When working with KVM virtual machines (especially Windows guests), many administrators encounter a frustrating reality: traditional shrinking methods like qemu-img convert -c often fail to deliver significant space savings. The ModernIE project demonstrates what's theoretically possible - Windows 7 VMs under 10GB - while our own attempts yield 80GB+ images even after extensive optimization.

The key to effective compression lies in understanding how KVM handles storage:

1. Guest OS layer (NTFS allocation)
2. Virtual disk format (qcow2/raw)
3. Host filesystem (ext4/xfs/zfs)
4. Physical storage (SSD/HDD)

Here's the complete workflow I've developed through extensive testing:

# Inside Windows guest (PowerShell):
Optimize-Volume -DriveLetter C -ReTrim -Defrag -SlabConsolidate
sdelete64.exe -z c:

# On KVM host:
virsh shutdown win7-vm
qemu-img convert -O qcow2 -c \
    -o cluster_size=512K,preallocation=metadata \
    win7-original.qcow2 win7-compressed.qcow2
virt-sparsify --compress --tmp ./tempdir \
    win7-compressed.qcow2 win7-final.qcow2

For truly professional results, consider these additional steps:

# 1. Remove Windows features (PowerShell):
Get-WindowsOptionalFeature -Online | 
    Where-Object {$_.State -eq "Enabled"} |
    Disable-WindowsOptionalFeature -Online -NoRestart

# 2. Clean update backups:
DISM /Online /Cleanup-Image /StartComponentCleanup /ResetBase

# 3. Optimize pagefile:
reg add "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management" \
    /v PagingFiles /t REG_MULTI_SZ /d "C:\pagefile.sys 1024 1024" /f

The snapshot preservation challenge requires a different approach:

# Create a new base image while maintaining snapshot chain:
virsh snapshot-create-as win7-vm temp-snapshot --disk-only
qemu-img convert -O qcow2 win7-original.qcow2 win7-newbase.qcow2
virsh blockpull --domain win7-vm --path /var/lib/libvirt/images/win7-original.qcow2
virsh snapshot-delete win7-vm temp-snapshot --metadata

Test results from my production environment:

The enterprise method includes Sysprep generalization, component removal, and LZMA compression at the host level.

For regular maintenance, I use this bash script:

#!/bin/bash
VM_NAME="win7-prod"
SNAPSHOT_PREFIX="temp-optimize-"

# Create temporary snapshot
virsh snapshot-create-as $VM_NAME $SNAPSHOT_PREFIX$(date +%s) \
    --disk-only --atomic --quiesce

# Get image path
IMG_PATH=$(virsh domblklist $VM_NAME | grep vda | awk '{print $2}')

# Compress image
qemu-img convert -O qcow2 -c -o cluster_size=512K "$IMG_PATH" \
    "${IMG_PATH}.optimized"

# Swap images
mv "$IMG_PATH" "${IMG_PATH}.old"
mv "${IMG_PATH}.optimized" "$IMG_PATH"

# Commit changes
virsh blockcommit $VM_NAME vda --active --pivot

# Cleanup
rm "${IMG_PATH}.old"

When working with KVM virtual machines running Windows guests, many administrators hit the same frustrating wall: traditional methods like zero-filling and qemu-img convert -c often yield disappointing results. The gap between theoretical compression potential and actual output can be massive - as seen when a 100GB disk only shrinks to 91GB despite appearing to have 60GB free space.

The secret behind ModernIE's compact Windows 7 VMs (under 10GB) lies in their specialized preparation process:

# Example of ModernIE-style optimization steps
1. sysprep /generalize /oobe /shutdown
2. sdelete -z c:
3. DISM /Online /Cleanup-Image /StartComponentCleanup /ResetBase
4. compact /compactos:always
5. Manual removal of unnecessary components (language packs, etc.)

Here's my tested approach that consistently delivers better results than basic methods:

# Inside Windows guest:
sdelete -z c:
diskpart
  select volume c
  shrink desired=10240 # Target size in MB
  exit

# On KVM host:
virt-sparsify --compress --tmp /path/to/tmp win7.qcow2 win7-compressed.qcow2
qemu-img convert -O qcow2 -c win7-compressed.qcow2 win7-final.qcow2

The snapshot preservation challenge requires a different approach. Here's how to handle it:

# Create a new base image incorporating snapshots
virsh snapshot-create-as --domain vm1 snap1
qemu-img rebase -b new_base.qcow2 vm1.qcow2
qemu-img commit vm1.qcow2
virt-sparsify --in-place vm1.qcow2

For extreme cases, consider these additional steps:

1. Convert from qcow2 to raw format before compression
2. Use xz compression with custom settings: qemu-img convert -O qcow2 -o compression_type=zstd,compression_level=19
3. Pre-allocate storage to eliminate fragmentation: fallocate -l 10G new_image.qcow2

Always verify your results with:

qemu-img info vm-image.qcow2
virt-filesystems --long -h --all -a vm-image.qcow2
du -h --apparent-size vm-image.qcow2