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

[Stefan Hajnoczi]

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.

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

Stefan