On Thu, 2017-08-03 at 00:48 -0700, Steven Swanson wrote: > +### DAX Support > + > +Supporting DAX efficiently is a core feature of NOVA and one of the challenges > +in designing NOVA is reconciling DAX support which aims to avoid file system > +intervention when file data changes, and other features that require such > +intervention. > + > +NOVA's philosophy with respect to DAX is that when a program uses DAX mmap to > +to modify a file, the program must take full responsibility for that data and > +NOVA must ensure that the memory will behave as expected. At other times, the > +file system provides protection. This approach has several implications: > + > +1. Implementing `msync()` in user space works fine. > + > +2. While a file is mmap'd, it is not protected by NOVA's RAID-style parity > +mechanism, because protecting it would be too expensive. When the file is > +unmapped and/or during file system recovery, protection is restored. > + > +3. The snapshot mechanism must be careful about the order in which in adds > +pages to the file's snapshot image. Hello Steven, Thank you for having shared this very interesting work. After having read the NOVA paper and patch 01/16 I have a question for you. Does the above mean that COW is disabled for writable mmap-ed files? If so, what is the reason behind this? Is there a fundamental issue that does not allow to implement COW for writable mmap-ed files? Or have you perhaps tried to implement this and was the performance not sufficient? Please note that I'm neither a filesystem nor a persistent memory expert. Thanks, Bart.
