When working with Hyper-V dynamic virtual disks (VHDs) containing Linux guests, you'll encounter a common issue: Windows' native compaction tools only recognize zeroed blocks on NTFS filesystems. For Ubuntu systems using ext4, we need a different approach.

Before proceeding, ensure you have:

• A Hyper-V host running Windows Server 2008 R2 or later
• Ubuntu guest VM with ext4 filesystem
• SystemRescueCD ISO (latest version)
• Sufficient free space for temporary operations

# 1. Prepare your Ubuntu VM
sudo apt-get update
sudo apt-get install zerofree -y
sudo poweroff

Mount the SystemRescueCD ISO to your VM and boot into it. Then execute:

# 2. Run zerofree on the target partition
zerofree -v /dev/sda1

The -v flag provides verbose output so you can monitor progress. For larger disks, this may take several hours.

For servers with multiple partitions or complex setups:

# Identify all partitions
fdisk -l

# Example for multi-partition systems
zerofree /dev/sda2
zerofree /dev/sda5

After zeroing out free space:

1. Shut down the VM completely
2. In Hyper-V Manager, right-click the VM
3. Select "Edit Disk"
4. Choose "Compact" option

For frequent compaction needs, consider this bash script:

#!/bin/bash
# Ubuntu VHD compaction helper script

echo "Preparing system for compaction..."
sudo apt-get -qq update
sudo apt-get -qq install zerofree

echo "Creating zerofree script..."
cat > /tmp/zerofree_script.sh << 'EOF'
#!/bin/bash
zerofree /dev/sda1
poweroff
EOF

chmod +x /tmp/zerofree_script.sh
echo "System ready for SystemRescueCD boot"
echo "Run: /tmp/zerofree_script.sh after booting into rescue mode"

Problem: Zerofree fails with "filesystem is mounted read-write"
Solution: Ensure you're running from SystemRescueCD or another live environment

Problem: Compaction results in minimal size reduction
Solution: Check for swap partitions (use swapoff before zerofree)

When working with Hyper-V dynamic disks containing Linux partitions (particularly ext4), Windows Server's native compaction tools fail because they expect NTFS filesystems. Here's a proven workflow to reclaim disk space from Ubuntu VHDs:

• SystemRescueCD ISO (latest version)
• Hyper-V Manager with edit permissions
• Basic Linux command-line knowledge

1. Prepare the Rescue Environment:

# Download SystemRescueCD
wget https://sourceforge.net/projects/systemrescuecd/files/sysresccd-x86/latest/systemrescuecd-x86-*.iso/download

2. Attach ISO to VM:
In Hyper-V Manager:

1. Right-click Ubuntu VM → Settings
2. Add SCSI DVD Drive if not present
3. Mount SystemRescueCD ISO

3. Boot into Rescue Mode:
Set BIOS boot order to prioritize CD-ROM, then run:

# After booting, identify your root partition
fdisk -l

# Typical output for reference:
# /dev/sda1  *        2048   104857599   52427776   83  Linux

4. Zero Out Free Space:

# Unmount filesystem first
umount /dev/sda1

# Run zerofree (may take hours for large disks)
zerofree -v /dev/sda1

# Alternative if zerofree isn't available:
dd if=/dev/zero of=/zero.file bs=4M; sync; rm /zero.file

5. Finalize the Process:

1. Power off VM completely (not save state)
2. In Hyper-V Manager: Edit Disk → Compact
3. Monitor progress in Windows Disk Manager

For frequent compaction needs:

#!/bin/bash
# Auto-compact script for Hyper-V Ubuntu VHDs

echo "Preparing for VHD compaction..."
sudo apt-get install zerofree -y

echo "Creating maintenance mode script..."
cat < /usr/local/bin/prep_compact
#!/bin/sh
service apache2 stop
service mysql stop
service cron stop
umount /dev/sda1
zerofree -v /dev/sda1
shutdown -h now
EOF

chmod +x /usr/local/bin/prep_compact
echo "System ready for next Hyper-V compaction cycle"

• If compaction fails, check for:
  • LVM volumes (requires additional steps)
  • Swap partition (disable with swapoff -a)
• For LUKS encrypted partitions, decrypt before zerofree