the database files to complete the write operations. When the files are on separate disks, one drive head stays in position to write to the transaction log file, so transactions can be completed more quickly than when the files are on a single disk. The difference in performance for a single transaction is small, but it can become noticeable when the system load is high.

To correct your configuration, you can either move the transaction log files or the database files.

  1. In Exchange System Manager, expand the appropriate Exchange server object, right-click the storage group you want, and then click Properties.

  2. On the General tab, specify a new location for the files.

    For example, if the E:\ drive will contain only log files for this storage group, in Transaction log location, click Browse, and then choose the E:\ drive.

  1. Follow the guidance in the core Exchange Server 2007 documentation, "How to Set or Change the Location of Storage Group Log Files" (http://go.microsoft.com/fwlink/?LinkId=80702).

  1. In Exchange System Manager, expand the Exchange server object on which the store resides, expand the appropriate storage group container, right-click the mailbox store object you want, and then click Properties.

  2. Specify new file locations on the Database tab of the Properties dialog box.

CautionCaution:
To move a database file, the database must be temporarily dismounted and then remounted. This is done automatically by the wizard, but the database is inaccessible to all users during the process.

  1. Start the Exchange Management Console on the server on which the storage group is located.

  2. In the console tree, expand Server Configuration, and then click Mailbox.

  3. In the results pane, expand the appropriate storage group container, right-click the mailbox database object you want, and then click Move Database Path. The Move Database Path wizard appears.

  4. On the Introduction page, the Database files path box displays the location where your database files are currently stored. Click Browse to specify a location to which to move these files.

  5. Click Move.

  6. On the Completion page, confirm whether the database file path was changed successfully. A status of Completed indicates that the wizard completed the task successfully. A status of Failed indicates that the task was not completed. If the task fails, review the summary for an explanation, and then click Back to make any configuration changes. Click Finish to complete the Move Database Path wizard.

noteNote:
You can also use the Exchange Management Shell to move database files in Exchange Server 2007. For more information, see Move-DatabasePath.

For more information about Exchange Server database files, see the following topics:

For more information about managing Exchange Server database files, see the Microsoft Knowledge Base article 328794, "How to Protect Exchange Data from Hard Disk Failure" (http://go.microsoft.com/fwlink/?LinkId=3052&kbid=328794)

For more information about hard repairing Exchange database stores, see the Knowledge Base article 296788, "Offline Backup and Restoration Procedures for Exchange" (http://go.microsoft.com/fwlink/?LinkId=3052&kbid=296788).

For more information about performing a hard repair on an Exchange database store file, see the Knowledge Base article 810190, "XADM: 'the information store terminated abnormally' error message and event ID 447 is logged" (http://go.microsoft.com/fwlink/?LinkId=3052&kbid=810190).

 
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