BTW, here is how FSX calls CreateFileMappingW. https://github.com/billziss-gh/secfs.test/blob/60f9f44b8773122bc3873b162b4136d57f60a131/fstools/src/fsx/win/mman

BTW, here is how FSX calls CreateFileMappingW. https://github.com/billziss-gh/secfs.test/blob/60f9f44b8773122bc3873b162b4136d57f60a131/fstools/src/fsx/win/mman.c#L115 https://github.com/billziss-gh/secfs.test/blob/60f9f44b87731... I do not see a big difference to the fsbench call. It may be that fsbench attempts to make a mapping that is 4MB long. Now you can fix that problem in Dokany ;)