The big disadvantage of SR-IOV, as described above, is that LPARs with logical SR-IOV ports cannot be moved (LPM). After the introduction of SR-IOV on POWER systems, there were a number of suggestions for workarounds. However, all of these workarounds require, on the one hand, a special configuration and, on the other hand, a number of reconfigurations to be carried out before and after an LPM operation. In everyday practice, however, this unnecessarily complicates LPM operations.
With the introduction of vNICs, client LPARs can use SR-IOV adapters and still support LPM. As with VSCSI and VFC, a pair of adapters is used for this purpose: the so-called vNIC adapter is used in a virtual slot on the client LPAR and an associated vNIC server adapter is used on a virtual I/O server. The logical SR-IOV port is assigned to the virtual I/O server. The vNIC server adapter, also known as the vNIC backing device, serves as a proxy for the logical SR-IOV port. The interaction of the various adapters is shown in figure 7.19.
In order to achieve good performance, only control information is transmitted from the vNIC adapter of the client to the vNIC server adapter on the virtual I/O server, which is transmitted in turn from the vNIC server adapter, via the associated logical SR-IOV port (ent adapter), to the corresponding logical port (virtual function) of the SR-IOV adapter. The data itself is transferred between the vNIC client adapter and the logical port of the SR-IOV adapter via DMA (Direct Memory Access) with the help of the hypervisor. In particular, there is no copying of the data via the virtual I/O server. The vNIC adapter on the client is a purely virtual adapter, so LPM works without any problems. The client does not own the logical SR-IOV port and does not access it itself via the PCIe bus (switch).