blktrace vs ftrace

[Matthias Brugger]

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

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