Microsoft Volume Shadow Copy Service (VSS) is built into the Microsoft Windows XP Professional, Windows Server 2003, and Windows Server 2008 operating systems. VSS works with Arcserve Backup, the Agent for Open Files, the Enterprise Option for VSS Hardware Snap-Shot, and VSS-aware applications and services to create shadow copies of single volumes or multiple volumes.
Note: Arcserve Backup does not support Writers under Windows XP. This is because some of the necessary Writer support in Windows Server 2003 is not included in the Windows XP operating system.
A shadow copy is a read-only, frozen view of a volume's file system as it exists when the copy is taken, and can reside on a volume separate from the one that was copied. The shadow copy volume can exist on the same server as the original data but in a different share or mount point, or on another network-attached volume.
The shadow copy of the volume is created at a single point in time and is synchronized across the whole volume set. To avoid inconsistencies, shadow copies cannot be taken of individual files.
Regardless of where the shadow copy volume is created, Arcserve Backup uses the shadow copy as the source for the backup. Shadow copy volumes can also be backed up (migrated) to another disk or tape device.
Using the shadow copy method to perform backups has two distinct advantages over other traditional backup methods:
Files that are open when a shadow copy is created appear closed on the shadow volume. When the shadow copy data is backed up, these open files are included.
Since the data backed up by Arcserve Backup exists on the shadow volume, work can continue on the original volume without affecting the integrity of the backup data.
In traditional backup operations, open files are skipped and do not appear in the backup. To get a better quality backup, administrators often implemented backup windows when the applications being backed up are unavailable to users. Unavailable applications prevented work from continuing, forcing users to wait until after the backup completed to finish their tasks.
If applications included in a VSS backup are not enabled for VSS, all of the application’s data (including open files) is written to the shadow copy. However, because the application cannot prepare its files before the backup begins, the data may not be consistent and any transactions may be incomplete.
A shadow copy of a volume, even in a crash-consistent state, still contains all files. A backup set created without a shadow copy does not contain the files that were open at the time of the backup. These open files are excluded from the backup.
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