Windows Volume Shadow Service usually known as VSS runs as a service on Windows servers and clients.
VSS provides an interface among different components that need and create snapshots.
The components involved in snapshot creation:
1) VSS
2) Shadow requestor processes/applications (usually backup software)
3) Writer Processes
4) Shadow Providers
Shadow requestor application requests VSS that a shadow copy be created of the volume that has to be protected.
Shadow writer is a subsystem of software applications whose persistent data need to be backed-up, like, SQL server, Exchange server, etc.
The writers as soon as they receive a signal from VSS that a shadow copy needs to be created, they sync the data in memory to the disk and block all I/Os to the data to be backed-up essentially making data consistent for a brief period of time. Without this the backup data will be incomplete hence useless to apps like databases.
Shadow providers are the software or hardware that provide, i.e. create, the snapshots.
Once VSS confirms the I/O has been blocked by writers it communicates to the providers to create a shadow copy. A shadow copy can be created at the storage array side or in the OS itself.
Hardware providers ship drivers that can be installed on the OS for creating h/w shadow copies. These are created at the array side and whole LUN is copied.
Software providers employ their own way to create a copy of the volume on the OS itself.
System providers are shipped with Windows OS developed by Microsoft. These too create copy on the OS itself but are based on Copy-On-Write, i.e. they are differential copies and never full copy snapshots. (For more info on Full copy/COW snapshots, search for Snapshot technology online.)
To view shadow copies, use the command vssadmin.exe List Shadows, or vshadow.exe from VSSSDK.
VSS provides an interface among different components that need and create snapshots.
The components involved in snapshot creation:
1) VSS
2) Shadow requestor processes/applications (usually backup software)
3) Writer Processes
4) Shadow Providers
Shadow requestor application requests VSS that a shadow copy be created of the volume that has to be protected.
Shadow writer is a subsystem of software applications whose persistent data need to be backed-up, like, SQL server, Exchange server, etc.
The writers as soon as they receive a signal from VSS that a shadow copy needs to be created, they sync the data in memory to the disk and block all I/Os to the data to be backed-up essentially making data consistent for a brief period of time. Without this the backup data will be incomplete hence useless to apps like databases.
Shadow providers are the software or hardware that provide, i.e. create, the snapshots.
Once VSS confirms the I/O has been blocked by writers it communicates to the providers to create a shadow copy. A shadow copy can be created at the storage array side or in the OS itself.
Hardware providers ship drivers that can be installed on the OS for creating h/w shadow copies. These are created at the array side and whole LUN is copied.
Software providers employ their own way to create a copy of the volume on the OS itself.
System providers are shipped with Windows OS developed by Microsoft. These too create copy on the OS itself but are based on Copy-On-Write, i.e. they are differential copies and never full copy snapshots. (For more info on Full copy/COW snapshots, search for Snapshot technology online.)
To view shadow copies, use the command vssadmin.exe List Shadows, or vshadow.exe from VSSSDK.
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