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Take this (certainly simplified) description from the HoboCopy tool:

HoboCopy is a backup/copy tool. (...) It uses the Volume Shadow Service (VSS) to "snapshot" the disk before copying. It then copies from the snapshot rather than the "live" disk.

Since Windows allows applications to fully lock files for read+write access I was wondering how Volume Shadow Copy Service handles locked files. (Locked as in an application does not share any access to a file it has opened, thereby essentially preventing any xcopy style backup of said file.)

Since Windows apps do their fair amount of locking, is there any chance to ever get a consistent backup this way?

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Sounds like VSS will copy locked files just fine. You should get copy of the file as it was when the shadow copy was created.

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Shadow Copy creates a read-only snapshot of the filesystem exactly as it looked at that time, irrespective of any file locks. (They are usually intended to prevent programs from using a file that is being modified by another program; this is not an issue with VSS as you get a static copy.)

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Volume Snapshot Service (VSS) relies on application cooperation in order to get a data-consistent snapshot. "Providers" register with the service and are called at the time of snapshot creation in order to create consistent disk images. Since this is a bit of work, the typical set of providers includes the registry and a number of databases.

Typical desktop applications don't participate.

So the answer to your question is something like: VSS creates copies of locked files and the writers of those files are responsible for making them consistent.

YMMV.

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