Adapter placement

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Network performance is dependent on the hardware you select, like the adapter type, and the adapter placement in the machine.

To ensure best performance, you must place the network adapters in the I/O bus slots that are best suited for each adapter.

When attempting to determine which I/O bus slot is most suitable, consider the following factors:

  • PCI-X versus PCI adapters
  • 64-bit versus 32-bit adapters
  • supported bus-slot clock speed (33 MHz, 50/66 MHz, or 133 MHz)

The higher the bandwidth or data rate of the adapter, the more critical the slot placement. For example, PCI-X adapters perform best when used in PCI-X slots, as they typically run at 133 MHz clock speed on the bus. You can place PCI-X adapters in PCI slots, but they run slower on the bus, typically at 33 MHz or 66 MHz, and do not perform as well on some workloads.

Similarly, 64-bit adapters work best when installed in 64-bit slots. You can place 64-bit adapters in a 32-bit slot, but they do not perform at optimal rates. Large MTU adapters, like Gigabit Ethernet in jumbo frame mode, perform much better in 64-bit slots.

Other issues that potentially affect performance are the number of adapters per bus or per PCI host bridge (PHB). Depending on the system model and the adapter type, the number of high speed adapters might be limited per PHB. The placement guidelines ensure that the adapters are spread across the various PCI buses and might limit the number of adapters per PCI bus. Consult the PCI Adapter Placement Reference for more information by machine model and adapter type.

The following table lists the types of PCI and PCI-X slots available in IBM® System p machines:
Slot type Code used in this topic
PCI 32-bit 33 MHz A
PCI 32-bit 50/66 MHz B
PCI 64-bit 33 MHz C
PCI 64-bit 50/66 MHz D
PCI-X 32-bit 33 MHz E
PCI-X 32-bit 66 MHz F
PCI-X 64-bit 33 MHz G
PCI-X 64-bit 66 MHz H
PCI-X 64-bit 133 MHz I

The newer IBM Power Systems processor-based servers only have PCI-X slots. The PCI-X slots are backwards-compatible with the PCI adapters.

The following table shows examples of common adapters and the suggested slot types:
Adapter type Preferred slot type (lowest to highest priority)
10/100 Mbps Ethernet PCI Adapter II (10/100 Ethernet), FC 4962 A-I
IBM PCI 155 Mbps ATM adapter, FC 4953 or 4957 D, H, and I
IBM PCI 622 Mbps MMF ATM adapter, FC 2946 D, G, H, and I
Gigabit Ethernet-SX PCI Adapter , FC 2969 D, G, H, and I
IBM 10/100/1000 Base-T Ethernet PCI Adapter, FC 2975 D, G, H, and I
Gigabit Ethernet-SX PCI-X Adapter (Gigabit Ethernet fibre), FC 5700 G, H, and I
10/100/1000 Base-TX PCI-X Adapter (Gigabit Ethernet), FC 5701 G, H, and I
2-Port Gigabit Ethernet-SX PCI-X Adapter (Gigabit Ethernet fibre), FC 5707 G, H, and I
2-Port 10/100/1000 Base-TX PCI-X Adapter (Gigabit Ethernet), FC 5706 G, H, and I
10 Gigabit-SR Ethernet PCI-X Adapter, FC 5718 I (PCI-X 133 slots only)
10 Gigabit-LR Ethernet PCI-X Adapter, FC 5719 I (PCI-X 133 slots only)

The lsslot -c pci command provides the following information:

  • The PCI type of the slot
  • The bus speed
  • Which device is in which slot
The following is an example of the lsslot -c pci command on a 2-way p615 system with six internal slots: 
# lsslot -c pci
# Slot      Description                         Device(s)
U0.1-P1-I1  PCI-X capable, 64 bit, 133 MHz slot  fcs0
U0.1-P1-I2  PCI-X capable, 32 bit, 66 MHz slot   Empty
U0.1-P1-I3  PCI-X capable, 32 bit, 66 MHz slot   Empty
U0.1-P1-I4  PCI-X capable, 64 bit, 133 MHz slot  fcs1
U0.1-P1-I5  PCI-X capable, 64 bit, 133 MHz slot  ent0
U0.1-P1-I6  PCI-X capable, 64 bit, 133 MHz slot  ent2
For a Gigabit Ethernet adapter, the adapter-specific statistics at the end of the entstat -d en[interface-number] command output or the netstat -v command output shows the PCI bus type and bus speed of the adapter. The following is an example output of the netstat -v command: 
# netstat -v

10/100/1000 Base-TX PCI-X Adapter (14106902) Specific Statistics:
--------------------------------------------------------------------
Link Status: Up
Media Speed Selected: Auto negotiation
Media Speed Running: 1000 Mbps Full Duplex
PCI Mode: PCI-X (100-133)
PCI Bus Width: 64 bit