RFE: Graphing and iteration support for fio

[Avi Kivity <avi@scylladb.com>]

Sometimes you want to run a set of experiments on a disk, varying a 
parameter between tests (in my case, iodepth, but buffer size is also a 
good candidate).  You then want to present the results in a nice graph.

I wrote a small wrapper around fio to do this 
(https://github.com/avikivity/diskplorer), but it occurs to me that 
generalized support for both in fio would be much more useful.

Possibly, you'd define a job as a template:

[aio-read]
template_start=1
template_end=100
template_step=1
(or template_ratio=1.05 for exponential growth)
iodepth=template_variable

(it's just possible that someone can come up with better syntax).

A few more options in the global section can then cause a graph to be 
generated.

btw, a fast disk can easily saturate a single core using libaio, so a 
multithreaded libaio ioengine would be welcome (I am currently emulating 
it using multiple jobs and new_group).