Re: [Qemu-devel] [Bug 939027] Re: Combining direct_io and non-direct_io leads to hang

[Stefan Hajnoczi]

On Thu, Feb 23, 2012 at 9:39 PM, Avleen Vig <address@hidden> wrote:
> Nothing else at all accesses those files, which is one of the things that 
> concerned me the most :-/
> It looks like it call came from the same place.

It's possible to check the file descriptor flags using
/proc/$PID/fdinfo/$FD.  So for example, you could find the qemu-kvm
process and list its open file descriptors using ls -l /proc/$(pgrep
qemu-kvm)/fd.  Find the file descriptor to the image file and look up
its flags in /proc/$PID/fdinfo/$FD.

If the file descriptor has bit 040000 set then it is O_DIRECT.  If
not, it uses buffered I/O.

Using this technique you could determine whether or not your system is
accessing the file using both direct and buffered I/O.  It would also
tell you which processes are doing this.

Stefan