IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

394 IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

logical volumes. This does not change with multipath support. When deciding how many I/O

adapters to use, your first priority should be to consider performance throughput of the IOA,

since this limit may be reached before the maximum number of logical units.

11.3.1 Multipath compared to mirroring

Prior to multipath being available, some customers used OS/400 mirroring to two sets of

disks, either in the same or different external disk subsystems. This provided implicit

dual-path as long as the mirrored copy was connected to a different IOP/IOA, bus, or I/O

tower. However, this required two copies of data, and the data was sent from the iSeries

server to the external storage server twice, using additional resources. Since disk level

protection is already provided by RAID 5 or RAID 10 in the external disk subsystem, we

recommend that you use the data protection features in the DS6000, rather than iSeries

mirroring.

11.3.2 Multipath rules for multiple iSeries systems or partitions

When you use multipath disk units, you must consider the implications of moving IOPs and

multipath connections between nodes. You must not split multipath connections between

nodes, either by moving IOPs between logical partitions or by switching expansion units

between systems. If two different nodes both have connections to the same LUN in the

DS6000, both nodes might potentially overwrite data from the other node.

The system enforces the following rules when you use multipath disk units in a

multiple-system environment:

򐂰 If you move an IOP with a multipath connection to a different logical partition, you must

also move all other IOPs with connections to the same disk unit to the same logical

partition.

򐂰 When you make an expansion unit switchable, make sure that all multipath connections to

a disk unit will switch with the expansion unit.

򐂰 When you configure a switchable independent disk pool, make sure that all of the required

IOPs for multipath disk units will switch with the independent disk pool.

If a multipath configuration rule is violated, the system issues warnings or errors to alert you

of the condition.

11.3.3 Changing from single path to multipath

If you have a configuration where the logical units were only assigned to one I/O adapter, you

can easily change to multipath. Simply assign the logical units in the DS6000 to another I/O

adapter and the existing DDxxx drives will change to DMPxxx and new DMPxxx resources

will be created for the new path.

11.4 iSeries performance and monitoring tools

There are numerous products which aid in capacity planning and performance management

for iSeries. Most are comprehensive planning tools in that they address the entire spectrum of

workload performance on iSeries including CPU, system memory, disks, and adapters.

Although none has external storage performance as its primary focus, each offers some

information which can aid in that area.