On Wed, Feb 26, 2014 at 05:01:52PM +0100, Peter Lieven wrote: > On 26.02.2014 16:41, Stefan Hajnoczi wrote: > >On Wed, Feb 26, 2014 at 11:14:04AM +0100, Peter Lieven wrote: > >>I was wondering if it would be a good idea to set the O_DIRECT mode for the > >>source > >>files of a qemu-img convert process if the source is a host_device? > >> > >>Currently the backup of a host device is polluting the page cache. > >Points to consider: > > > >1. O_DIRECT does not work on Linux tmpfs, you get EINVAL when opening > > the file. A fallback is necessary. > > > >2. O_DIRECT has no readahead so performance could actually decrease. > > The question is, how important is reahead versus polluting page > > cache? > > > >3. For raw files it would make sense to tell the kernel that access is > > sequential and data will be used only once. Then we can get the best > > of both worlds (avoid polluting page cache but still get readahead). > > This is done using posix_fadvise(2). > > > > The problem is what to do for image formats. An image file can be > > very fragmented so the readahead might not be a win. Does this mean > > that for image formats we should tell the kernel access will be > > random? > > > > Furthermore, maybe it's best to do readahead inside QEMU so that even > > network protocols (nbd, iscsi, etc) can get good performance. They > > act like O_DIRECT is always on. > your comments are regarding qemu-img convert, right? > How would you implement this? A new open flag because > the fadvise had to goto inside the protocol driver. > > I would start with host_devices first and see how it performs there. > > For qemu-img convert I would issue a FADV_DONTNEED after > a write for the bytes that have been written > (i have tested this with Linux and it seems to work quite well). > > Question is, what is the right paramter for reads? Also FADV_DONTNEED?
I think so but this should be justified with benchmark results. Stefan