Re: testing stable pages being modified

[Josef Bacik <jbacik@fusionio.com>]

On Fri, May 31, 2013 at 12:24:30AM -0600, Zach Brown wrote:
> > Changing O_DIRECT in flight has always been a deep dark corner case, and
> > crc errors are the expected result.  Have you found anyone doing this in
> > real life?
> 
> Agreed; and no, I haven't heard of people accidentally modifying stable
> pages.
> 

Windows does this, it also will send down the same page for different offsets
which is why we have that special check in check_direct_IO for reads because
that would cause lots of csum errors too.  I tried to fix the modified in flight
problem by checking the csums of the pages in the io completion handler and
re-submitting the IO if the pages had changed, but this of course dramatically
reduced performance for all of those well behaved O_DIRECT applications, so in
the end I just set nodatasum for that vm image and carried on.  I'm not sure
what the solution is for this problem.  Thanks,

Josef