VSS or volume Snapshot as the name indicates makes snapshots of volumes not individual files/folders. Later on the files which are related to application requesting backup are preserved as there is no use of keeping the files of the whole volume.
VSS operates at the block level of volumes.
Service and Applications keep on functioning normally while backup is being performed.
VSS has the ability to backup open files.
VSS backups are much more rapid as compared to traditional backups.
VSS solves the inconsistent data problem by creating and maintaining a point–in–time snapshot of the volume to be backed up. The backup job can then use this snapshot.
VSS can only be called by one requester at a time.
How Volume Shadow Copy Service Works
VSS service makes the backup components communicate and work together.
VSS requester The software that requests the actual creation of shadow copies like System Center Data Protection Manager application or non-microsoft backup applications like Tivoli backup application are VSS requesters.
VSS writer The component that guarantees we have a consistent data set to back up. VSS writers for various Windows components, such as the registry, are included with the Windows operating system. Other VSS writers are like IIS writer, Exchange writer, SQL writer etc.
VSS provider The component that creates and maintains the shadow copies. This can occur in the software or in the hardware. The Windows operating system includes a VSS provider that uses copy-on-write. If you use a storage area network (SAN), it is important that you install the VSS hardware provider for the SAN, if one is provided. A hardware provider offloads the task of creating and maintaining a shadow copy from the host operating system.