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When you try to run a backup, it fails with the error "Writers status is not stable. Try after restarting application Writer service."

Internal log files will show the error code: ZWC_ENG_SNAPSHOT_WRITER_FAILED


If the unstable writer error appears only for Differential or Incremental backups of databases, including but not limited to Exchange, Microsoft SQL Server, Oracle, and Sharepoint, please review the following article instead:

Info: How do databases handle backups from multiple different backup applications?

What's going on?

Zmanda Cloud Backup utilizes a built-in Windows function known as Volume Shadowcopy Services, or VSS. 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. Cancel the backup that is running, or wait for it to complete, before running your backups.
    • If no backups are running, the writer may be stuck, and can be reset like Failed or Unstable writers.


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

A list of writers and their controlling services can be found here:

Info: List of VSS Writers and Services


If the controlling service cannot be restarted, a reboot of the machine is required.

The writers associated with System State backups frequently require a reboot when they become unstable.

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