🧹 Ubuntu Disk Cleanup Guide (Docker + KVM/libvirt)

📌 Overview

This guide helps you quickly identify and clean up disk space on an Ubuntu system, focusing on:

• Docker

• KVM / libvirt virtual machines

• System logs

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🔍 1. Identify Disk Usage

Check top-level disk usage

sudo du -h / --max-depth=1 | sort -hr

Drill into /var

sudo du -h /var --max-depth=1 | sort -hr

Drill into /var/lib

sudo du -h /var/lib --max-depth=1 | sort -hr

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🐳 2. Docker Cleanup

Check Docker size

sudo du -sh /var/lib/docker

Clean unused resources

sudo docker system prune -a -f

Clear container logs

sudo truncate -s 0 /var/lib/docker/containers/*/*.log

Limit future log growth

Edit:

sudo nano /etc/docker/daemon.json

Add:

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "100m",
    "max-file": "3"
  }
}

Restart Docker:

sudo systemctl restart docker

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🖥️ 3. KVM / libvirt Disk Management

Check VM disk usage

sudo du -h /var/lib/libvirt/images | sort -hr | head -20

List virtual machines

virsh list --all

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🧹 Delete unused VM (frees space immediately)

virsh undefine <vm-name> --remove-all-storage

⚠️ This permanently deletes the VM and its disk.

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🔍 Inspect disk usage (important)

qemu-img info /var/lib/libvirt/images/<file>.qcow2

Key fields:

• virtual size → max allocated size

• disk size → actual used space

Interpretation:

Case	Meaning
disk size ≈ virtual size	Disk is full (no easy recovery)
disk size << virtual size	Can reclaim space

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🧊 Compact a VM disk

Step 1: Clean inside VM

Inside the VM:

df -h
du -h / --max-depth=1 | sort -hr

Remove unnecessary files.

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Step 2: Zero free space

dd if=/dev/zero of=zero.fill bs=1M
rm zero.fill

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Step 3: Compact image

qemu-img convert -O qcow2 old.qcow2 compacted.qcow2

Replace original file after verification.

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🧪 Find orphaned disks

ls /var/lib/libvirt/images
virsh list --all

👉 Delete disks not attached to any VM (after verification).

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📦 4. System Logs Cleanup

Check journal size

journalctl --disk-usage

Clean logs (time-based)

sudo journalctl --vacuum-time=7d

Clean logs (size-based)

sudo journalctl --vacuum-size=500M

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📁 5. General Cleanup Targets

Logs

sudo du -sh /var/log

Cache

sudo du -sh /var/cache

Clean:

sudo apt clean

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Snap cleanup

snap list --all

Remove old revisions:

sudo snap remove <package> --revision=<old_revision>

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🔧 6. Useful Tools

ncdu (interactive disk analyzer)

sudo apt install ncdu
sudo ncdu /

👉 Best tool for quickly spotting large files and directories.

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🚀 7. Quick Cleanup Script

# Docker logs
sudo truncate -s 0 /var/lib/docker/containers/*/*.log

# Docker unused data
sudo docker system prune -a -f

# System logs
sudo journalctl --vacuum-time=7d

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⚠️ Important Warnings

• ❌ Do NOT delete /var/lib/docker manually

• ❌ Do NOT delete .qcow2 files blindly

• ❌ Always verify before removing VMs

• ✅ Use virsh for VM management

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💡 Key Takeaways

• Large /var → usually Docker or VM disks

• Large /var/lib/libvirt → VM storage (real data)

• Logs are rarely the main issue at TB scale

• Biggest gains come from:

• deleting unused VMs

• cleaning inside VMs

• pruning Docker

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✅ Recommended Workflow

1. Identify large directories (du, ncdu)

2. Check /var/lib

3. Investigate:

4. Docker

5. libvirt

6. Remove unused VMs or data

7. Clean logs and caches

8. Prevent future growth (log rotation)

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End of Guide