6.5. Reflection Checkpoint

System and Process:

• What is the difference between high %cpu and high %iowait in top? Which diagnostic tool distinguishes them?
• A process is in D state (uninterruptible sleep). Can you kill it? What usually causes this?
• What command shows logs from the boot before the current one? Why is this useful?
• How does the OOM killer decide which process to kill? How do you protect a critical process?

Storage:

• A disk shows 60% full with df -h but writes fail with "No space left on device." What do you check next?
• cat /proc/mdstat shows a RAID1 array as "degraded." What does this mean and what are the next steps?
• Before running fsck on a corrupted filesystem, what must you do first and why?

Network:

• "Connection refused" vs "Connection timed out" — what does each indicate about where the problem is?
• DNS resolution fails but ping 8.8.8.8 works. What does this narrow the problem to?
• Write the sequence of commands to diagnose a "cannot reach server.example.com" error, layer by layer.

Application:

• A service fails to start and you see "Address already in use." What two commands identify the conflicting process?
• What command tests nginx configuration syntax before reloading?
• What is RTO and how does backup strategy affect it?