On Tue, Jul 19, 2011 at 10:08:21AM -0600, Eric Blake wrote: > On 07/19/2011 10:06 AM, Daniel P. Berrange wrote: > > > >The motivation for using O_DIRECT is that allowing pollution of the > >host cache causes stability problems for the host as a whole. As > >such IMHO, apps would likely want an error back if O_DIRECT cannot > >be supported, > > > >NB, even some Linux filesystems can't do O_DIRECT, so this isn't an > >obscure mingw32 issue. > > Conversely, open() on Linux silently ignores unknown flags - so if > you are using a really old kernel but newer glibc headers, then > O_DIRECT is non-zero and open() succeeds, but you _don't_ get direct > I/O. > > If O_DIRECT is 0, then it is pretty easy to diagnose that the > request is unsupported. But if O_DIRECT is non-zero, then how do I > tell whether the open(O_DIRECT) really meant that I have direct I/O, > or whether it was a nice hint but still ignored and I'm still > polluting the file system cache? Hmm, I could have sworn we've seen QEMU itself fail to start when requesting O_DIRECT on say, tmpfs. Perhaps open() isn't failing, but rather read/write fail ? Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list