Exchange 2007 / 2010: Windows Server Backup and Backup Completion Times
Administrators are choosing Windows Server Backup to backup their Exchange 2007 and Exchange 2010 installations. In some cases administrators are reporting that backups are not completing in a timely fashion.
Remember that a Windows Server Backup of an Exchange solution is basically a 4 step process:
1) Volume Shadow copy is made of the Exchange data on the host where the backup is initiated.
2) All Exchange log files and database files in the backup set must have a consistency check performed.
3) The data on the drives is written to the desired backup media.
4) Backup complete is performed allowing for log truncation activities (if applicable).
The first step of this process can always be expected to complete fairly quickly. A matter of fact this step must complete within 30 seconds of being initiated. If the process cannot be completed within 30 seconds the volume shadow copy framework will abort the backup operation.
The second steps is the consistency check of Exchange data. This ensures that the log files and databases in the backup set are free from page level corruption prior to transfer to the backup destination. This process is not optional and must complete. It is important for an administrator to realize that the consistency check is performed against every log file that existed on the disk at the time the backup is taken. For example, if a recent move mailbox operation was performed that resulted in 10,000 logs being generated and included with the backup all 10,000 logs must have a consistency check performed against them. Therefore the number of logs generated due to operations or on a daily basis effect the time it takes for the backup to complete since they directly impact the amount of time it takes for consistency check to complete. Also each database within the backup must be consistency checked. Larger databases will obviously take longer to have a consistency check performed then smaller databases. Another contributing factor to the time it takes consistency check to be completed is the disk performance of where both the shadow copy is created and the production data hosted.
The third step is the transfer of data to the desired backup media. Administrators can increase the performance of this step by utilizing backup media that is locally mounted on the server. For example, an iscsi drive or locally attached disk may be a good performing candidate as the storage location for backup data. Backup to network location, although supported, can be slower than utilizing locally attached disks. In Windows 2008 R2 administrators also have an additional backup location “Back up to a hard disk that is dedicated for backups (recommended). By using this option rather then a lettered volume the administrator may realize faster backup times since the disk is dedicated solely to backups and may not have additional data stored on it.
With an understanding of these factors and administrator should be able to understand the time to backup completion and the factors that may cause backups to run longer then expected.