Note in docs about interpreting IO stats when running in docker

[lukewhiting]

This PR adds documentation describing possible inaccuracies in the FS statistics from the _nodes/stats endpoint when running inside a container

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[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[DaveCTurner]

I think it'd be better to use more neutral wording here. The stats we expose aren't inaccurate, they are an accurate reflection of something that might not be exactly what you expect.

Maybe we should call out explicitly that these stats come from /proc/diskstats and leave it up to the user to worry about whether that's what they want or not.

[lukewhiting]

I think it'd be better to use more neutral wording here. The stats we expose aren't inaccurate, they are an accurate reflection of something that might not be exactly what you expect.

Maybe we should call out explicitly that these stats come from /proc/diskstats and leave it up to the user to worry about whether that's what they want or not.

I think you are right about more natural language but I do want to try and highlight this specific example as not all users may be immediately familiar with /proc/diskstats and the implications of accessing it within a container.

How about:

NOTE: These statistics are derived from `proc/diskstats` and represent the underlying host
filesystem even when running within a container therefore extra care is needed to ensure
they are correctly interpreted in such circumstances.

[DaveCTurner]

Nit: these stats relate to block devices rather than filesystems. Also I'd rather avoid the passive voice ("care is needed", "are correctly interpreted"). How about this?

NOTE: These statistics are derived from the `/proc/diskstats` kernel interface.
This interface accounts for IO performed by all processes on the system, even
if you are running {es} within a container.

[lukewhiting]

Looks good. Have made that change. Thanks for the assist!

[DaveCTurner]

LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x