Skip to content

Level 4 · Performance, Troubleshooting & Capstone

Level 4 is what you reach for when something is wrong and you don't yet know what: the performance tools that tell you where time is going, the tracers that show you every syscall a process makes, the kernel module system, and the boot-recovery skills for when the system won't even start. It ends with a capstone that builds a server, breaks it on purpose, and gets it back.

What was run, and what wasn't

top, vmstat, iostat, free, strace, perf (where the kernel/VM supported perf events), module insmod/lsmod commands, and the boot-recovery steps were run on a real Linux VM. Distro comparisons in module 8 and kernel release/LTS details in module 9 are documentation-based (you can't "run" a comparison of five distributions) and are dated to October 2026; check current LTS status before relying on it long-term.

Prerequisites: Levels 1–3. The capstone assumes you're comfortable combining filesystem, process, systemd, network and security content from every earlier level.

Modules

  1. Performance Tools: top/htop, vmstat, iostat, free, and reading load average correctly (it's not CPU percent).
  2. Tracing & Profiling: strace for syscalls, ltrace for library calls, and perf for sampling where CPU time actually goes.
  3. Filesystem Performance & I/O Scheduling: I/O schedulers, iostat queue depth, and why a "slow disk" is sometimes a slow filesystem choice.
  4. Kernel Modules: loading, unloading, and building a trivial "Hello, kernel" module.
  5. /proc and /sys as Debugging Tools: reading a hung process's state, open files and memory map without a debugger.
  6. Troubleshooting Boot Failures & Recovery Mode: single-user/rescue targets, a broken fstab, and recovering from it.
  7. Containers vs VMs: where the namespaces/cgroups from Level 3 end and a hypervisor begins.
  8. Linux Distros Compared: Debian/Ubuntu, RHEL/Fedora, Arch and Alpine — init systems, package managers and release models.
  9. Kernel Versions, LTS & Staying Current: the mainline/stable/LTS split and how to tell which one you're running.
  10. Capstone: Build, Harden & Diagnose a Linux Server From Scratch: provision a VM, apply Level 1–3 hardening, then diagnose three injected faults.