Diagnose a slow Linux server: CPU, disk, network and logs

A troubleshooting round in several scenarios. For each, say which tools you reach for, what the numbers tell you and how you get from the host down to the cause.

1. CPU looks high: go from an overview such as htop to the process, thread and code path responsible.
2. Storage I/O is suspected: read iostat-style metrics (queue depth, latency, utilization), find the workload behind the I/O and decide whether the disk is really the bottleneck.
3. Network throughput is low, including when the service runs in a container: isolate where the slowdown is.
4. A server is being overwhelmed and logging may be the cause: trace it, stabilize the service, and keep it from happening again.