When migrating a VM, vSphere will take advantage of various ways of offloading the transfer of the data involved.
In order of preference this could include
- VASA-based offloads of the complete virtual disk duplication (applicable only on VVol storage).
- VAAI-based offload of the copying of the virtual disks's data using XCOPY (applicable only on SAN arrays, and only if supported by the array)
Before reverting to explicit host based copying of the virtual machines data during migrations to VVol datastores, vSphere has been enhanced to use XCOPY using the VAAI offload.
XCOPY Supported:
- VMFS to RDM/VVol
- VVol to RDM
- RDM to VVol
- VVol to VVol
- VMFS to VMFS
XCOPY Not Supported:
Note: Some of SAN arrays that support VVols may support XCOPY for non-VVol LUNs but not to or from VVol LUNs, which may separately affect the overall migration performance in an array-specific way. vSphere will use XCOPY on arrays that support it for VVols only when moving to a VVol virtual datastore (not from VVol to another type of file system like VMFS). This is a result of the implementation of ESXi's datamover.