7.5. Virtual SCSI
Another possibility for the virtualization of storage under PowerVM is the use of virtual SCSI adapters. As with virtual FC, a virtual SCSI client adapter is connected to a virtual SCSI server adapter on a virtual I/O server by the hypervisor. The virtual SCSI client adapter has the role of a SCSI initiator, the virtual SCSI server adapter on the virtual I/O server is the SCSI target. In contrast to physical SCSI, where several SCSI targets can be connected to a physical SCSI host bus adapter (HBA), a maximum of one SCSI target is connected to virtual SCSI. The connection of the virtual SCSI client adapter to the virtual SCSI server adapter by means of the hypervisor uses a so-called logical channel. This is a point-to-point connection between the two adapters.
In contrast to virtual FC, in the case of virtual SCSI, all devices (disks, tapes, etc.), that the virtual SCSI client adapter has to access, must be assigned individually to the virtual SCSI server adapter. With a few hundred devices (in larger environments this can be more than a thousand devices), this can quickly become very confusing. Figure 7.14 shows the communication path from the virtual SCSI client adapter via the hypervisor to the virtual SCSI server adapter and the hard disks.
The hard disks are assigned to the virtual SCSI server adapter on the virtual I/O servers by creating a so-called virtual target device that points to the target device. The target device is also called the backing device. Hard disks (hdisk), tapes (rmt) or logical volumes can be used as backing devices. A backing device does not necessarily have to be a device, but can also be a file in the file system, see Virtual Optical Media Library and Storage Pools later.