A memory-only snapshot of a file is disclosed that can be used by a system to perform a read of the file without disrupting other systems from reading or writing to the file. The in-memory snapshot structure includes a copy of allocation information of the file that points to the file data and be can be used by a backup application to read a file. A lock manager is provided that is configured to include an output parameter for file locks that includes allocation information associated with the in-memory file snapshot, thereby notifying a writer to the file that it must perform copy-on operations. A free space manager is also provided that is configured to track pinned blocks so as to both allow a write to a file to free snapshot blocks, while also preventing the freed blocks from being allocated to other files until backup read processing is completed.