Most storage systems provide some mechanism to create multiple logical disks from a single array. This is useful when the storage system presents storage directly to the hosts.
However, in a virtualized SAN, use a one-to-one mapping between arrays and logical disks so that the subsequent load calculations and the managed disk (MDisk) and MDisk group configuration tasks are simplified.
In this scenario, you have two RAID-5 arrays and both contain 5 + P components. Array A has a single logical disk that is presented to the SAN Volume Controller cluster. This logical disk is seen by the cluster as mdisk0. Array B has three logical disks that are presented to the cluster. These logical disks are seen by the cluster as mdisk1, mdisk2 and mdisk3. All four MDisks are assigned to the same MDisk group that is named mdisk_grp0. When a virtual disk (VDisk) is created by striping across this group, array A presents the first extent and array B presents the next three extents. As a result, when the system reads and writes to the VDisk, the loading is split 25% on the disks in array A and 75% on the disks in array B. The performance of the VDisk is about one third of what array B can sustain.
The uneven logical disks cause performance degradation and complexity in a simple configuration. You can avoid uneven logical disks by creating a single logical disk from each array.