Virtual Scsi - IBM p5 590 System Handbook

Although the VIOS is a component of the standard Advanced POWER
Virtualization feature, which includes Shared Ethernet Adapter (SEA) service,
the VIOS must be installed and configured by the client using the hardware
management console (HMC). The VIOS is created using the HMC and is not
intended for running applications or for general user logins. It is strictly designed
to provide virtual I/O resources to logical client partitions.

3.6 Virtual SCSI

When each partition typically requires one I/O slot for disk attachment and
another one for network attachment, this puts a constraint on the number of
partitions. To overcome these physical limitations, I/O resources have to be
shared. Virtual SCSI provides the means to do this for SCSI storage devices.
Virtual I/O allows the POWER5 to support more partitions than it has slots for I/O
devices by enabling the sharing of I/O adapters amongst partitions. Virtual SCSI
(VSCSI) will enable a partition to access block-level storage that is not a physical
resource of that partition. The VSCSI design is that the virtual storage be backed
by a logical volume on a portion of a disk rather than an entire physical disk.
These logical volumes appear to be the SCSI disks on the client partition, which
gives the system administrator maximum flexibility in configuring partitions.
Furthermore virtual I/O allows attachment of previously unsupported storage
solutions. As long as the Virtual I/O Server supports the attachment of a storage
resource, any client partition can access this storage by using virtual SCSI
adapters. For example, if there is no native support for EMC storage devices on
Linux, by running Linux in logical partition of a POWER5 server with virtual I/O
make it possible.
Virtual I/O will provide a high performance I/O mechanism by minimizing the
number of times data is copied within the memory of the physical system. The
virtual I/O model described herein allows for either zero copy, if data is being
retrieved from a physical device and DMAed directly to the memory of the
partition using virtual I/O using the redirected DMA, or single copy of the data is
first moved to the memory space of the I/O server before being DMAed to the I/O
client.
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Chapter 3. POWER5 virtualization capabilities