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Zmanda Cloud Backup utilizes a built-in Windows function known as Volume Shadowcopy Services, or VSS.

VSS allows backups to be taken of in-use, locked, or inaccessible files without interrupting whatever process or user is currently accessing those files. This is especially useful for databases, as it means backups can be taken while the database is active, without requiring downtime.

Every backup type in ZCB relies upon one or more VSS Writers. Each writer is controlled by a Windows Service.

A list of currently available writers can be seen via the command line on any Windows machine. Open a command prompt as an Administrator and run the command:

vssadmin list writers

This command will list all of the writers currently available on the machine as well as their current state. A writer in the Stable state is ready and waiting to take a backup. Writers in the Failed or Unstable states have encountered a problem, and may need to be reset. Writers that are currently In-Progress are being used for a backup. If no backups are running, the writer may be stuck, and can be reset like Failed or Unstable writers.

Restarting the service that controls the writer will reset the writer and, in most cases, bring it back to a stable state.

It is important to note that not all controlling services can be restarted. If a service cannot be restarted, a reboot of the machine is required.

List of VSS Writers

The following list shows the VSS Writers required for each backup type, as well as the name of the controlling service.

Backup TypeRequired VSS WriterName of Controlling ServiceService Display Name
System StateASR WriterVSSVolume Shadow Copy
 BITS WriterBITSBackground Intelligent Transfer Service
 COM+ REGDB WriterVSSVolume Shadow Copy
 IIS Config WriterAppHostSvcApplication Host Helper Service
 IIS Metabase WriterIISADMIN

IIS Admin Service

 Registry WriterVSSVolume Shadow Copy
 System WriterCryptSvc

Cryptographic Services

 WMI WriterWinmgmtWindows Management Instrumentation
 NTDS Writer<unknown>Active Directory
 Shadow Copy Optimization WriterVSSVolume Shadow Copy
Microsoft ExchangeMicrosoft Exchange WriterMSExchangeISMicrosoft Exchange Information Store
Hyper-VMicrosoft Hyper-V VSS WritervmmsHyper-V Virtual Machine Management
Microsoft SQL ServerSQL Server VSS WriterSQL Server VSS WriterSQL Server VSS Writer
 MSDE WriterMSDE Writer

MSDE Writer

SharepointSharepoint VSS WriterSharepoint VSS WriterSharepoint VSS Writer
  • Sharepoint 2007:
SPSearch VSS WriterWindows Sharepoint Services SearchSPSearch VSS Writer
  • Sharepoint 2007:
OSearch VSS WriterOffice Sharepoint Services Search

OSearch VSS Writer

  • Sharepoint 2010:
SPSearch4 VSS WriterSharepoint Foundation Search V4SPSearch 4 VSS Writer
  • Sharepoint 2010:
OSearch14 VSS WriterSharepoint Server Search 14OSearch VSS Writer