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

[Stefan Hajnoczi]

On Wed, Feb 27, 2013 at 03:50:53PM +0000, Dietmar Maurer wrote:
> > NBD enables interprocess communication - any form of IPC requires a protocol
> > and NBD is quite a trivial one.  What is a simpler way of talking to a 
> > backup
> > server?
> 
> Unfortunately, NBD add considerable overheads. I guess the socket 
> communications copies data.
> This is really unnecessary if I can write directly to the output stream.

The disk is the bottleneck, not memory bandwidth.  Hard disks only do
10-100 MB/sec and SSDs only do a couple 100 MB/sec.  Memory copy is
insignificant compared to the I/O activity required to copy out the
entire disk image, not to mention delaying guest writes until we read
the original data from the disk.

Unless there's a concrete performance problem here this is premature
optimization.

Stefan