[dm-devel] dm stats

[Bryn M. Reeves]

On Wed, Dec 21, 2016 at 10:49:06AM +0000, A M, Ashalatha wrote:
> In this documentation, would like to know what is the command to be used to create/print/list stats.
> 
> @stats_create <range> <step> [<program_id> [<aux_data>]]
> 
> @stats_print <region_id> [<starting_line> <number_of_lines>]
> 
> @stats_list [<program_id>]

These are device-mapper messages that you can send to a device to
create, print, or list statistics regions.

All range and step arguments are in sectors and the messages are
issued with the 'dmsetup message' command.

For example:

  # dmsetup message vg_hex/root 0 "@stats_list" 
  0: 35325952+2048 2048 dmstats -
  1: 35325952+2048 2048 dmstats -
  2: 44908544+16384 16384 dmstats DMS_GROUP=vm.img:2#-

This is however a very low-level interface to the dmstats facility;
users are encouraged to use the userspace library (part of libdevmapper
- see libdevmapper.h for API documentation), or the 'dmstats' command
to manage device-mapper statistics.

  # dmstats list
  Name             GrpID RgID ObjType RgStart RgSize  #Areas ArSize  ProgID 
  vg_hex-root          -    0 region   16.84g   1.00m      1   1.00m dmstats
  vg_hex-root          -    1 region   16.84g   1.00m      1   1.00m dmstats
  vm.img               2    2 region   21.41g   8.00m      1   8.00m dmstats
  
This has the benefit of a simpler command interface, automatic rate
conversion for derived metrics (MB/s, queue length, etc.), configurable
time and size units, flexible field control, grouping, aliasing and many
other convenient features - see man 8 dmstats for further details of the
command interface.

> Also, it is documented that 11 stats(Documentation/iostats.txt) would be reported and additional counters as well:
> 
> 12. the total time spent reading in milliseconds
> 
> 13. the total time spent writing in milliseconds
> 
> So are these same as "read_ticks" and "write_ticks" else how different from the ticks?

Yes: these are the fields reported by dmstats as 'read_ticks' and
'write_ticks'. They differ from 'read_time' and 'write_time' in that
they are the time spent _servicing_ (i.e. actively processing) read and
write requests - the two '_time' fields instead track the total
accumulated time for all requests (the time from the request being
submitted to the time when it is completed, summed for all requests).

Regards,
Bryn.