A developer recently asked us this question: “I seem to recall that VSS cannot handle memory mapped files, am I right? I need to instrument a consistent backup procedure across MSMQ and SQL. I thought of doing both within a shadow copy set but MSMQ engine uses memory mapped files.” The answer: The built-in filesystem consistence for snapshots does not extend to memory-mapped files.  If you want a memory-mapped file to be consistent, you must create a writer that puts it into a consistent state during its OnFreeze method.