Various storage functions, such as cloning and snapshots, are performed more efficiently by the storage array (target) than by a host (initiator). In a virtualized environment, since virtual disks are files on VMFS and disk, arrays cannot interpret the VMFS on-disk data layout; you cannot leverage hardware functions on a per-VM or per virtual disk (file) basis.

vStorage APIs for Array Integration (VAAI) are a set of new protocol interfaces between ESXi and storage arrays and new application programming interfaces in VMkernel. Using a small set of primitives (fundamental operations) that can be issued to the array using these interfaces, ESXi is able to improve its offering of storage services.

The fundamental operations are:

• Atomic test & set (ATS)--new locking mechanism
• Clone blocks/full copy/XCOPY
• Zero blocks/write same

The goal of VAAI is to help storage vendors provide hardware assistance to speed up VMware I/O operations that are more efficiently accomplished in the storage hardware. VAAI plugins can improve the performance of data transfer and are transparent to the end user.