Raw Device Mapping (RDM) is a VMware feature that allows a virtual machine (VM) to directly access a physical storage device (LUN - Logical Unit Number). Instead of a virtual disk (.vmdk), the VM can use a raw disk for better performance, flexibility, and integration with certain storage-level applications.
🔧 How to Configure RDM in VMware
Step 1: Identify the LUN
- Ensure the storage device (LUN) is properly connected to the ESXi host.
- Use
esxcli storage core device listto check available LUNs.
Step 2: Attach RDM to a VM
- Open VMware vSphere Client and navigate to the VM.
- Right-click on the VM → Edit Settings.
- Click Add New Device → RDM Disk.
- Select the LUN from the available list.
- Choose Compatibility Mode:
- Physical (Pass-through) Mode – Allows direct access to SAN features.
- Virtual Mode – Provides snapshot and cloning capabilities.
- Confirm settings and save.
Step 3: Verify Configuration
- SSH into the ESXi host and use
ls -l /vmfs/devices/disks/to check mappings. - Inside the VM, use
fdisk -l(Linux) or Disk Management (Windows) to verify disk availability.
✅ Pros of RDM
💡 Better Performance – Direct access to physical LUNs reduces overhead.
🔗 SAN-Level Features – Leverages native SAN functionalities like replication and snapshots.
🛠 Cluster Compatibility – Works well with Microsoft Clustering Services (MSCS).
🗄 Large Disk Support – Overcomes VMFS file size limitations.
❌ Cons of RDM
📦 Storage Complexity – Requires careful planning for LUN assignments.
🚀 Less vSphere Integration – Some vSphere features (e.g., vMotion in physical mode) may be restricted.
⚙ Backup Limitations – VMware snapshots do not work in Physical Mode.
📊 Scalability Issues – Managing multiple RDMs can become cumbersome.
🎯 When to Use RDM?
✅ Databases that require direct disk access for high performance.
✅ SAN-based replication where VMFS is not suitable.
✅ Microsoft Failover Clusters (MSCS) requiring shared disks.
🎨 Conclusion
RDM is a powerful feature in VMware that offers direct disk access for VMs, enabling better performance and SAN integration. However, it comes with management complexities and limited VMware functionality in certain cases. Use it wisely based on your workload requirements! 🚀🔍
