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Planning Volume 2, Control Workstation and Software Environment

Chapter 5. Planning for Virtual Shared Disks

Without special programming, a physical disk connected to a node can only be accessed by applications running on that node. The IBM Virtual Shared Disk component lets you define virtual shared disks. It provides the special programming that allows applications running on multiple nodes within the same switch partition to access the data on a raw logical volume as if it were local at each of the nodes. Actually, the logical volume is located at one of the nodes called a server node. The concurrent virtual shared disk support added in PSSP 3.2 allows you to use the concurrent disk access environment supplied by AIX.

The Recoverable Virtual Shared Disk subsystem is a required component for virtual shared disk support that enhances virtual shared disk availability by twin-tailing the physical disk to another node which takes over the I/O service if the virtual shared disk node or communication adapter fails. You configure nodes as primary and secondary server nodes of virtual shared disks. It offers continuous access to data with transparent recovery in the event of the failure of an SP node, disk, disk adapter, disk cable or communication adapter. Recovery from SP switch adapter or Ethernet adapter failure is the same as from a node failure: control of connected twin-tailed volumes is passed to the secondary server.

|IBM virtual shared disks are supported only on an SP system or |clustered enterprise servers with an SP switch. In an SP Switch2 system |where some nodes are not on the switch, virtual shared disks can work only |with those nodes that are on the switch.
Enhanced security options:

As of PSSP 3.2 you have the option of running PSSP with an enhanced level of security. The restricted root access option removes the dependency PSSP system management software has to internally issue rsh and rcp commands as a root user from a node. Any such actions can only be run from the control workstation or from nodes configured to authorize them. PSSP does not automatically grant authorization for a root user to issue rsh and rcp commands from a node. If you enable this option some procedures might not work as documented. For example, to run HACMP an administrator must grant the authorizations for a root user to issue rsh and rcp commands that PSSP would otherwise grant automatically. See Considering restricted root access for a description of this option.

With PSSP 3.4 you can have a secure remote command process to run in place of the rsh and rcp commands. See Considering a secure remote command process for a description of this option.

Virtual shared disks are not supported in this enhanced security environment. See Limitations when using restricted root access and Considering choosing none for AIX remote command authorization for a complete list of limitations.

This chapter is the first step in planning to use the optional components of PSSP that help you create and use virtual shared disks. After reading this chapter, if you plan to use virtual shared disks on an SP system you already have, see Chapter 11, "Planning for migration" for versions supported, coexistence, and migration information. Whether you plan to use them on an existing or a new SP system, see the book PSSP: Managing Shared Disks for additional planning information, to prepare for migrating to or installing PSSP 3.4, for creating and using virtual shared disks, |and for more information about running under various PSSP security |configurations.

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