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Non-single level stores are copy-on-demand. Typically, a process runs a program by starting with an empty address space and mapping the executable code into that space. The program is started, the first instruction fetched; the address space is empty a page fault occurs and the page is copied in. If the page is modified, that is local to the process and the disk image is unchanged. Each process has its own copy of writable pages.

With single level store, the program pages are mapped in, not copied. Writing to the page alters the disk image. All processes running the same program share the memory pages.

[zasdffaa]

> With single level store, the program pages are mapped in, not copied. Writing to the page alters the disk image. All processes running the same program share the memory pages.

Umm, mmap'ed files on linux allow both https://www.man7.org/linux/man-pages/man2/mmap.2.html

       MAP_SHARED
              Share this mapping.  Updates to the mapping are visible to
              other processes mapping the same region, and (in the case
              of file-backed mappings) are carried through to the
              underlying file.  (To precisely control when updates are
              carried through to the underlying file requires the use of
              msync(2).)

       MAP_PRIVATE
              Create a private copy-on-write mapping.  Updates to the
              mapping are not visible to other processes mapping the
              same file, and are not carried through to the underlying
              file.  It is unspecified whether changes made to the file
              after the mmap() call are visible in the mapped region.

and AFAIK on Windows the same.