The trade-off between memory mirroring and usable capacity is essentially:

🔐 Higher reliability and fault tolerance vs 📦 Reduced available memory for workloads

Let’s break it down clearly in the context of enterprise systems like IBM POWER10 servers.

🧠 What memory mirroring does

Memory mirroring maintains two identical copies of data in separate memory regions (or DIMMs):

• Every write → duplicated to both copies
• Reads → typically from one copy (or best-performing one)

👉 If one memory module fails, the system continues using the other copy without interruption.

📉 1. Usable capacity trade-off (the biggest cost)

• With full mirroring:
  • 50% of physical memory is usable
  • Example:
    • 1 TB installed → only ~512 GB usable

👉 You sacrifice half your RAM for redundancy.

🔐 2. Reliability gain

• Protects against:
  • DIMM failures
  • Memory controller/channel faults
• Enables:
  • Zero downtime for memory failures
  • Higher system availability (critical for banking, telecom, etc.)

👉 This is why it's common in mission-critical workloads.

⚡ 3. Performance trade-offs

🟢 Potential benefits:

• Some systems can:
  • Read from the “best” copy (lower latency path)
  • Improve resilience to soft errors

🔴 Costs:

• Write overhead:
  • Every write must go to two locations
• Memory bandwidth consumption doubles for writes

👉 Usually a small overhead, but noticeable in memory-intensive workloads.

🔄 4. Impact on memory bandwidth

• Write bandwidth is effectively split across mirrored copies
• Under heavy write workloads:
  • Memory subsystem may become a bottleneck sooner

🧩 5. Interaction with virtualization (PowerVM)

With PowerVM:

• Mirroring is handled at hardware/firmware level
• Partitions (LPARs) see:
  • Reduced available memory
  • But higher reliability guarantees

👉 No application changes needed

⚖️ Trade-off summary

🧠 When to use memory mirroring

✔ Recommended for:

• Banking systems
• Core databases (e.g., OLTP)
• Systems requiring continuous uptime

❌ Avoid for:

• Memory-heavy analytics (where capacity matters more)
• HPC workloads
• Cost-sensitive environments

🧠 Big insight

Memory mirroring is a classic enterprise trade-off: