[linux-lvm] dmioscope: visualise IO distribution for DM devices

["Bryn M. Reeves" <bmr@redhat.com>]

I wanted to give a quick plug for a little tool I've been working on for
the last few weeks: dmioscope is a python script that uses device-mapper
statistics to generate histograms of IO distribution on a DM device:

# python dmioscope.py -C READS -b 6 3 0 vg_hex/root
Initialised READS adaptive histogram with 6 bins,
min_size=1677721.
vg_hex/root: current IO distribution
        0.0            2000.0         4000.0         6000.0         8000.0         
         +-------------------------------------------------------------
0.0B     |#######
5.3GiB   |#####################################################
10.7GiB  |#####
16.0GiB  |###
21.3GiB  |
26.7GiB  |
90.00% of IO reaches 50.00% of disk.
[...]

(watch reads, six bins, interval of 3s, carry on forever)

It's useful for finding and tracking hot-spots in real time, and for
getting a general picture of where IO is going to on a device. I'm
hoping it can also be used to get an idea of the benefits that can
be achieved with caching.

I've pushed it to GitHub - if anyone finds any bugs, or has any
suggestions, I'm very interested to hear them: there's a README.md with
usage information and more details on the histogram implementation on
the main page:

  https://github.com/bmr-cymru/dmioscope

Regards,
Bryn.