blktrace vs ftrace

Hi all, I'm trying to analize I/O behavior of Qemu on the host. I used blktrace and the blk tracer for ftrace but I see different I/O behavior. I use 4 threads in the VM each reading sequentially a file. Readahead is turned off. With blktrace reads are transmitted in a sequential way, which means the next read will be executed when the first one has completed. In comparison with ftrace, up to 4 reads are issued to the disk in parallel. I suppose the different behaviors are due to the overhead introduced by the tracers. My doubt is, which tracer has less influence on the I/O behaviour and therefore reflects the system behaviour without tracing. Thanks a lot, Matthias --- http://motzblog.wordpress.com/