Re: [fuse] Effects of opening with O_DIRECT

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, Mar 2, 2020 at 9:29 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
>
> On Mar 02 2020, Miklos Szeredi <miklos@szeredi.hu> wrote:
> > On Sun, Mar 1, 2020 at 2:20 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
> >>
> >> Hi,
> >>
> >> What happens if a file (on a FUSE mountpoint) is opened without
> >> O_DIRECT, has some data in the page cache, and is then opened a second
> >> with O_DIRECT?
> >>
> >> Will reads with O_DIRECT come from the page cache (if there's a hit), or
> >> be passed through to the fuse daemon?
> >
> > O_DIRECT read will try first directly, and fall back to the cache on
> > short or zero return count.
> >
> >>
> >> What happens to writes (with and without O_DIRECT, and assuming that
> >> writeback caching is active)? It seems to me that in order to keep
> >> consistent, either caching has to be disabled for both file descriptors
> >> or enabled for both...
> >
> > This is not a fuse specific problem.   The kernel will try to keep
> > things consistent by flushing dirty data before an O_DIRECT read.
> > However this mode of operation is not recommended.  See open(2)
> > manpage:
> [...]
>
> Is there currently any other way to execute a read request while making
> sure that data does not end-up in the page cache (unless it happens to
> be there already)?

Hmm, that sounds something like posix_fadvise(..., POSIX_FADV_DONTNEED).

Thanks,
Miklos