Inter-disk settings for a single copy of the logical volume
If you select the minimum inter-disk setting
(Range = minimum),
the physical partitions assigned to the logical volume are located on a single
disk to enhance availability. If you select the maximum inter-disk setting
Range = maximum), the physical partitions are located on
multiple disks to enhance performance.
For nonmirrored logical volumes, use the minimum setting to provide the greatest availability (access to data in case of hardware failure). The minimum setting indicates that one physical volume contains all the original physical partitions of this logical volume if possible. If the allocation program must use two or more physical volumes, it uses the minimum number, while remaining consistent with other parameters.
By using the minimum number of physical volumes, you reduce the risk of losing data because of a disk failure. Each additional physical volume used for a single physical copy increases that risk. A nonmirrored logical volume spread across four physical volumes is four times as likely to lose data because of one physical volume failure than a logical volume contained on one physical volume.
The following figure illustrates a minimum inter-disk allocation policy.
This illustration shows three disks. One disk contains three physical partitions; the others have no physical partitions.
The maximum setting, considering other constraints, spreads the physical partitions of the logical volume as evenly as possible over as many physical volumes as possible. This is a performance-oriented option, because spreading the physical partitions over several disks tends to decrease the average access time for the logical volume. To improve availability, the maximum setting is only used with mirrored logical volumes.
The following figure illustrates a maximum inter-disk allocation policy.
This illustration shows three disks, each containing one physical partition.
These definitions are also applicable when extending or copying an existing logical volume. The allocation of new physical partitions is determined by your current allocation policy and where the existing used physical partitions are located.