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
