IBM DS6000 Series manual Vertical growth and scalability, 229

2 Gbps Fibre Channel ports				2 Gbps Fibre Channel ports
Host adapter	Controller		Controller	Host adapter
	Card 0		Card 1
chipset				chipset
	Server 0		Server 1
Power PC	Processor	Interconnect	Processor	Power PC
chipset	memory		memory	chipset
	Volatile		Volatile
Device adapter			Persistent	Device adapter
chipset	Persistent			chipset
	2 Gbps Fibre Channel ports		2 Gbps Fibre Channel ports
		Fibre Channel switch
	To next	ooo
	switch	16 DDM
		Fibre Channel switch	Server enclosure
		Fibre Channel switch
		ooo
		16 DDM
		Fibre Channel switch	Expansion enclosure

Figure 11-8 DS6800 with one DS6000 expansion enclosure

Note that each Fibre Channel switch in the disk subsystems from here on connects to the next pair of Fibre Channel switches through its two remaining ports. This is similar to inter-switch links between Fibre Channel switches.

Through the affinity of extent pools to servers, the DA in a server is used to drive the I/O to the disk drives in the host extent pools owned by its server.

When creating volumes in extent pools, these volumes get an affinity to a certain server through the extent pool affinity to a server (see Chapter 4, “Virtualization concepts” on page 65). This suggests even distribution of volumes across all ranks in the disk subsystems and all loops to balance the workload. Although each HA port can reach any volume in the disk subsystem, Figure 11-8indicates also a server affinity to its local HA and its Fibre Channel ports. This introduces the concept of a preferred path. When a volume has an affinity, for example, to server 0, and is accessed through a port in the HA of server 0, then the I/O is locally processed. When this volume is accessed through the HA of the other server, in this example from server 1, then the I/O is routed to the server which owns the extent pool, which here is server 0.

11.3.5 Vertical growth and scalability

Figure 11-9 on page 230 shows a simplified view of the basic DS6000 structure and how it accounts for scalability.

Chapter 11. Performance considerations 229