HP B6960-96035 Concepts Manual page 290

VSS
Volume Shadow Copy service (VSS) is a software service introduced by Microsoft on
Windows operating systems. This service collaborates with the backup application,
applications to be backed up, shadow copy providers, and the operating system
kernel to implement the management of volume shadow copies and shadow copy
sets.
The idea of the Volume Shadow Copy service is to provide a unified communication
interface that can coordinate backup and restore of any application regardless of
their specific features. With this approach, a backup application does not need to
handle each application to be backed up specifically. However, this approach is
applicable to a backup application only in case it conforms to the VSS specification.
What is a shadow copy?
A shadow copy refers to a volume that represents a duplicate of the original volume
at a particular moment in time. The volume shadow copy technology provides a copy
of the original volume at a certain point in time. The data is then backed up from the
shadow copy, not from the original volume. The original volume continues to change
as the backup process continues, but the shadow copy of the volume remains constant.
Shadow copy is basically a snapshot backup, which allows applications and users
to continue writing to data volumes, even if they are in the middle of a backup
process, while the backup is getting data from a shadow copy of the original volume.
A shadow copy set is a collection of shadow copies created in the same point in
time.
What is a writer?
A writer refers to any process that initiates change of data on the original volume.
Writers are typically applications (for example, MSDE Writer for MS SQL Server) or
system services (for example, System Writer and Registry Writer) that write persistent
information on a volume. Writers participate in the shadow copy synchronization
process by assuring data consistency.
What is a shadow copy provider?
A shadow copy provider refers to some entity that performs the work involved in
creating and representing the volume shadow copies. Shadow copy providers own
the shadow copy data and expose the shadow copies. Shadow copy providers can
be software (including a system provider, MS Software Shadow Copy Provider) or
hardware (local disks, disk arrays).
290 Microsoft Volume Shadow Copy service