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This discussion of vSCSI devices applies only to SAN-based LUNs presented through VIO. Internal devices, volumes, and files presented by VIO as vSCSI devices are not recommended for use with Storage Foundation.
Virtual SCSI uses a client/server model. A Virtual I/O server partition owns the physical I/O devices, and exports the devices as virtual SCSI (vSCSI) resources to the client partitions. The Virtual I/O client is a logical partition that has a virtual client adapter node defined in its device tree. The VIO client uses the vSCSI resources provided by the Virtual I/O Server partition to access the storage devices.
If redundant SAN connections exist to the VIO server, the VIO server provides multi-pathing to the array. Client partitions can also perform multi-pathing between VIO servers in an active/standby configuration. This configuration provides extended protection from VIO configuration and maintenance. Redundant VIO servers are recommended for production workloads.
A virtual SCSI (vSCSI) disk is a resource which can be a SCSI disk, or a volume or file in a VIO Server (VIOS) that is exported to a virtual IO client (VIOC). IBM vSCSI LUNs implement a sub-set of the SCSI protocol. The two main limitations are:
Persistent reservations (SCSI3 - PGR) are not implemented.
The lack of SCSI reservations means that I/O Fencing is not supported. Storage Foundation Cluster File System High Availability (SFCFSHA) and Storage Foundation for Oracle RAC (SFRAC) do not support vSCSI disks, because SFCFSHA and SFRAC require I/O fencing.
Device inquiry limitations.
Storage Foundation (SF) cannot directly fetch the inquiry data, as is done from a physical SCSI disk. However, if the vSCSI disk in VIOC is backed by a dmpnode in VIOS, then all the inquiry data that can be fetched from a physical disk can be fetched.
Cross-platform data sharing (CDS) functionality is supported.