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docs: surface the cgroup v2 limitation earlier in runtime metrics #24914

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16 changes: 13 additions & 3 deletions content/manuals/engine/containers/runmetrics.md
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Expand Up @@ -68,6 +68,14 @@ cgroup v2 is used by default on the following distributions:
- Debian GNU/Linux (since 11)
- Ubuntu (since 21.10)

> [!IMPORTANT]
>
> The detailed metrics examples later in this page describe the cgroup v1 file
> layout. On cgroup v2 hosts, use this page to find the container's cgroup
> directory and then inspect the controller files in that directory (for
> example, `memory.*`, `cpu.*`, and `io.*`). For controller semantics, refer to
> the [cgroup v2 kernel documentation](https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html).

#### cgroup v1

You can look into `/proc/cgroups` to see the different control group subsystems
Expand Down Expand Up @@ -137,12 +145,14 @@ container, take a look at the following paths:
- `/sys/fs/cgroup/docker/<longid>/` on cgroup v2, `cgroupfs` driver
- `/sys/fs/cgroup/system.slice/docker-<longid>.scope/` on cgroup v2, `systemd` driver

### Metrics from cgroups: memory, CPU, block I/O
### Metrics from cgroups: memory, CPU, block I/O (cgroup v1)

> [!NOTE]
>
> This section isn't yet updated for cgroup v2.
> For further information about cgroup v2, refer to [the kernel documentation](https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html).
> This section documents the cgroup v1 metric files and formats.
> For cgroup v2, inspect the relevant controller files in the container's
> cgroup directory and refer to
> [the kernel documentation](https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html).

For each subsystem (memory, CPU, and block I/O), one or
more pseudo-files exist and contain statistics.
Expand Down