Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image

[Kevin Wolf]

Am 18.03.2013 um 07:40 hat Wenchao Xia geschrieben:
> 于 2013-3-15 22:51, Stefan Hajnoczi 写道:
> > On Fri, Mar 15, 2013 at 03:24:38PM +0800, Wenchao Xia wrote:
> >>    I'd like to add a new way to save vmstate, which will based on the
> >> migration thread, but will write contents to block images, instead
> >> of fd as stream. Following is the method to add API:
> > 
> > Hi Wenchao,
> > What use cases are there besides saving vmstate to a raw image?
> > 
> > I'm curious if you're proposing this since there is no "file:" URI or
> > because you really want to do things like saving vmstate into a qcow2
> > file or over NBD.
> > 
> > Stefan
> > 
> Hi, Stefan
>   Most used cases would be "raw" and "qcow2", which is flex and can be
> chosen by user. In this way, existing block layer feature in qemu can
> be used, such as tagging zeros. I haven't check the buffer/cache status
> in qemu block layer, but if there is, it can also benefit.
>   User can specify "raw" or "qcow2" according to host configuration, If
> there is dedicated storage components underlining, he can use "raw" to
> skip qemu's block layer.

Oh, seems I misread this then. I thought this was about internal live
snapshots, which is a feature that I consider really useful. I'm not so
sure if saving the VM state as the disk contents of a qcow2 image is
really helpful.

If zero clusters help a lot, then there's clearly something to improve
in the migration protocol, because it shouldn't send so many zeros in
the first place.

Kevin