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AIX Version 4.3 System Management Guide: Operating System and Devices

Developing a Volume Group Strategy

Disk failure is the most common hardware failure in the storage system, followed by failure of adapters and power supplies. Protection against disk failure primarily involves the configuration of the logical volumes (see "Developing a Logical Volume Strategy"). However, volume group size also plays a part as is explained below.

To protect against adapter and power supply failure, you need to consider a special hardware configuration for any specific volume group. Such a configuration includes two adapters and at least one disk per adapter, with mirroring across adapters, and a nonquorum volume group configuration. The additional expense of this configuration is not appropriate for all sites or systems. It is recommended only where high (up-to-the-last-second) availability is a priority. Depending on the configuration, high availability can cover hardware failures that occur between the most recent backup and the current data entry. High availability does not apply to files deleted by accident.


It is important that you understand the material contained in the "Logical Volume Storage Overview".

When to Create Separate Volume Groups

You may want to organize physical volumes into volume groups separate from rootvg for the following reasons:

High Availability in Case of Disk Failure

The primary methods used to protect against disk failure involve logical volume configuration settings, such as mirroring. While the volume group considerations are secondary, they have significant economic implications because they involve the number of physical volumes per volume group:

When deciding on the number of disks in each volume group, you also need to plan for room to mirror the data. Keep in mind that you can only mirror and move data between disks that are in the same volume group. If the site uses large file systems, finding disk space on which to mirror could become a problem at a later time. Be aware of the implications on availability of inter-disk settings for logical volume copies and intra-disk allocation for a logical volume.

High Availability in Case of Adapter or Power Supply Failure

To protect against adapter or power supply failure, depending on the stringency of your requirements, do one or more of the following:

Decide on the Size of Physical Partitions

The physical partition size is set when the volume group is created. The default size is 4MB. The default is designed to suit most sites and systems but may not be appropriate in every case. You can choose a partition size as small as 1MB to gain flexibility in sizing but this requires more partitions. The additional partitions create more overhead for the Logical Volume Manager (LVM) and are likely to affect performance.

If you make the partitions larger than 4MB, you lose some sizing flexibility and may also waste space. For example, if you have 20MB partitions, then your JFS log will have to be 20MB when it only needs 4MB. Some waste may be an acceptable tradeoff if the particular site or system requires larger partitions.

Note that you may only create and extend physical partitions in increments that are a factor of their size; for example, 20MB partitions are created or extended in 20MB increments.

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