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We run two different backup services on certain critical servers at the same time. One is for offsite backup to a third party secure storage facility (just in case the data centre disappears into a hole in the ground), the other is for a local onsite copy for rapid recovery and general daily use.

Both these backup services use the Windows Volume Snapshot Service but are timed not to overlap.

My question is, is it possible for two different backup applications to use VSS at the same time?

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Yes. No reason that it wouldn't work.

Edit - to clarify, VSS creates disk snapshots; both on schedules (Shadow Copies in the Explorer UI), and when VSS-aware applications request snapshots. Then, when those applications want access to those snapshots, VSS presents them.

There's nothing that prevents more than one snapshot being used at the same time - if that were the case, Shadow Copy would be a real crappy technology, for one thing. Furthermore, I'm dead certain that nothing prevents more than one snapshot being created at the same time.

So - yes, two different backup applications should be able to use VSS at the same time. The only problems you might run into are disk space or performance (CPU or IO); but that can apply to anything you might ever change on a computer system, so it doesn't really need to be underlined in this context.

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Multiple VSS snapshots on the same volume are possible.

If both backup applications call the VSS writer at the same time there will be a conflict and cause the VSS snapshot to fail.

I suspect additional snapshots can create Disk+CPU loading because the copy-on-write files will need to be accessed and compared.
C:\Windows\system32>vssadmin list shadows vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool (C) Copyright 2001-2013 Microsoft Corp.

Contents of shadow copy set ID: {ee9dc68e-04c4-4dfa-be1f-e026775adbee} Contained 1 shadow copies at creation time: 4/29/2019 3:43:36 PM Shadow Copy ID: {510074c4-d6e7-466c-9f05-61fe4a2938c9} Original Volume: (D:)\?\Volume{328a461b-0000-0000-0000-501f00000000}\ Shadow Copy Volume: \?\GLOBALROOT\Device\HarddiskVolumeShadowCopy1 Originating Machine: AETW-HV-01.aeviso.com.tw Service Machine: AETW-HV-01.aeviso.com.tw Provider: 'Microsoft Software Shadow Copy provider 1.0' Type: ApplicationRollback Attributes: Persistent, No auto release, Differential

Contents of shadow copy set ID: {9737a8c7-4f8a-48a7-8924-dd813ec1f8c1} Contained 1 shadow copies at creation time: 4/29/2019 3:46:35 PM Shadow Copy ID: {cc6051c3-7659-4c0a-a537-a38bffdd8f22} Original Volume: (D:)\?\Volume{328a461b-0000-0000-0000-501f00000000}\ Shadow Copy Volume: \?\GLOBALROOT\Device\HarddiskVolumeShadowCopy3 Originating Machine: AETW-HV-01.aeviso.com.tw Service Machine: AETW-HV-01.aeviso.com.tw Provider: 'Microsoft Software Shadow Copy provider 1.0' Type: ApplicationRollback Attributes: Persistent, No auto release, Differential

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