How does modern disk image software make accurate images of drives in use?

Since Windows XP there is the Volume Shadow Copy Service which can show a "point in time" consistent backup capable view of a system or disk that is in use without it needing to be unmounted first. It appears to have been variously enhanced and and used by other applications as a backup service.

Backing up and restoring critical business data can be very complex due to the following issues:

• The data usually needs to be backed up while the applications that produce the data are still running. This means that some of the data files might be open or they might be in an inconsistent state.
• If the data set is large, it can be difficult to back up all of it at one time.

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VSS coordinates the actions that are required to create a consistent shadow copy (also known as a snapshot or a point-in-time copy) of the data that is to be backed up.

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System provider

One shadow copy provider, the system provider, is supplied in the Windows operating system. Although a default provider is supplied in Windows, other vendors are free to supply implementations that are optimized for their storage hardware and software applications.

To maintain the "point-in-time" view of a volume that is contained in a shadow copy, the system provider uses a copy-on-write technique. Copies of the blocks on volume that have been modified since the beginning of the shadow copy creation are stored in a shadow copy storage area.

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What is the difference between a shadow copy and a backup?

In the case of a hard disk drive backup, the shadow copy created is also the backup. Data can be copied off the shadow copy for a restore or the shadow copy can be used for a fast recovery scenario—for example, LUN resynchronization or LUN swapping.

When data is copied from the shadow copy to tape or other removable media, the content that is stored on the media constitutes the backup. The shadow copy itself can be deleted after the data is copied from it.

When was Volume Shadow Copy Service introduced? On which Windows operating system versions is it available?

VSS was introduced in Windows XP. It is available on Windows XP, Windows Server 2003, Windows Vista®, Windows Server 2008, Windows 7, and Windows Server 2008 R2.