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Oracle RAC on OCI vs On-Premises: Architecture, Performance, Cost & HA Explained

Oracle Real Application Clusters (RAC) has always been the backbone of mission-critical database availability. Banks, telecoms, ERPs, and payment platforms still rely on RAC to survive node failures without business impact.


But today, DBAs face a serious architectural decision: Do you continue running Oracle RAC in your own data center, or move it to Oracle Cloud Infrastructure (OCI)?

This is not a superficial cloud comparison. RAC is extremely sensitive to network latency, storage behavior, cluster timing, and failure handling. A wrong assumption can destabilize production systems.


In this blog, we break down Oracle RAC on OCI vs On-Premises with real DBA logic, real performance behavior, and real operational experience — not marketing slides 🚀


Oracle RAC on OCI vs On-Premises
Oracle RAC on OCI vs On-Premises


🔹 Section 1: Architecture Deep Dive – OCI RAC vs On-Prem RAC


🔸 1. Physical Infrastructure Control


On-Premises RAC


  • You own:

    • Servers

    • Network switches

    • Storage arrays

  • Full control over:

    • BIOS settings

    • NUMA layout

    • NIC bonding

    • Firmware versions

  • Full responsibility for:

    • Hardware failures

    • Vendor coordination

    • Capacity planning


OCI RAC

  • Runs on:

    • Bare Metal shapes

    • Or Exadata Cloud Service

  • Hardware is owned and managed by Oracle Cloud Infrastructure

  • DBAs still get:

    • Full root access

    • Full Grid + Database control

  • Oracle owns:

    • Physical hosts

    • Network fabric

    • Storage backend


📌 Key Difference:On-prem RAC gives control, OCI RAC gives control without hardware ownership.


🔸 2. Interconnect & Cache Fusion Behavior

RAC performance depends heavily on cache fusion latency.

On-Prem RAC

  • Typically uses:

    • InfiniBand

    • High-speed Ethernet (25/40/100 Gb)

  • Predictable latency

  • Vulnerable to:

    • Switch misconfiguration

    • NIC driver issues

    • Micro-bursts


OCI RAC

  • Uses:

    • OCI high-bandwidth private network

    • RDMA-enabled fabric

  • Extremely fast

  • Slightly less deterministic than top-tier InfiniBand

  • Far more consistent than most enterprise on-prem setups

📌 Reality Check:A well-designed OCI interconnect is often better than an average on-prem network.


🔸 3. Shared Storage Architecture


On-Prem

  • ASM on:

    • SAN (FC / iSCSI)

    • NVMe

  • DBA must manage:

    • Multipathing

    • Storage firmware

    • Performance tuning


OCI

  • Two primary models:

    • OCI Block Volumes (VM/BM RAC)

    • Exadata Smart Storage (ExaCS)

  • Storage benefits:

    • Elastic scaling

    • Predictable IOPS

    • No firmware firefighting


📌 DBA Relief:Storage issues are no longer a midnight blame game.

🔹 Section 2: Real-World Production Scenario – Node Evictions

🔸 On-Prem Scenario


A 3-node OLTP RAC system starts experiencing random node evictions.

Symptoms


  • ORA-29740

  • CSS misscount exceeded

  • Sudden instance reboots

Root Cause

  • Switch firmware bug

  • Packet loss during peak load

Resolution

  • Network team escalation

  • Vendor coordination

  • Weeks of analysis and fixes


🔸 OCI Scenario

Same workload, same RAC size, running on OCI.

What Happens

  • Node eviction detected

  • Oracle SR raised

  • Oracle identifies host-level issue

  • Host replaced

  • Cluster stabilized within hours


📌 Key Lesson:OCI drastically reduces Mean Time To Repair (MTTR).


🔹 Section 3: SQL Examples & Performance Observations

🔸 Cache Fusion Timing


SELECT inst_id,

       name,

       value

FROM gv$sysstat

WHERE name LIKE 'gc%time%'

ORDER BY inst_id;


Typical Observations

Platform

GC CR Receive Time

On-Prem High-End

~0.4 ms

OCI Bare Metal

~0.6–0.8 ms

Exadata Cloud

~0.4–0.5 ms

📌 Interpretation:OCI is slightly higher in micro-latency but extremely stable under load.

🔸 AWR Behavior Differences

OCI RAC often shows:

  • Lower DB CPU

  • Slightly higher GC waits during peaks

  • Better sustained throughput

Why?

  • CPU scheduling is optimized

  • Network fabric is shared but predictable

🔹 Section 4: Cost, Pitfalls & Best Practices

🚫 Common Mistakes

  • Treating OCI RAC like a VM-only solution

  • Ignoring fault domains

  • Over-provisioning RAC nodes

  • Migrating bad SQL and blaming cloud


✅ Best Practices

  • Use Bare Metal for real RAC workloads

  • Distribute nodes across fault domains

  • Use:

    • Services

    • FAN

    • Application Continuity

  • Monitor:

    • gc buffer busy

    • CSS misscount

  • Choose Exadata Cloud Service for extreme OLTP


📌 Golden Rule:RAC is not for scaling bad design.



🔹 Section 5: Advanced DBA Insights

  • RAC is about survivability, not node count

  • Cloud RAC shifts DBA focus:

    • Less hardware firefighting

    • More SQL engineering

  • On-prem RAC still makes sense when:

    • Regulatory isolation is mandatory

    • Ultra-low latency trading systems exist

  • OCI RAC wins when:

    • Predictable HA matters

    • Faster provisioning is required

    • Hardware lifecycle pain must disappear

📌 Hard Truth:Bad SQL performs badly everywhere.



🔹 Conclusion / Key Takeaways

  • OCI RAC is real RAC — not a compromise

  • On-prem RAC still has niche dominance

  • OCI dramatically improves operational stability

  • Exadata Cloud Service matches on-prem Exadata performance

  • The decision must be workload-driven, not emotional


Modern RAC is less about hardware and more about resilience engineering ⚙️


🔹 Learn From An Expert

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