Re: [Qemu-devel] [PATCH v4 3/6] add backup related monitor commands

[Stefan Hajnoczi]

On Mon, Mar 04, 2013 at 02:33:16PM +0000, Dietmar Maurer wrote:
> > > > Is it using 64 KB writes and have you tried 256 KB writes?
> > >
> > > I use a modified 'qemu-img convert' at 64KB block size (I need 64KB for
> > backup).
> > 
> > Maybe you'd better use a different output format that doesn't restrict you 
> > to
> > 64k writes.
> 
> The output format is not really the restriction. The problem is that an 
> additional
> IPC layer add overhead, an d I do not want that (because it is totally 
> unnecessary).

I missed the reason why you cannot increase the block size.  At least
increase the NBD write size - the VMA process can break up a large write
however it likes.  Do this and the overhead is divided by 4.

Stefan