4.8. VSS Transaction Flow

In processing a backup, the requester and the writers coordinate to do several things: to provide a stable system image from which to back up data (the shadow copied volume), to group files together on the basis of their usage, and to store information on the saved data. This must all be done with minimal interruption of the writer's normal work flow.
A requester (in our case the Backup Vendor) queries writers for their metadata, processes this data, notifies the writers prior to the beginning of the shadow copy and of the backup operations, and then notifies the writers again after the shadow copy and backup operations end.
Here is how the QEMU VSS provider is registered in Windows OS after the Guest Tools installation:
C:\Users\Administrator>vssadmin list providers
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2005 Microsoft Corp.

Provider name: 'QEMU Guest Agent VSS Provider'
   Provider type: Software
   Provider Id: {3629d4ed-ee09-4e0e-9a5c-6d8ba2872aef}
   Version: 0.12.1