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Are there any limitations when implementing VMware VAAI?

While vSphere administrators will want to be sure their storage array supports VMware's vStorage APIs for Array Integration, there are some restrictions to keep in mind when setting up storage acceleration.

Generally speaking, SAN storage acceleration requires a compliant hypervisor and storage array. For a vSphere shop, ESXi/ESX 4.1 and later versions support vStorage APIs for Array Integration (VAAI) as long as you have enterprise or enterprise-plus licensing. If you deploy network-attached storage (NAS) instead, plan on using ESXi 5.1 or later. Storage array vendors have been integrating VMware VAAI storage acceleration support since about 2010 and include products from HP 3PAR, Dell Compellent, EMC, IBM and NetApp. It's important to check with each prospective vendor to verify current VAAI support.

Keep in mind that any storage subsystems that do not support VMware VAAI will still work in a virtualized environment just as they had before. Host systems will use traditional data movement methods to accomplish storage tasks and there will be no benefits of storage acceleration.

However, storage acceleration does have numerous limitations, which should be considered carefully when planning to implement or expand acceleration in the data center. First, source and destination files must share similar characteristics such as zeroing scheme, block sizes, raw disk mapping (RDM). For example, a thick-zeroed VMDK file cannot be copied or cloned to a thin-provisioned VMDK. In addition, source and destination VMFS volumes must have the same block sizes; both -- or neither -- should use RDM. The logical storage address or length of the copy process must be aligned properly; this happens automatically in vSphere, but may not occur with other platforms. And it is never possible to clone or migrate between arrays or spread LUNs across multiple arrays -- storage acceleration can only work when data remains on the same storage array.

Storage acceleration has evolved to a point where it is an attractive and reliable feature of modern virtualized data centers, but it takes a certain amount of care and planning to take full advantage of the technology. Compliant storage hardware is essential, though any storage hardware suitable for virtualization will still work without the benefits of acceleration. It is often best to introduce storage acceleration in phases, benchmarking storage performance before and after implementation so that IT professionals can obtain an objective assessment of benefits. Periodic storage testing can then help to identify performance declines or shifts in acceleration status that may suggest a problem to be resolved.

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