PhysicalDisk → CellDisk → GridDisk on Exadata

Here’s a clear explanation of the relationship between physical disks, cell disks, and grid disks in Oracle Exadata:

🔁 Relationship Overview

PhysicalDisk → CellDisk → GridDisk

Think of it as a layered storage abstraction:

📦 1. PhysicalDisk

• This is the real hardware inside the storage server.
• Types: High Capacity (HC), High Performance (HP), or Flash.
• You can view them with: cellcli> list physicaldisk

🧱 2. CellDisk

• Created on top of PhysicalDisks by Exadata’s cell services.
• Represents a managed region of the physical disk.
• Automatically created when the Exadata cell is deployed.
• It is not directly used by ASM or databases.
• View with: cellcli> list celldisk

🧩 3. GridDisk

• Created from CellDisks.
• These are the actual units presented to ASM (Automatic Storage Management).
• GridDisks are what the database sees and uses for storage.
• View with: cellcli> list griddisk

📊 Real-World Example

Imagine a 1.2TB disk:

• PhysicalDisk: 1.2TB hardware disk.
• CellDisk: 1.2TB logical unit managed by Exadata.
• GridDisk: This 1.2TB can be sliced into:
  • 512GB for DATA
  • 512GB for RECO
  • 176GB left for FLASH or reserved

So one physical disk may result in:

• One CellDisk
• Multiple GridDisks (e.g., DATA_CD_01, RECO_CD_01, etc.)

💡 Summary Table