Learn about different types of PCI slots, connectivity versus performance, slot restrictions, and adapter labels.
See the PCI adapters topic in Removing and Replacing parts to determine if your system unit supports the hot-plugging of adapters.
Select the appropriate information from this list:
Each PCI bus has a limit on the number of adapters it can support. Typically, this limit can range from two adapters to six adapters for each bus. To overcome this limit, the system design can implement multiple PCI buses. You can use either of the following methods to add PCI buses to your system:
To increase the number of PCI slots when designing a system, add a secondary PCI bus. A PCI-to-PCI bridge chip can connect a secondary bus to a primary bus. The following illustration shows how to use a primary PCI bus to increase the total number of PCI slots.
Because the slots on the secondary PCI bus must pass through the bridge chip, certain adapters on a secondary PCI bus might experience lower performance.
Some systems implement a secondary PCI bus. On these systems, place higher-speed adapters on the primary bus to optimize performance.
Another way to add more PCI slots, design the system with two or more primary PCI buses. This design requires a more complex I/O interface with the system memory. The following illustration shows another method of increasing the number of PCI slots.
This design can improve I/O performance over the secondary bus method because the I/O interface has created multiple parallel paths into the system memory.
If your system contains hot-plug enabled PCI slots. These systems dedicate a PCI bus to each PCI slot, which allows the adapter to be removed or added without affecting other adapters. This architecture uses one or more PCI primary buses that are bridged to multiple PCI secondary buses. Each PCI secondary bus has a single PCI slot.
The main processor board integrates a number of devices, but they physically connect to one of the PCI buses. For this reason, some of the buses might have only two or three slots available to install adapters. Integrated PCI adapters include SCSI adapters and Ethernet adapters.
Choosing between 32-bit and 64-bit slots influences slot placement and affects performance. Higher-speed adapters use 64-bit slots because they can transfer 64 bits of data for each data-transfer phase.
The 32-bit adapters can typically function in 64-bit PCI slots. However, 32-bit adapters still operate in 32-bit mode and offer no performance advantage in a 64-bit slot. Likewise, most 64-bit adapters can operate in 32-bit PCI slots, but the 64-bit adapter operates in 32-bit mode and reduces performance potential.
Some systems offer 50 MHz capability on 64-bit slots. Adapters capable of functioning at 50 MHz might take advantage of this available operating speed. If you plug a 33 MHz adapter into a 50 MHz 64-bit slot, the slot switches to 33 MHz and also switches the remaining slots on this PCI bus to 33 MHz. For systems with hot-plug PCI slots, adapters are not affected by the clock rate of other adapters because each adapter has its own PCI bus.
You must consider some performance implications when configuring your system. Installing the maximum number of adapters might affect system performance.
Connectivity limits define how many specified adapters can be physically plugged into a system. This limit defines how many adapters the software and hardware can support. Connectivity limits define the maximum number of adapters for connecting to networks or disks. In many cases, a disk or network has a low duty cycle and the system needs additional adapters to retain the physical connection to all resources. In these cases, follow the connectivity limits.
This information also provides suggested performance limits, established to determine how many concurrently running adapters can provide good performance. As you add adapters (with each adapter performing at close to its rated speed), additional adapters continue to provide an incremental performance increase. After the system reaches its performance limit, adding more adapters does not provide an increase in I/O throughput.
Bus speed, memory speed, adapter design, or processor speed can influence performance. The system processor's speed can often limit how many adapters of a given type the system can support while maintaining maximum performance. After a system uses 90 percent of its system processor, adding more adapters only provides a minor throughput increase.
Because of the wide variety of workloads, this information provides performance-limit guidelines only. The guidelines are based on I/O streaming of large reads or writes to a disk or network. They are not based on small I/Os, which are more transaction-rate limited. Small I/O workloads probably use more system processor capacity and result in fewer supported adapters for maximum performance.
These guidelines are based on the maximum number of processors supported for multiprocessor systems. If your system runs fewer than the maximum number of processors supported, then typically you must reduce the maximum number of adapters by the same ratio. For example, if a system with a maximum of 12 processors can support 12 ATM adapters for maximum performance, the same system with eight processors can support only eight ATM adapters for maximum performance.
If your system uses disk and communication adapters concurrently, use a more conservative estimate of the number of supported adapters.
If your configured system runs close to its performance limits, ensure that your system type or configuration provides the desired performance. In these cases, you might need to contact your marketing support personnel for more detailed information.
Subscribe to this information
