With vVols, an individual virtual machine, not the datastore, becomes a unit of storage management, while storage hardware gains complete control over virtual disk content, layout, and management.
Historically, vSphere storage management used a datastore-centric approach. With this approach, storage administrators and vSphere administrators discuss in advance the underlying storage requirements for virtual machines. The storage administrator then sets up LUNs or NFS shares and presents them to ESXi hosts. The vSphere administrator creates datastores based on LUNs or NFS, and uses these datastores as virtual machine storage. Typically, the datastore is the lowest granularity level at which data management occurs from a storage perspective. However, a single datastore contains multiple virtual machines, which might have different requirements. With the traditional approach, it is difficult to meet the requirements of an individual virtual machine.
The vVols functionality helps to improve granularity. It helps you to differentiate virtual machine services on a per application level by offering a new approach to storage management. Rather than arranging storage around features of a storage system, vVols arranges storage around the needs of individual virtual machines, making storage virtual-machine centric.
vVols maps virtual disks and their derivatives, clones, snapshots, and replicas, directly to objects, called virtual volumes, on a storage system. This mapping allows vSphere to offload intensive storage operations such as snapshot, cloning, and replication to the storage system.
By creating a volume for each virtual disk, you can set policies at the optimum level. You can decide in advance what the storage requirements of an application are, and communicate these requirements to the storage system. The storage system creates an appropriate virtual disk based on these requirements. For example, if your virtual machine requires an active-active storage array, you no longer must select a datastore that supports the active-active model. Instead, you create an individual virtual volume that is automatically placed to the active-active array.