IBM DS6000 Series manual 225

To host servers

Adapter		Adapter
Processor	Memory	Processor
		Storageserver
Adapter		Adapter
	To next
	switch
Fibre Channel switch
	oo o
	16 DDM
Fibre Channel switch

Figure 11-3 High availability and increased bandwidth connecting both DA to two logical loops

These two switched point-to-point loops to each drive, plus connecting both DAs to each switch, accounts for the following:

￿There is no arbitration competition and interference between one drive and all the other drives because there is no hardware in common for all the drives in the FC-AL loop. This leads to an increased bandwidth utilizing the full speed of a Fibre Channel for each individual drive. Note that the external transfer rate of a Fibre Channel DDM is 200 MB/sec.

￿Doubles the bandwidth over conventional FC-AL implementations due to two simultaneous operations from each DA to allow for two concurrent read operations and two concurrent write operations at the same time.

￿Despite the superior performance, don’t forget the improved RAS over conventional FC-AL. The failure of a drive is detected and reported by the switch. The switch ports distinguish between intermittent failures and permanent failures. The ports understand intermittent failures which are recoverable and collect data for predictive failure statistics. If one of the switches itself fails, a disk enclosure service processor detects the failing switch and reports the failure using the other loop. All drives can still connect through the remaining switch.

Chapter 11. Performance considerations 225