vStorage API for Array Integration (VAAI) is an application program interface (API) framework from VMware that enables certain storage tasks, such as thin provisioning, to be offloaded from the VMware server virtualization hardware to the storage array.
Offloading these tasks lessens the processing workload on the virtual server hardware. For a storage administrator to make use of VAAI, the manufacturer of his storage system must have built support for VAAI into the storage system.
Introduced in vSphere 4 with support for block-based (Fibre Channel or iSCSI) storage systems, VAAI consisted of a number of primitives, or parts. “Copy offload” enables the storage system to make full copies of data within the array, offloading that chore from the ESX server. “Write same offload” enables the storage system to zero out a large number of data blocks to speed the provisioning of virtual machines (VMs) and reduce I/O. Hardware-assisted locking allows vCenter to offload SCSI commands from the ESX server to the storage system so the array can control the locking mechanism while the system does data updates.
In vSphere 5, vStorage APIs for Array Integration were enhanced. The most notable new functionality addresses thin provisioning of storage systems and expands support to network-attached storage (NAS) devices.
VAAI’s thin provisioning enhancements allow storage arrays that use thin provisioning to reclaim blocks of space when a virtual disks is deleted, to mitigate the risk of a thinly provisioned storage array running out of space. Thin provisioned storage systems that support vSphere 5’s VAAI are given advance warnings when space thresholds are reached. In addition, in that version, VAAI enables mechanisms to temporarily pause virtual machines when space runs out, giving admins time to add storage or migrate the virtual machine to a different array.
VAAI’s “hardware acceleration of NAS” component has two primitives, according to VMware. Full file clone enables the NAS device to clone virtual disks to speed the process of creating virtual machines on NAS systems. “Reserve space” enables the creation of a thick virtual disk on a NAS device.
Qualified storage array vendors can partner with VMware to develop the firmware and plug-ins required by their arrays.