Virtual volumes are encapsulations of virtual machine files, virtual disks, and their derivatives.
Virtual volumes are stored natively inside a storage system that is connected through Ethernet or SAN. They are exported as objects by a compliant storage system and are managed entirely by hardware on the storage side. Typically, a unique GUID identifies a virtual volume. Virtual volumes are not preprovisioned, but created automatically when you perform virtual machine management operations. These operations include a VM creation, cloning, and snapshotting. ESXi and vCenter Server associate one or more virtual volumes to a virtual machine. The system creates the following types of virtual volumes for the core elements that make up the virtual machine:
A data virtual volume that corresponds directly to each virtual disk .vmdk file. As virtual disk files on traditional datastores, virtual volumes are presented to virtual machines as SCSI disks.
A configuration virtual volume, or a home directory, represents a small directory that contains metadata files for a virtual machine. The files include a .vmx file, descriptor files for virtual disks, log files, and so forth. The configuration virtual volume is formatted with a file system. When ESXi uses SCSI protocol to connect to storage, configuration virtual volumes are formatted with VMFS. With NFS protocol, configuration virtual volumes are presented as an NFS directory.
Additional virtual volumes can be created for other virtual machine components and virtual disk derivatives, such as clones, snapshots, and replicas. These virtual volumes include a swap virtual volume to hold virtual machine swap files and a virtual memory volume to hold the contents of virtual machine memory for a snapshot.
By using different virtual volumes for different VM components, you can apply and manipulate storage policies at the finest granularity level. For example, a virtual volume that contains a virtual disk can have a richer set of data services and performance levels than the virtual volume for the VM boot disk. Similarly, a snapshot virtual volume can use a different storage tier compared to a current virtual volume.