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

[Peter Lieven]

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?

Peter
> It seems reasonable to investigate this stuff more.  Please run
> benchmarks so we have justification to merge patches.
>
> Stefan
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