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Implementing a Backup Plan

It is important to perform backups to protect your organization's information and messaging system. Your backup plan should include:

Understanding Transaction Log Files

Windows NT Backup backs up the Microsoft Exchange Server directory and information stores differently from normal file system backups. To understand how directory and information store backups work, it is important to understand how transaction log files work.

Microsoft Exchange Server uses write-ahead transaction log files to improve server data write performance and to provide fault tolerance for the directory and information stores. Data for the directory or information store is written synchronously at high speed to a sequential transaction log file and to a memory cache simultaneously. The data in the cache is later written to the directory database or the information store database files as necessary. When a log file has 5 megabytes (MB) of transactions, a new log file is generated. Log files are named with hexadecimal serial numbers.

Note   The size of log files is always 5 MB, so you can't determine whether a log file is filled to capacity by checking the file size.

If a power outage or abnormal system shutdown occurs before the data in the cache is written to the database, Microsoft Exchange Server reconstructs the database upon restarting by reading from the transaction log files. If a directory or information store database becomes corrupted, the transaction log files remaining on the hard disk can be used to reconstruct the database from a backup if an uninterrupted sequence of transaction log files exist from the time of the backup.

Normally, transaction log files accumulate sequentially until a full or incremental backup is performed, which deletes log files that have all transactions committed to the database. Differential backups back up the transaction log files but leave the log files on the hard disk.

Using Circular Logging

Circular logging recycles transaction log files by overwriting logs that have been committed to the database with new transactions. This prevents the continuous buildup of transaction log files and reduces the disk space required to store them (typically less than 100 MB). By default, circular logging is turned on in Microsoft Exchange Server. However, you can use the Advanced property page on the server object to turn off circular logging for the directory and information store.

When developing your backup plan, you should consider the implications of using circular logging. Because circular logging overwrites transaction log files, it reduces your ability to recover data. In most cases, you should turn off circular logging to increase your backup options and your ability to recover data.

With circular logging turned on, you can restore information only up to the last full backup¾not to the last transaction. When circular logging is turned on, you can only perform full backups, not incremental or differential backups. This is because Windows NT Backup relies on having complete transaction log files to perform incremental and differential backups of the directory and information store databases.

You may want to keep circular logging turned on for some configurations. For example, you could keep circular logging turned on because your server computer has limited hard disk space or because the server is being used as a news server that contains only noncritical data.

For more information about circular logging, see Microsoft Exchange Server Getting Started.

Choosing Backup Devices

You should choose backup devices and reliable backup media that best meet the needs of your organization. In addition, you should choose reliable backup media that best meets your needs. For example, you should choose a tape drive that is large enough and fast enough to efficiently support the amount of data you need to back up. Most storage device manufacturers publish recommended media brands that have been tested and certified on their storage devices. Media reliability reports are also available from many computer trade publishers.

Choosing a Backup Routine

Backup routines include full (normal), incremental, and differential. Most organizations use a combination of these routines.

A full (normal) backup backs up the entire directory or information store. It also backs up the transaction log files and deletes those transaction log files that have all transactions committed to the database. Restoring from a full backup requires only the full backup.

An incremental backup backs up data in the directory or information stores that has changed since the last full or incremental backup. It also backs up the transaction log files and then deletes those that have all transactions committed to the database. Restoring from an incremental backup requires the last full backup and each incremental backup that has taken place since then.

A differential backup backs up data in the directory or information stores that has changed since the last full backup. It also backs up the transaction log files, but does not delete them. Restoring from a differential backup requires the last full backup and the differential backup.

Backup Type

Easy to schedule.

Easy to restore data.

Removes transaction log files.

Allows circular logging.

Can affect server performance.

Consumes more time.

Requires the most tape space and frequent tape replacement.

Incremental Minimal effect on server performance.

Removes transaction log files.

Requires minimal tape space.

Requires a more complex restore process.

Circular logging must be turned off.

Differential Minimal effect on server performance.

Easy to restore data.

Requires minimal tape space.

Does not remove transaction log files.

Circular logging must be turned off.

Requires more tape space than an incremental backup but less than a full backup.

Determining Factors   You should consider the following factors when choosing a backup routine:

Full daily backups may be feasible if you're backing up a small amount of data, but not if you're backing up a large amount of data.

Backup Rotations   Most organizations rotate full backups with differential or incremental backups. A daily rotation requires full backups every day. A weekly rotation requires one full backup followed by either differential or incremental backups on the remaining days of the week. A three-day backup rotation requires a full backup followed by two days of either differential or incremental backups.

Note   Mixing differential and incremental backups in the same weekly backup set is not recommended because the rotation and restore process becomes too complicated.

The following chart illustrates several backup rotations.

Sample Backup Rotations
F = Full, I = Incremental, D = Differential
Rotation Sunday Monday Tuesday Wednesday Thursday Friday Saturday
Daily F F F F F F F
Weekly F I or D I or D I or D I or D I or D I or D
3-day F I or D I or D F I or D I or D Repeat cycle

Performing Timely Backups

Timely backups include:

Verifying and Validating Backups

Your ability to recover servers and restore data depends on the quality of your backups. Therefore, you should always use the Verify feature of the Backup program when making backups. Also, periodically perform restores from backups to nonproduction servers to ensure that the backup process is working. Routinely review the daily backup logs to ensure that backups have been completed as scheduled. Follow up on any errors or inconsistencies in the backup logs.

Documenting and Archiving Backups

Before you can restore data, you need to know the backup strategy, including the backup types and the rotation. It is important to document the backup strategy and provide guidelines on how backups can be used to restore data. Label backups carefully and store them in a safe and secure location. For maximum reliability, archive full backups to a different location, preferably offsite.

Tip   You can use the Windows NT Backup command Copy option to create full backups without disturbing the state of ongoing incremental or differential backups. You should consider creating periodic copy backups and adding them to your archives to provide more protection. For example, you could create and archive duplicate copy backups once a month and after changing server configurations significantly.