I really cannot give you an answer to "why", but I can give you some information on the advantages of both IOP...
and bus-level LPARs:
With bus-level I/O partitioning, you dedicate an I/O bus and all resources on the bus to the same partition. A partition using the bus-level configuration, all I/O (included the alternate IPL device, console, and electronic customer support device) is dedicated and no resources are dynamically switched into or out of the partition. On a server that has partitions at the bus level, all buses are owned dedicated by their respected partitions and no devices are switched.
Bus-level logical partitions allow for: * Better problem isolation and therefore higher availability. * Better performance. * Simplified hardware management.
When you partition a bus at the IOP level, you share the bus and divide up the I/O resources by IOP. This type of logical partitions allows for:
* Greater flexibility with partition I/O subsystems. * Potential cost reduction by eliminating some expansion units that you may need to support additional buses. * Optimization of hardware resources to avoid server limits such as 19 buses per server (only on AS/400e models). * The ability to dynamically switch an IOP from one logical partition to another without the need to restart the server. * Simplified configuration planning since hardware movement is not necessary.
However, it is possible to configure a partition solely of dedicated buses to create a bus-level partition or solely of shared buses to create an IOP-level partition.
When setting up a secondary partition, additional considerations for card locations need to be made. If the IOP you select for the console also has a LAN card and the LAN card is not intended for use with Operations Console, it will be activated for use by the console and you may not be able to use it for your intended purposes.
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