Re: [Qemu-devel] [PATCHv2] block: introduce BDRV_O_SEQUENTIAL

[Peter Lieven]

On 21.03.2014 13:06, Paolo Bonzini wrote:
> Il 21/03/2014 12:49, Peter Lieven ha scritto:
> > A 10G logical volume was created and filled with random data.
> > Then the logical volume was exported via qemu-img convert to an iscsi target.
> > Before the export was started all caches of the linux kernel where dropped.
> >
> > Old behavior:
> >  - The convert process took 3m45s and the buffer cache grew up to 9.67 GB close
> >    to the end of the conversion. After qemu-img terminated all the buffers were
> >    freed by the kernel.
> >
> > New behavior with the -N switch:
> >  - The convert process took 3m43s and the buffer cache grew up to 15.48 MB close
> >    to the end with some small peaks up to 30 MB durine the conversion.
> >
> > Signed-off-by: Peter Lieven <address@hidden>
> > ---
> > v1->v2: - added test example to commit msg
> >         - added -N knob to qemu-img
>
> I'm sorry, I cannot find the original discussion.  Why is the new knob needed?

The thread was named "qemu-img convert cache mode for source".

I think the 2 points (mainly by Marcus) were that you would not expect qemu-img
to mangle with the page cache by default as you would not expect it from cp or 
dd.
And secondly, if a running vServer and the image that is converted share pages 
it
can ruin the vServers cache.

Peter