As a point of education, what is the architectural and functional relationship between the StorageCraft Volume Snapshot Driver (stcvsm.sys) and Volume Shadow Copy Service (VSS) on Windows XP/Vista?

More specifically, what is the architectural and functional relationship between the StorageCraft Volume Snapshot Driver and the “StorageCraft Volume Snapshot Software Provider” (displayed by executing the “vssadmin list providers” command)?  Is the “driver” the same as the “provider” when VSS is enabled?

Thank you for the clarification.

Comments from StorageCraft?

There is some very good information on the VSS framework on Microsoft's knowledge base so I won't bore you with the full details.  My intent is not to educate you fully on VSS but rather a give you a quick overview so you can see how ShadowProtect fits into the framework.

The VSS framework has 3 basic components:

1.  Providers
2.  Writers
3.  Requesters

Providers are what actually implement the snapshots.  There are 3 types of providers, they are System, Software and Hardware.  Microsoft volsnap is he sytem provider,  stcvsm.sys is a software provider.  Hardware providers generally are expensive external RAID systems that implement snapshot within their own firmware/hardware.

Writers are applications that have registered components that allow them to be notified of an impending backup and snapshot request.  Once notified that a backup and generally a snapshot is pending, the application can prepare its data for snapshotting.  This generally involves cache flushing and database quiesing. 

Requesters are the backup applications that use the VSS Framework to take backups.

The ShadowProtect product contains both provider and requester components.

stcvsm.sys is what implements the actual snapshot.  While ShadowProtect can use stcvsm.sys directly without the framework (like on Windows 2000), we prefer to use it with the framework on xp, 2003 and above.  In order to do this, certain components must be registered to be a provider.  This is what the "StorageCraft Shadow Copy Provider" Service is. 

This service interacts with the VSS framework and our snapshot driver. 

1.  ShadowProtect makes a backup request to the VSS. (I will refer to the VSS framework simply as VSS for this discussion).
2.  VSS notifies the appications that a backup and snapshot have been requested.
3.  The VSS aware applications then prepare their data for snapshotting.
4.  THe applications then notifiy VSS they are ready for a snapshot.
5.  VSS requests the provider to take the snapshot
6.  Snapshot is taken
7.  VSS is notified that snapshot was taken.
8.  VSS notifies the application that the snapshot was taken
9.  Applications then release what that were holding or resume what they were doing. 
10.  VSS notifies the ShadowProtect requester that a snapshot is available.
11.  ShadowProtect then begins the backup process of reading the image from the snapshot.

There is much more detail than this.  I tried to keep it to a minimum.

Hope this answers your questions.

STC