Chapter 4: Comparison

VM vs Container: Side-by-Side Comparison

A comprehensive comparison of Virtual Machines and Containers across performance, security, use cases, and practical considerations.

VM vs Container: Side-by-Side Comparison

Now that you understand both technologies, let's compare them directly to help you make informed decisions.

Architecture Comparison

        VIRTUAL MACHINE                    CONTAINER
┌─────────────────────────┐    ┌─────────────────────────┐
│      Application        │    │      Application        │
├─────────────────────────┤    ├─────────────────────────┤
│      Guest OS           │    │      Libraries Only     │
│   (Full Linux/Windows)  │    │    (No full OS)         │
├─────────────────────────┤    ├─────────────────────────┤
│    Virtual Hardware     │    │   Container Runtime     │
├─────────────────────────┤    ├─────────────────────────┤
│      Hypervisor         │    │      Host OS Kernel     │
├─────────────────────────┤    │      (Shared)           │
│      Host OS            │    │                         │
├─────────────────────────┤    ├─────────────────────────┤
│      Hardware           │    │      Hardware           │
└─────────────────────────┘    └─────────────────────────┘

Performance Comparison

Metric Virtual Machine Container
Startup Time 30 seconds - 5 minutes < 1 second
Memory Overhead 512MB - 2GB per VM 5-50MB per container
Disk Space 10-100GB per VM 100MB - 1GB per image
CPU Overhead 5-15% < 1%
Network Latency Higher (virtualized NIC) Near-native
Disk I/O 80-95% of native 95-100% of native

Resource Density

On a server with 128GB RAM and 32 CPU cores:

VMs (with 4GB RAM each):
┌──────┬──────┬──────┬──────┐
│ VM1  │ VM2  │ VM3  │ ...  │  ≈ 25-30 VMs max
│ 4GB  │ 4GB  │ 4GB  │      │
└──────┴──────┴──────┴──────┘

Containers (with 512MB each):
┌────┬────┬────┬────┬────┬────┬────┬────┐
│ C1 │ C2 │ C3 │ C4 │ C5 │ C6 │...│    │  ≈ 100-200 containers
└────┴────┴────┴────┴────┴────┴────┴────┘

Security Comparison

Isolation Level

Aspect VM Container
Kernel Separate Shared
Attack Surface Smaller Larger
Escape Difficulty Very Hard Harder but possible
CVE Impact Limited to VM May affect all containers

When Security Favors VMs

  • Multi-tenant environments with untrusted workloads
  • Running legacy or unpatched software
  • Compliance requirements (PCI-DSS, HIPAA)
  • Isolating different customers completely

When Security is Adequate with Containers

  • Single-tenant environments
  • Trusted code from your organization
  • Modern, patched container images
  • With additional hardening (SELinux, AppArmor, gVisor)

Portability Comparison

VMs

Export VM → OVA/VMDK file → Import to new hypervisor
                             (May need conversion)
  • Portable between same hypervisor type
  • Conversion needed between different platforms
  • Large file sizes make transfers slow

Containers

Build image → Push to registry → Pull anywhere
(docker build)  (docker push)    (docker pull)
  • Runs anywhere Docker/containerd exists
  • Small images transfer quickly
  • "Build once, run anywhere"

Development Workflow

With VMs

  1. Request VM from IT (hours/days)
  2. Install OS and dependencies
  3. Configure development environment
  4. Hope production matches

With Containers

  1. Write Dockerfile
  2. Build image locally
  3. Same image runs in dev/staging/production
  4. Share via container registry
# Development = Production
FROM python:3.11
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . /app
CMD ["python", "/app/main.py"]

Operational Comparison

Operation VM Container
Scaling Out Minutes Seconds
Updates In-place updates Replace with new image
Rollback From snapshots Pull previous image version
Monitoring Traditional tools Container-native tools
Logging Per-VM log files Centralized (stdout)

Cost Comparison

VMs

  • OS licensing costs (Windows)
  • Lower density = more hardware
  • More management overhead

Containers

  • No OS licensing per container
  • Higher density = less hardware
  • Requires orchestration expertise

Decision Framework

Choose VMs When:

  • Running different operating systems (Windows + Linux)
  • Strong isolation is required (multi-tenant)
  • Legacy applications that need specific OS versions
  • Full OS access is required
  • Compliance mandates VM-level isolation

Choose Containers When:

  • Deploying microservices
  • Need fast scaling and updates
  • Development and production parity is important
  • Maximizing hardware utilization
  • Building cloud-native applications

Hybrid Approach

Many organizations use both:

┌──────────────────────────────────────────┐
│              Physical Server              │
├──────────────────────────────────────────┤
│    VM 1 (Production)    │   VM 2 (Dev)   │
│  ┌─────────────────┐    │  ┌──────────┐  │
│  │   Kubernetes    │    │  │  Docker  │  │
│  │  ┌─────┬─────┐  │    │  │  ┌────┐  │  │
│  │  │ C1  │ C2  │  │    │  │  │ C1 │  │  │
│  │  └─────┴─────┘  │    │  │  └────┘  │  │
│  └─────────────────┘    │  └──────────┘  │
└──────────────────────────────────────────┘

VMs provide the isolation layer, containers run inside for efficiency.

What's Next?

In the final chapter, we'll focus specifically on GPU workloads - how VMs and Containers handle GPUs, and what's best for AI/ML applications.