Docker Input Plugin Measure Thin Pool Minimum Free Space

[GeorgeMac]

Closes #5758

This adds a new measurement docker_thin_pool with the field minimum_free_space measured in bytes.

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

I think we may want to reorganize the storage driver metrics. Right now the devicemapper metrics are spread out over several measurements:

- docker
  - fields:
  	- pool_blocksize

- docker_data
  - tags:
    - unit
    - engine_host
    - server_version
  - fields:
    - available
    - total
    - used

- docker_metadata
  - tags:
    - unit
    - engine_host
    - server_version
  - fields:
    - available
    - total
    - used

We can't get rid of these, since we don't want to break any queries, dashboards, alerts, etc. However, we can deprecate them and try again. I think this layout would be preferable:

- docker_devicemapper
  - tags:
    - engine_host
    - server_version
    - pool_name
  - fields:
    - data_space_available_bytes
    - data_space_total_bytes
    - data_space_used_bytes
    - metadata_space_available_bytes
    - metadata_space_total_bytes
    - metadata_space_used_bytes
    - thin_pool_minimum_free_space_bytes
    - (other stuff)

I like this layout more because it gives us room to grow, without a measurement explosion, and it groups all the devicemapper related values into a single measurement.

[GeorgeMac]

I like it @danielnelson

update: as I implement this I am just adding a note here that I will start with precisely the list of fields you describe there (minus "other stuff") with the addition of pool_blocksize. Since there are all byte measurements which I imagine might be the most useful. Some other possibilities are:

• base_device_size_bytes (e.g. 10MB)
• backing_filesystem (e.g. ext4)
• udev_sync_supported (e.g. true)
• data_file (e.g. /dev/loop1)
• metadata_file (e.g. /dev/loop2)
• data_loop_file (e.g. /var/lib/docker/devicemapper/data)
• metadata_loop_file (e.g. /var/lib/docker/devicemapper/metadata)
• deferred_removal_enabled (e.g. true)
• deferred_deletion_enabled (e.g. true)
• deferred_deleted_device_count (e.g. 1)
• library_version (e.g. 1.02.145 (2017-11-03))

Do we want any or all of these to be included in this PR?

[danielnelson]

It's hard to know where to draw the line between collecting everything and not enough, there isn't a correct answer really. My instinct is to add only base_device_size_bytes now and leave the rest off for later.

@akhon Do you need to monitor any of these other values?

[danielnelson]

You don't need to fix this, but this is good opportunity to show an obscure README issue. In Markdown, or maybe it's only Github Markdown, if you have multiple nested lists you will need to alter the first list character when repeating them, otherwise you get a weird bit of inconsistent vertical whitespace. To see this, in the current plugin README compare the vertical whitespace difference between the docker measurement and the docker_data. We usually use - and +.

Bad:

- foo
  - bar
- foo2
  - bar2
- foo3
  - bar3
- foo4
  - bar4

Good:

- foo
  - bar
+ foo2
  - bar2
- foo3
  - bar3
+ foo4
  - bar4

[GeorgeMac]

Oh yeah that is weird. Nice I did not know this!

[GeorgeMac]

What have you done. Now I can't unsee the whitespace 😂