Using Volume Shadow Copy Service (VSS) with and without explicit usage of writers

The Volume Shadow Copy Service (VSS) of Windows has been available since Windows XP/2003.
 It can be used by application to create a shadow copy of one or more partitions.
The components that communicate with the VSS are called VSS writer.
You can before and after a backup perform actions.
 A list of the available system writers can obtained by the command:
 

vssadmin list writers

Snapshot can use VSS in two different modes.

VSS without explicit usage of writers

  • Creates a shadow copy, that contains a konsistent copy of all existing data.
  • Snapshot uses this mode automatically if more then one partition is selected, or MS Exchange is detected.
    (in Windows SBS2003 the necessary Exchange writer is disabled by default, and must be enabled; see  http://support.microsoft.com/kb/838183)
  • writers are not used in an explicit way, and no specific actions are taken.
  • Transaction logs on an Exchange server will not be truncated after the backup, and a Hyper-V host does not notify the virtual machines running on the scheduled backup.


    • VSS mit expliziter Beteiligung von Writern

  • This mode uses snapshot only if instructed (Advanced Backup Options dialog).
    The explicit involvement of all writers forcing by ticking at Explicitly include all VSS writers.
    This means that all the started VSS writers are notified about the starting backup, and can go ahead and carry out special operations    .
  • In this mode a Hyper-V Server would stop virtual machines running on it for a short time until the the shadow copy has been taken.
  • An Exchange server will - after snapshot signals a successful backup - truncate transaction logs
  • Each writer is responsible for each owns actions.
  • Snapshot has no effect on the actions carried out by the writers.
    • A detailed control of the Writer is possible by using the command line, for example if you already have a dedicated Exchange backup feature and do not want that snapshot triggers the deletion of the transaction logs: