vSphere supports various storage options and functionalities in traditional and software-defined storage environments. A high-level overview of vSphere storage elements and aspects helps you plan a proper storage strategy for your virtual data center.

Traditional Storage Virtualization Models in vSphere Environment

Generally, storage virtualization refers to a logical abstraction of physical storage resources and capacities from virtual machines and their applications. ESXi provides host-level storage virtualization.

In vSphere environment, a traditional model is built around the following storage technologies and ESXi and vCenter Server virtualization functionalities.
Local and Networked Storage
In traditional storage environments, the ESXi storage management process starts with storage space that your storage administrator preallocates on different storage systems. ESXi supports local storage and networked storage.
See What Types of Physical Storage Does ESXi Support.
Storage Area Networks
A storage area network (SAN) is a specialized high-speed network that connects computer systems, or ESXi hosts, to high-performance storage systems. ESXi can use Fibre Channel or iSCSI protocols to connect to storage systems.
See Using ESXi with a SAN.
Fibre Channel
Fibre Channel (FC) is a storage protocol that the SAN uses to transfer data traffic from ESXi host servers to shared storage. The protocol packages SCSI commands into FC frames. To connect to the FC SAN, your host uses Fibre Channel host bus adapters (HBAs).
See Using ESXi with Fibre Channel SAN.
Internet SCSI
Internet iSCSI (iSCSI) is a SAN transport that can use Ethernet connections between computer systems, or ESXi hosts, and high-performance storage systems. To connect to the storage systems, your hosts use hardware iSCSI adapters or software iSCSI initiators with standard network adapters.
See Using ESXi with iSCSI SAN.
Storage Device or LUN
In the ESXi context, the terms storage device and LUN are used interchangeably. Typically, both terms mean a storage volume that is presented to the host from a block storage system and is available for formatting.
See Target and Device Representations and Managing Storage Devices.
Virtual Disks
A virtual machine on an ESXi host uses a virtual disk to store its operating system, application files, and other data associated with its activities. Virtual disks are large physical files, or sets of files, that can be copied, moved, archived, and backed up as any other files. You can configure virtual machines with multiple virtual disks.
To access virtual disks, a virtual machine uses virtual SCSI controllers. These virtual controllers include BusLogic Parallel, LSI Logic Parallel, LSI Logic SAS, and VMware Paravirtual. These controllers are the only types of SCSI controllers that a virtual machine can see and access.
Each virtual disk resides on a datastore that is deployed on physical storage. From the standpoint of the virtual machine, each virtual disk appears as if it were a SCSI drive connected to a SCSI controller. Whether the physical storage is accessed through storage or network adapters on the host is typically transparent to the VM guest operating system and applications.
VMware vSphere ® VMFS
The datastores that you deploy on block storage devices use the native vSphere Virtual Machine File System (VMFS) format. It is a special high-performance file system format that is optimized for storing virtual machines.
See Understanding VMFS Datastores.
NFS
An NFS client built into ESXi uses the Network File System (NFS) protocol over TCP/IP to access an NFS volume that is located on a NAS server. The ESXi host can mount the volume and use it as an NFS datastore.
See Understanding Network File System Datastores.
Raw Device Mapping
In addition to virtual disks, vSphere offers a mechanism called raw device mapping (RDM). RDM is useful when a guest operating system inside a virtual machine requires direct access to a storage device. For information about RDMs, see Raw Device Mapping.

Software-Defined Storage Models

In addition to abstracting underlying storage capacities from VMs, as traditional storage models do, software-defined storage abstracts storage capabilities.

With the software-defined storage model, a virtual machine becomes a unit of storage provisioning and can be managed through a flexible policy-based mechanism. The model involves the following vSphere technologies.

VMware vSphere ® Virtual Volumes™ (vVols)
The Virtual Volumes functionality changes the storage management paradigm from managing space inside datastores to managing abstract storage objects handled by storage arrays. With Virtual Volumes, an individual virtual machine, not the datastore, becomes a unit of storage management. And storage hardware gains complete control over virtual disk content, layout, and management.
See Working with VMware vSphere Virtual Volumes.
VMware vSAN
vSAN is a distributed layer of software that runs natively as a part of the hypervisor. vSAN aggregates local or direct-attached capacity devices of an ESXi host cluster and creates a single storage pool shared across all hosts in the vSAN cluster.
See Administering VMware vSAN.
Storage Policy Based Management
Storage Policy Based Management (SPBM) is a framework that provides a single control panel across various data services and storage solutions, including vSAN and Virtual Volumes. Using storage policies, the framework aligns application demands of your virtual machines with capabilities provided by storage entities.
See Storage Policy Based Management.
I/O Filters
I/O filters are software components that can be installed on ESXi hosts and can offer additional data services to virtual machines. Depending on implementation, the services might include replication, encryption, caching, and so on.
See Filtering Virtual Machine I/O.

vSphere Storage APIs

Storage APIs is a family of APIs used by third-party hardware, software, and storage providers to develop components that enhance several vSphere features and solutions.

This Storage publication describes several Storage APIs that contribute to your storage environment. For information about other APIs from this family, including vSphere APIs - Data Protection, see the VMware website.

vSphere APIs for Storage Awareness

Also known as VASA, these APIs, either supplied by third-party vendors or offered by VMware, enable communications between vCenter Server and underlying storage. Through VASA, storage entities can inform vCenter Server about their configurations, capabilities, and storage health and events. In return, VASA can deliver VM storage requirements from vCenter Server to a storage entity and ensure that the storage layer meets the requirements.

VASA becomes essential when you work with Virtual Volumes, vSAN, vSphere APIs for I/O Filtering (VAIO), and storage VM policies. See Using Storage Providers.

vSphere APIs for Array Integration

These APIs, also known as VAAI, include the following components:

  • Hardware Acceleration APIs. Help arrays to integrate with vSphere, so that vSphere can offload certain storage operations to the array. This integration significantly reduces CPU overhead on the host. See Storage Hardware Acceleration.
  • Array Thin Provisioning APIs. Help to monitor space use on thin-provisioned storage arrays to prevent out-of-space conditions, and to perform space reclamation. See ESXi and Array Thin Provisioning.

vSphere APIs for Multipathing

Known as the Pluggable Storage Architecture (PSA), these APIs allow storage partners to create and deliver multipathing and load-balancing plug-ins that are optimized for each array. Plug-ins communicate with storage arrays and determine the best path selection strategy to increase I/O performance and reliability from the ESXi host to the storage array. For more information, see Pluggable Storage Architecture and Path Management.