Administration Guide

Terminology

Physical Volume

A physical volume is an individual hardware disk drive; it might be internal, SCSI-attached or an external SSA drive.

Volume Group

A volume group is a collection of physical volumes. A physical volume can be in one and only one volume group.

Root Volume Group

A root volume group is a special volume group that contains the AIX base operating system.

Logical Volume

A logical volume is disk space that can be used for file systems, paging, or dump. A logical volume can only be in one volume group, but can span physical volumes.

Logical Partition

A logical partition is a convenient measure of space and is a block of space in a logical volume. Logical partitions can be 1 MB in size and up to 128 MB in size, in powers of two.

Physical Partition

A physical partition is, like a logical partition, a unit of measure. Logical partitions and physical partitions are in a one to one ratio in an environment without mirroring. If mirroring is being used, there will be 2 or 3 physical partitions for each logical partition, depending on the number of copies.

Mirroring

The ability to make a copy of a logical volume or volume group to provide redundancy. AIX allows one (the original), two (the original plus a mirror image), or three (the original plus two mirror images) copies of a logical volume or volume group. The copies are made below the application layer, transparent to the typical user.

Quorum

A quorum is a method used to determine whether or not there are enough physical volumes remaining in a volume group for that volume group to remain online (varied on). Quorum can be turned on or off by the user. If quorum is on, at least 51 percent of the physical volume votes must be present or the volume group will be varied offline to maintain data integrity. If quorum is off, only one vote of all the physical volumes is needed to keep the volume group online.

Note:

In general, quorum should be turned off in a mirrored environment and turned on in an environment without mirroring. Quorum is turned on by default during node installation. You can set a mirroring option to turn quorum off when mirroring is initiated and you can change the option to have it turned on when you suspend mirroring.

Mirroring support in AIX

Redundancy is an important element of modern computer system availability. AIX provides redundant copies of the operating system through disk mirroring. Disk mirroring provides for one or two identical copies of AIX, in addition to the original. A mirrored root volume group means that there are multiple copies of the operating system image available to a workstation or node. Mirrored system images are distributed so that a node can remain in operation even after one of the mirrored units fails. If a physical volume which contains the original copy of AIX should fail, a mirrored copy takes over. This take over is transparent to the user and to any user applications that happen to be running. Through mirroring of the operating system environment, AIX eliminates the possibility that a single physical volume can be a single point of failure for the SP system. PSSP 3.1 provides full support for mirroring on all PSSP 3.1 SP nodes.

Root volume group support in PSSP

PSSP 3.1 supports AIX mirroring and alternate root volume groups on SP nodes by providing commands to:

• Maintain a database of information about the root volume group
• Initiate or suspend mirroring on the nodes
• Maintain the bootlist on the nodes
• Define alternate root volume groups

PSSP 3.1 uses the Volume_Group class of the SDR to store mirroring information in volume group objects. Each volume group object contains information that describes a root volume group on a node. When you install PSSP on the control workstation and initialize the SDR, a single volume group object is created with the name of rootvg for each node. You can use the spmkvgobj command to create additional volume group objects to represent alternate root volume groups for a node. For more information on the Volume_Group class, refer to Class = Volume_Group.

An attribute of the Node object, the selected_vg attribute, designates one of the volume group objects as the currently selected volume group. When the SDR is initialized, all the selected_vg attributes are set to rootvg, the default root volume group object.

PSSP 3.1 provides the following commands for managing root volume groups:

• Commands to manipulate root volume group information:

  spmkvgobj

  Create a new Volume_Group object.

  spchvgobj

  Change Volume_Group information.

  sprmvgobj

  Remove a Volume_Group object that is not the current volume group.

  spbootins

  Set boot/install configuration data in the SDR.

  Note:

  Many attributes that are now associated with the Volume_Group object have been moved from the spbootins command to the spchvgobj command.

• Commands to initiate or suspend mirroring or to switch between alternate root volume groups::

  spmirrorvg

  Initiate mirroring.

  spunmirrorvg

  Suspend mirroring.

  spbootlist

  Set the bootlist based on specifications in the Node and Volume_Group objects.

Each of these commands accepts a list of nodes as input. See the book PSSP: Command and Technical Reference for details of how to use these commands.

While these commands comprise the base support for mirroring and alternate root volume groups in PSSP, you can accomplish some tasks using SP Perspectives, or SMIT. They are interfaces between you and the commands, each being more or less appropriate for you depending on your level of experience. Example tasks in this appendix are shown using the base line commands.