I am going to quote ChatGPT (be aware ChatGPT makes mistakes):
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When a program is said to be "VSS-aware," it means that the program is designed to interact properly with the Volume Shadow Copy Service (VSS) in Windows. VSS is a framework that allows applications to create consistent and point-in-time copies (snapshots) of data on a volume, even while the data is in use.
Being VSS-aware involves following specific guidelines and APIs (Application Programming Interfaces) provided by Microsoft for interacting with VSS. Here are some key aspects of VSS-awareness:
Coordination with VSS Writers: VSS-aware applications need to coordinate with VSS writers. VSS writers are components that ensure the consistency of data during a snapshot. For example, databases or file system writers can ensure that their data is in a consistent state when a snapshot is taken.
Freezing and Thawing of Applications: VSS-aware applications should freeze and thaw their activities when a snapshot is taken. This helps ensure that the data captured in the snapshot is in a consistent state.
Integration with VSS API Calls: VSS-aware applications use the VSS API calls to request and manage snapshots. These API calls include operations like querying for supported capabilities, requesting the creation of a snapshot, and releasing the snapshot when it's no longer needed.
If a program is not VSS-aware, it might not follow these guidelines, which could lead to issues such as:
Inconsistent snapshots: The captured snapshot might not represent a consistent state of the data.
Data corruption: Unaware applications may not handle freezing and thawing of data, leading to potential data corruption in the snapshot.
Conflicts with other VSS-aware applications: Multiple VSS-aware applications might interfere with each other if they are not properly coordinating their activities through VSS.
In the context of backup programs, being VSS-aware is crucial to ensure reliable and consistent backups, especially in scenarios where multiple backup applications might be running on the same system. It's always recommended to use applications that explicitly state their VSS-awareness and adhere to best practices for VSS integration.
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I have seen issues with AllSync and Macrium Reflect, and AllSync and Hasleo. It might be that AllSync is not fully compliant with VSS-aware requirements. It might also be that Hasleo/Macrium are not fully compliant. (In any case I find it weird if a program that IS compliant can be disturbed by a NON-complient program, if this is indeed the case.)
In any case it seems to me that you cannot take it for granted that two VSS-using backup programs will work with no isssues.
By "issue" I mean: failure of backup (error message).
But it is good news that apparently it IS possible to program backup programs so they will work simultaneously!
>>>
When a program is said to be "VSS-aware," it means that the program is designed to interact properly with the Volume Shadow Copy Service (VSS) in Windows. VSS is a framework that allows applications to create consistent and point-in-time copies (snapshots) of data on a volume, even while the data is in use.
Being VSS-aware involves following specific guidelines and APIs (Application Programming Interfaces) provided by Microsoft for interacting with VSS. Here are some key aspects of VSS-awareness:
Coordination with VSS Writers: VSS-aware applications need to coordinate with VSS writers. VSS writers are components that ensure the consistency of data during a snapshot. For example, databases or file system writers can ensure that their data is in a consistent state when a snapshot is taken.
Freezing and Thawing of Applications: VSS-aware applications should freeze and thaw their activities when a snapshot is taken. This helps ensure that the data captured in the snapshot is in a consistent state.
Integration with VSS API Calls: VSS-aware applications use the VSS API calls to request and manage snapshots. These API calls include operations like querying for supported capabilities, requesting the creation of a snapshot, and releasing the snapshot when it's no longer needed.
If a program is not VSS-aware, it might not follow these guidelines, which could lead to issues such as:
Inconsistent snapshots: The captured snapshot might not represent a consistent state of the data.
Data corruption: Unaware applications may not handle freezing and thawing of data, leading to potential data corruption in the snapshot.
Conflicts with other VSS-aware applications: Multiple VSS-aware applications might interfere with each other if they are not properly coordinating their activities through VSS.
In the context of backup programs, being VSS-aware is crucial to ensure reliable and consistent backups, especially in scenarios where multiple backup applications might be running on the same system. It's always recommended to use applications that explicitly state their VSS-awareness and adhere to best practices for VSS integration.
<<<
END QUOTE
I have seen issues with AllSync and Macrium Reflect, and AllSync and Hasleo. It might be that AllSync is not fully compliant with VSS-aware requirements. It might also be that Hasleo/Macrium are not fully compliant. (In any case I find it weird if a program that IS compliant can be disturbed by a NON-complient program, if this is indeed the case.)
In any case it seems to me that you cannot take it for granted that two VSS-using backup programs will work with no isssues.
By "issue" I mean: failure of backup (error message).
But it is good news that apparently it IS possible to program backup programs so they will work simultaneously!