ESX/ESXi provides host-level storage virtualization, which logically abstracts the physical storage layer from virtual machines.

An ESX/ESXi virtual machine uses a virtual disk to store its operating system, program files, and other data associated with its activities. A virtual disk is a large physical file, or a set of files, that can be copied, moved, archived, and backed up as easily as any other file. 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 that a virtual machine can access through one of the virtual SCSI controllers resides on a VMware Virtual Machine File System (VMFS) datastore, an NFS-based datastore, or on a raw disk. 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 actual physical disk device is being accessed through parallel SCSI, iSCSI, network, or Fibre Channel adapters on the host is transparent to the guest operating system and to applications running on the virtual machine.

iSCSI SAN Storage Virtualization gives an overview of storage virtualization. The diagram illustrates storage that uses VMFS and storage that uses raw device mapping. The diagram also shows how iSCSI storage is accessed through either iSCSI HBAs or by using a general-purpose NIC that uses iSCSI initiator software.

iSCSI SAN Storage Virtualization
This image gives an overview of storage virtualization. It illustrates storage using VMFS and storage using raw device mapping. It also shows how iSCSI storage is accessed through either iSCSI HBAs or by using a general-purpose NIC that uses iSCSI initiator software.