The performance difference between shared and dedicated I/O adapters in IBM Power environments mainly comes down to latency, throughput, CPU usage, and contention. Here’s a clear breakdown of the real impact.

⚙️ 1. Architecture Difference

Shared I/O (via Virtual I/O Server (VIOS))

• Multiple LPARs share the same physical adapter

• I/O path:

  LPAR → Hypervisor → VIOS → Adapter
  

Dedicated I/O

• Adapter is exclusively assigned to one LPAR

• I/O path:

  LPAR → Adapter (direct)
  

🚀 2. Performance Impact

🔹 Latency

• Shared I/O:
  • Extra hops + software stack
  • ~10–50 µs additional latency
• Dedicated I/O:
  • Near hardware latency (lowest possible)

👉 Impact:

• Critical for OLTP, real-time, low-latency apps

🔹 Throughput

• Shared I/O:
  • Slightly reduced due to VIOS mediation
  • Possible contention between LPARs
• Dedicated I/O:
  • Full adapter bandwidth available

👉 Typical difference:

• Shared: ~90–95% of max
• Dedicated: ~100% (line rate)

🔹 CPU Overhead

• Shared I/O:
  • VIOS consumes CPU cycles
  • Extra context switching
• Dedicated I/O:
  • Minimal CPU overhead (no VIOS layer)

👉 Impact:

• High packet-rate workloads benefit from dedicated

🔹 Contention & Scalability

• Shared I/O:
  • Performance depends on:
    • Number of LPARs sharing
    • VIOS sizing
  • Risk of bottlenecks if oversubscribed
• Dedicated I/O:
  • No contention
  • Predictable performance

📊 Summary Table

⚡ When Each Makes Sense

✅ Shared I/O is best for:

• Consolidated environments
• Cloud / multi-tenant setups
• Workloads needing:
  • Live Partition Mobility
  • Dynamic scaling
• Efficient adapter utilization

✅ Dedicated I/O is best for:

• High-performance databases (Oracle, SAP HANA)
• Low-latency trading systems
• High-throughput networking/storage
• Workloads sensitive to jitter

🔄 Middle Ground: SR-IOV

Between these two extremes, SR-IOV offers:

• Near-dedicated performance
• Some virtualization flexibility
• Lower overhead than shared I/O

🧠 Key Insight

• Shared I/O = efficiency + flexibility (small performance cost)
• Dedicated I/O = maximum performance + isolation (less flexibility)

Modern systems using IBM PowerVM often combine all three:

• Shared I/O for general workloads
• SR-IOV for high-speed networking
• Dedicated I/O for critical applications