Googler here, I think OP is referring to the relationship between colossus and bigtable [1]. Basically, what happens is that colossus uses bigtable for storing metadata. So, when you issue a command like `ls /some-cluster-directory/myfolder/mysubfolder` you are really querying bigtable instead of a distributed ls on a cluster fs.
It is much more complicated than that, but the idea is that using bigtable you create a resemblance of a fs that feels like a fs to use for the most part.
(I'm an ex-Googler, but I am glad that my spotty memory gives me ample protection against accidentally giving out company secrets.)
I suspect Google Drive is also backed by something that's not a traditional file system, even thought Google Drive tries to look a bit like a file system to the user.
From the outside one clue is that you can have two files with the same name, another is the url to a file doesn't change when you move it. Also it's dog slow to list a directory, like 30sec for 50 files.
It is much more complicated than that, but the idea is that using bigtable you create a resemblance of a fs that feels like a fs to use for the most part.
[1] http://www.pdsw.org/pdsw-discs17/slides/PDSW-DISCS-Google-Ke...