How google doc are stored on disk?

Question

I know google docs are stored in google drive but what is the underlying storage used for google docs? My guesses are

1. GFS: But GFS is not optimized for small files and a large number of small writes.
2. BigTable: Each update is a new record. Every time someone requests a document it will create the doc from the sequence of updates and return, But how it will find the size of the document and other metadata required for google drive.
3. NFS: Not sure if will work here or not. I have 0 knowledge of NFS.