Dell PowerVault ME4 Series Storage System Administrator’s Guide

About virtual and linear storage

This product uses two different storage technologies that share a common user interface. One uses the virtual method while the other one uses the linear method.

Virtual storage is a method of mapping logical storage requests to physical storage (disks). It inserts a layer of virtualization such that logical host I/O requests are mapped onto pages of storage. Each page is then mapped onto physical storage. Within each page the mapping is linear, but there is no direct relationship between adjacent logical pages and their physical storage.

A page is a range of contiguous Logical Block Addresses (LBAs) in a disk group, which is one of up to 16 RAID sets that are grouped into a pool. Thus, a virtual volume as seen by a host represents a portion of storage in a pool. Multiple virtual volumes can be created in a pool, sharing its resources. This allows for a high level of flexibility, and the most efficient use of available physical resources.

Virtual storage provides the foundation for data-management features such as thin provisioning, automated tiered storage, SSD read cache, and the quick rebuild feature.

The linear method maps logical host requests directly to physical storage. In some cases the mapping is one-to-one, while in most cases the mapping is across groups of physical storage devices, or slices of them. This linear method of mapping is highly efficient. The negative side of linear mapping is lack of flexibility. This makes it difficult to alter the physical layout after it is established.