How ShadowSnap uses Shadow Storage



This article explains shadow copies, shadow storage, and how they interact with the Datto ShadowSnap backup solution. This article does not apply to the Datto Windows Agent.


Shadow Copies are native Windows VSS snapshots. Shadow storage is the location that Windows uses to store these snapshots.

The ShadowSnap backup process uses shadow storage; the amount of shadow storage space required by ShadowSnap depends on how quickly applications modify data after the backup begins, and how quickly ShadowProtect and ShadowSnap are able to copy the data into the backup.   

Datto recommends that you disable the Windows Shadow copy process when using ShadowProtect because of the difference between how ShadowProtect and Microsoft interact with the shadow storage space. Microsoft will fill up the shadow storage space until it is completely full, and then destroy the oldest backup to make room for the newest backup. This can have an adverse effect on ShadowProtect, because it uses the same storage space for its backups. Once we have written the backup to the destination, the software cleans up its shadow copies.

If you cannot disable Windows Shadow Copy, Datto recommends that you set your shadow storage space to unbounded. This will allow the space to grow as necessary to accommodate ShadowSnap backups.

Note that if your shadow storage space is unbounded, and the Microsoft provider is enabled, following this step will likely fill your hard drive.

Working with Shadow Storage

The following commands will allow you to manage shadow copies and shadow storage from the Windows Command Prompt. You will need to launch the Command Prompt session as an administrator.

View existing shadow copies:

vssadmin list shadows

View the size of the storage:

vssadmin list shadowstorage

Change the allocated size for your shadow storage:

vssadmin resize shadowstorage /for=<ForVolumeSpec> /on=<OnVolumeSpec> [/maxsize <MaxSizeSpec>]
  • If no value is specified for /maxsize, there is no limit placed on the amount of storage space that can be used.

Additional Resources

Microsoft TechNet: vssadmin resize shadowstorage

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