Re: [Qemu-devel] qemu-img convert cache mode for source

[Stefan Hajnoczi <stefanha@redhat.com>]

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