Command

Systems

HDFS NameNode Metadata Checkpointing: Resolving Sync Issues
ByEric Ma Sep 9, 2017Apr 11, 2026

The Secondary NameNode periodically merges the fsimage and edits log files to keep the edits log manageable and consolidate metadata. When this checkpointing fails, you’ll see errors like: ERROR org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode: Exception in doCheckpoint java.io.IOException: Inconsistent checkpoint fields. This typically indicates a mismatch between the NameNode and Secondary NameNode metadata states, often from unclean shutdowns or…

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Systems

QEMU/KVM Networking: TAP, virtio, and vhost Explained
ByWeiwei Jia Aug 26, 2017Apr 11, 2026

QEMU/KVM networking operates across multiple layers — TAP devices, QEMU’s packet queuing, virtio descriptors, and kernel bypass mechanisms. Understanding this stack is essential for debugging connectivity issues and tuning performance in production environments. TAP Device Fundamentals TAP (TUN/TAP) devices create a bridge between the guest and host network stack. A typical QEMU/KVM configuration looks like:…

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Linux

Building and Installing a Linux Kernel from Source
ByEric Ma Mar 18, 2017Apr 11, 2026

Grab a release from kernel.org. Recent stable versions (6.x and later) are recommended for production systems: wget https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.gz tar xf linux-6.12.tar.gz cd linux-6.12/ If wget isn’t available, use curl: curl -O https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.gz Verify the tarball Always verify the kernel’s integrity using checksums and signatures published on kernel.org. This is essential for production systems: wget https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.gz.asc…

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Linux

Linux Kernel’s taskstats: Per-Task Performance Metrics
ByWeiwei Jia Feb 14, 2017Apr 11, 2026

The Linux kernel provides the taskstats API to monitor detailed per-task statistics, including I/O wait time, CPU scheduling delays, and memory reclamation waits. Understanding how these metrics are collected at the kernel level is essential for diagnosing performance issues and understanding process behavior. How the Kernel Records I/O Delays The kernel records I/O wait delays…

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Programming

Getting Started: How to Choose Your First Programming Language
ByJoseph Macwan Feb 6, 2017Apr 11, 2026

Picking your first programming language matters, but it’s not the career-defining decision some make it out to be. What matters more is that you pick something reasonable and start building. That said, some choices will save you months of frustration. Know What You’re Optimizing For Before evaluating languages, answer these questions honestly: What do you…

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Linux

Mastering Vim: A Practical Guide to vimtutor
ByEric Ma Dec 31, 2016Apr 11, 2026

vimtutor is the most practical way to learn Vim. It’s an interactive lesson built directly into your editor that teaches through practice rather than passive reading. You launch it from the terminal with no setup required, and you get immediate feedback on every command you type. Running vimtutor From any terminal: vimtutor This opens an…

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Programming

Building Java CLI Applications with Spring Shell 3.x
ByJohnnymorgan Dec 23, 2016Apr 11, 2026

Spring Shell is a framework for building command-line interfaces (CLIs) in Java applications. It provides structured command definition, argument parsing, tab completion, and history management—all integrated with the Spring ecosystem. Requirements and Setup Spring Shell 3.2+ requires Java 17+ and Spring Boot 3.2+. If you’re still on older versions, Spring Shell 2.x supports Java 8…

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Linux

Reducing QEMU/KVM Lock Contention with IOThreads and Dataplane
ByWeiwei Jia Dec 18, 2016Apr 11, 2026

QEMU/KVM uses global locks to synchronize threads, which is necessary for correctness but causes performance problems at scale. When multiple I/O requests and vCPU threads contend for the same lock, you get lock contention that manifests as scheduling jitter, reduced system scalability, and degraded I/O performance. IOThreads (the modern implementation of dataplane) address this by…

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Linux

Measuring CFS Scheduler Timeslices in Linux
ByWeiwei Jia Dec 5, 2016Apr 11, 2026

CFS (Completely Fair Scheduling) is the default process scheduler in modern Linux kernels. Understanding actual timeslice allocation—how long a process runs before being context-switched out—is essential for performance debugging, verifying fairness assumptions, and identifying scheduling anomalies. The challenge is that /proc interfaces and ps commands show scheduling policy but not the actual on-CPU duration per…

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Programming

GNOME Shell Extensions to Enhance GNOME 45
ByEric Ma Oct 22, 2016Apr 11, 2026

GNOME Shell’s clean, minimal design works well for many users, but the extension ecosystem lets you adapt it to your actual workflow. Here are extensions worth considering for modern GNOME (45+). AlternateTab Changes Alt-Tab behavior from application-grouped switching to per-window switching — more like traditional window managers. If GNOME’s default grouping feels disruptive, this restores…

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Linux

Building 32-bit Wine on CentOS 7, Rocky Linux, and AlmaLinux
ByEric Ma Oct 16, 2016Apr 11, 2026

RHEL-based distributions only provide 64-bit Wine packages in their standard repositories. Many Windows applications—especially legacy software—require 32-bit Wine. Building Wine from source is the most reliable approach to get a working 32-bit installation alongside 64-bit support. Prerequisites and Resource Requirements Building Wine requires significant resources: ~2GB temporary disk space, ~500MB installed Compilation takes 20-40 minutes…

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Linux

Switching Linux Systemd Boot Targets
ByEric Ma Sep 17, 2016Apr 11, 2026

Systemd replaced traditional init on most modern Linux distributions, shifting terminology from “runlevels” to “targets.” If you’re migrating from older systems, here’s the mapping: Runlevel Systemd Target Purpose 0 poweroff.target Shutdown 1 rescue.target Single-user mode 2, 3, 4 multi-user.target Multi-user, no GUI 5 graphical.target Multi-user with GUI 6 reboot.target Reboot The two targets you’ll use…

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Linux

Changing Your Linux Password Over SSH
ByEric Ma Aug 7, 2016Apr 11, 2026

Open a terminal and connect to the server: ssh USER@HOST Replace USER with your username and HOST with the server’s hostname or IP address. On first connection, SSH will display the host key fingerprint: The authenticity of host ‘HOST (IP_ADDRESS)’ can’t be established. ECDSA key fingerprint is SHA256:xxxxxxxxxxxx. Are you sure you want to continue…

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Linux

How to Unmount a Busy Filesystem in Linux
ByEric Ma May 21, 2016Apr 11, 2026

When you try to unmount a filesystem with umount, Linux may report “device is busy.” This is a safety mechanism — the kernel blocks unmounting when processes or kernel threads are actively accessing the partition or holding open files on it. Forcing an unmount without understanding what’s using the filesystem risks data corruption and data…

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Linux

How to Enable and Disable Your Laptop Keyboard in Linux
ByEric Ma May 14, 2016Apr 11, 2026

When you dock a laptop with an external USB keyboard, the built-in keyboard becomes a nuisance—it’s too easy to brush keys while typing and trigger unwanted input. Rather than disabling the keyboard at the BIOS level or physically disconnecting it, you can use the xinput tool to disable and re-enable it from within your Linux…

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Programming

Finding Your Bash Script’s Location: $0 vs ${BASH_SOURCE[0]}
ByEric Ma Feb 14, 2016Apr 11, 2026

When a Bash script needs to reference its own location — for sourcing additional files, logging, or managing relative paths — $0 seems like the obvious choice. But it’s a trap: $0 behaves differently depending on whether your script is executed directly or sourced. The Problem with $0 When you execute a script directly: ./script.sh…

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Linux

Configuring a Static IP Address on Fedora, RHEL, and CentOS
ByEric Ma Jan 30, 2016Apr 11, 2026

NetworkManager is the default network management tool on modern Fedora, RHEL, and CentOS systems. You can configure static IPs either through the CLI or by editing configuration files directly. This guide covers both approaches for Fedora 41+ and RHEL 9+. Identify your network interface Start by listing all available network interfaces: ip link show or…

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Programming

Running Commands Across Multiple Linux Servers in Parallel
ByEric Ma Jan 3, 2016Apr 11, 2026

Running commands across multiple servers is fundamental to cluster management and infrastructure automation. While orchestration tools like Ansible and Terraform dominate modern deployments, sometimes you need lightweight solutions that work without dependencies—or you’re already invested in SSH-based infrastructure. When managing clusters, you typically work in two modes: serial execution (one node at a time) and…

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Programming

Getting Started with Linux for Developers
ByEric Ma Nov 29, 2015Apr 11, 2026

Before writing code, get comfortable with fundamental Linux commands. You’ll use these daily: File operations: ls, cd, cat, less Text search: grep, ripgrep (rg) Remote access: ssh, scp, rsync Session management: tmux or screen for persistent terminal sessions Shell: bash (default on most distributions), though zsh and fish are popular alternatives Start with built-in documentation….

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Programming

Optimizing Large File Uploads to S3 with AWS CLI
ByEric Ma Nov 29, 2015Apr 11, 2026

Install AWS CLI v2 through your system’s package manager for production deployments. On Debian/Ubuntu: sudo apt-get update sudo apt-get install awscli On RHEL/CentOS/Fedora: sudo dnf install awscli Or use the official installer directly: curl “https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip” -o “awscli-v2.zip” unzip awscli-v2.zip sudo ./aws/install Verify installation: aws –version Configure AWS CLI credentials Set up credentials interactively: aws configure…

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