Every Exadata storage server includes multiple physical disks, which can be hard disk drives (HDDs) or flash devices.
Each physical disk has a logical representation in the operating system (OS) known as a Logical Unit Number (LUN). Typically, there is a one-to-one relationship between physical disks and LUNs on all Exadata storage server models. However, on Exadata Extreme Flash (EF) storage servers with four 30.72 TB capacity-optimized flash devices, each capacity-optimized flash devices is configured with 2 LUNs, resulting in 8 LUNs on each storage server.
A cell disk reserves the space on a LUN for use by Exadata System Software. A LUN may contain only one cell disk.
With the introduction of Exascale, storage is physically organized in Exascale storage pools, each containing numerous pool disks. While one Exadata cell disk can accommodate multiple Exascale pool disks (as indicated in the diagram), this is usually unnecessary because Exascale provides additional facilities that securely share storage pool resources amongst numerous tenants.
On systems that also use Oracle ASM, you can continue to define multiple grid disks on the available space in each cell disk.