Re: [RFC patch v1 2/3] qemu-file: add buffered mode

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Apr 27, 2020 at 01:14:33PM +0100, Dr. David Alan Gilbert wrote:
> * Denis Plotnikov (dplotnikov@virtuozzo.com) wrote:
> > The patch adds ability to qemu-file to write the data
> > asynchronously to improve the performance on writing.
> > Before, only synchronous writing was supported.
> > 
> > Enabling of the asyncronous mode is managed by new
> > "enabled_buffered" callback.
> 
> It's a bit invasive isn't it - changes a lot of functions in a lot of
> places!
> The multifd code separated the control headers from the data on separate
> fd's - but that doesn't help your case.
> 
> Is there any chance you could do this by using the existing 'save_page'
> hook (that RDMA uses).
> 
> In the cover letter you mention direct qemu_fflush calls - have we got a
> few too many in some palces that you think we can clean out?

When I first introduced the QIOChannel framework, I hoped that we could
largely eliminate QEMUFile as a concept.  Thus I'm a bit suspicious of
the idea of introducing more functionality to QEMUFile, especially as the
notion of buffering I/O is rather generic. Is there scope for having a
QIOChannelBuffered object for doing buffering. Would that provide better
isolation from the migration code and thus be less invasive/complex to
maintain ?


Regards,
Daniel
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