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

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Feb 28, 2013 at 03:24:27PM +0000, Dietmar Maurer wrote:
> > > 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.
> 
> I currently test with about 700MB read speed, and get a slow down by factor 1.7.
> So memory copy is very significant , or there is something wrong with nbd.c.

What are the details of the test?

Is it using 64 KB writes and have you tried 256 KB writes?

Stefan