102 An Introduction to the New IBM Eserver pSeries High Performance Switch
3.4 IBM Virtual Shared Disks
This section contains a short description of the IBM Virtual Shared Disks 4.1 product in a
pSeries High Performance Switch environment.
In this section we include the following topics:
Restrictions for VSD
IBM Subsystem Device Driver (SDD) overview
High Performance Switch considerations and tuning recommendations
3.4.1 Overview of the Virtual Shared Disk components
IBM Virtual Shared Disk is a subsystem that lets application programs that are running on
different nodes of an RSCT peer domain access a raw logical volume as if it were local at
each of the nodes. Each Virtual Shared Disk corresponds to a logical volume that is actually
local at one of the nodes, which is called the server node.
The Virtual Shared Disk subsystem routes I/O requests from the other nodes, called client
nodes, to the server node and returns the results to the client nodes. The I/O routing is done
by the Virtual Shared Disk device driver that interacts with the AIX Logical Volume Manager
(LVM). The device driver is loaded as a kernel extension on each node. Thus, raw logical
volumes can be made globally accessible in the VSD nodeset (domain).
The application program interface to a Virtual Shared Disk is the raw device (or device special
file). This means application programs must issue requests to a Virtual Shared Disk using the
block size specified by the LVM (currently, requests are multiples of 512 bytes on 512-byte
block boundaries). Figure 3-15 shows a logical view of the the VSD subsystem.
Figure 3-15 Logical view of the VSD subsystem
3.4.2 Product packaging
VSD used to be a component of the PSSP, and only SP customers could take advantage of it.
VSD is now shipped as part of the RSCT Release 2.3, included with the AIX 5L operating
system. It can also be installed as a PTF to AIX Version 5.2.B (AIX 5.2 maintenance level 2).
VSD packaging is described in Table 3-3 on page 103.