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When using VSS, write operations on the data being backed up can continue while these open files are copied and backed up, eliminating locked file conditions.
VSS can be enabled for backups of local volumes in both clustered and non-clustered environments. If the operating system fails to create a shadow copy of the data a traditional backup of the data will be performed, and a corresponding message will appear in the Event Viewer.
During a VSS backup, the free space needed on a volume for the shadow copy is related to the duration of the backup and the amount changes being made to the data objects (files, databases) that are being backed up. VSS snapshots capture and copy disk blocks that are being changed by other processes while the backup for a particular object is running. If the backup runs longer, or there is a lot of disk activity (files or databases being changed), this would cause VSS to copy more data blocks and therefore, use more disk space.
Microsoft Volume Shadow Service is the default, and only supported VSS software provider. The VSS software provider takes a snapshot of the source volume, database or file, which is then used to create backups or QR Volumes (when used with the Quick Recovery Agent).
VSS can be enabled for backups of local NTFS volumes. If selected, the following will automatically occur:
Before selecting the Use VSS option with Exchange, you should review the VSS-related items in Advanced - Exchange Database iDataAgent Configuration.
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Using VSS enabled backups to secure Exchange data offers the following advantages:
Out-of-Place restores from VSS-enabled backups for the Exchange Database iDataAgent for Exchange Server 2007 allow the database stores to be restored to a different storage group (including Recovery Storage Groups) on the same Exchange Server, or to a different Exchange Server within the same Exchange organization, or to a physically different location on disk (i.e., non-Exchange location) with a choice of whether or not to replay the logs. For step-by-step instructions, see Restore Out-of-Place or Cross-Application.
The Consistency Check Throttling feature allows users to define pause points in the consistency check process for Volume Shadow Copy Service (VSS) enabled backups.
Pause points allow Exchange system resources to be made available periodically to other processes during prolonged consistency checks of large databases or when there is a high load on the Exchange Server. For example, establishing pause points in the consistency check process allows users to send and receive e-mail messages without a significant delay, by avoiding a bottleneck of I/O transactions from building up in the queue.
VSS requires the data to be in a consistent state to ensure application integrity before considering a VSS backup to be valid. However, consistency checks can degrade I/O transaction processing performance. To address this performance issue, while ensuring integrity of the backup data, this Agent for Exchange Server 2003 (with Service Pack 2) and Exchange Server 2007, provide the throttling option to specify the maximum throughput (in Megabytes) as a trigger to periodically pause the consistency check in order for other I/O transactions to occur. The following formula is used to calculate the throughput for the purpose of establishing the pause points:
Maximum throughput rate = 512KB per I/O * number of I/Os between 1-second pauses
Once the consistency check throttling has been configured, and a pause point is reached after the specified number of Megabytes, the consistency check process sleeps for one second to allow other processes use of Exchange system resources before continuing on. Exchange Server 2003 (with Service Pack 2) uses ESEUTIL to verify the integrity of the database, whereas Exchange Server 2007 uses the CHKSGFILES API to perform this function.
For step-by-step instructions, see Configure Consistency Check Throttling.
The following must be done before using VSS.