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 storage pool 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 Lenovo Storage V seriesclustered system. This logical disk is seen by the system as mdisk0. Array B has three logical disks that are presented to the system. These logical disks are seen by the system as mdisk1, mdisk2, and mdisk3. All four MDisks are assigned to the same storage pool that is named mdisk_grp0. When a volume is created by striping across this storage pool, 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 volume, the loading is split 25% on the disks in array A and 75% on the disks in array B. The performance of the volume 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.