Compaq ProLiant 1000 I/O Performance Tuning of Compaq Servers - Page 9

I/O Performance Tuning of Compaq Servers 9 PCI Bandwidth & Saturation While balancing the busses cannot increase your maximum throughput of 133 MB/s per bus, it can increase the potential sustained throughput of the server. By making sure that you use the maximum bandwidth on the two busses, you can increase overall performance. When a bus is carrying its maximum sustained throughput, the bus is said to be saturated. Once any bus has become saturated, it becomes a limiting factor in server performance. Balanced busses saturate at higher performance levels. Here again, the loads placed on your server will determine the importance of a balanced bus. In general, the high-speed PCI busses in Compaq servers are less likely to become saturated in environments where random, small-block transfers are the norm. Operating at 33 MHz and transmitting 32 bits in parallel, small files common in web serving and user storage are not usually on the bus long enough to cause any sustained saturation. However, exceptionally heavy loads of small block transfers or large block transfers, common in video streaming or CAD storage, can make a balanced bus a critical part of your server's performance. Compaq recommends that you always balance the loads on your PCI busses according to the guidelines above. However, the impact that balancing will have on performance will vary depending on the load placed on your server. SCSI Bus SCSI provides performance and features that have made it the interface of choice for Compaq servers. Originally, there was SCSI, which was renamed to SCSI-1 with the advent of SCSI-2. SCSI-1 suffered from many compatibility issues that were addressed in the next revision. At the same time SCSI-2 clarified and established SCSI standards, it extended the performance and functionality, both making SCSI more powerful and resolving compatibility issues. SCSI-3, the newest standard, extends the functionality of the SCSI bus to new devices such as Fibre Channel, SCSI-3 Parallel Interface, and the High Performance Serial Bus. Most importantly, SCSI-3 paves the way for a higher-performance bus interface. When referencing SCSI devices, prefixes, such as Wide, Narrow, and Fast, are used. Each of these prefixes gives some insight to the maximum performance of the SCSI device. There are two classes of prefixes; those that deal with the bus speed and those that deal with bus width. Table 1 summarizes and defines some common SCSI prefixes. Table 2. SCSI prefixes Bus speed Bus width Regular This term is no longer used. Regular, or the lack of Fast or Ultra, denotes the original 5 MHz SCSI bus speed. On a narrow bus, Regular SCSI could transmit 5 MB/s. Fast Defined in SCSI-2, the Fast protocol increases the speed of the SCSI bus to 10 MHz. On narrow busses, which transmit 1 byte per clock cycle, this gives a maximum throughput of 10 MB. Ultra The Ultra protocol, part of the SCSI-3 specification, builds on the performance of Fast SCSI, but doubles the clock again to 20 MHz. Ultra SCSI can transmit up to 20MB/s on a narrow bus. Narrow The original SCSI bus is capable of transmitting 8 bits per clock cycle. The term narrow is rarely used, but implied by the lack of the Wide prefix. Wide Introduced as part of the SCSI-2 specification, Wide busses allow the transmission of 2 bytes or 16 bits per clock cycle. By doubling the data bus width, the throughput of the SCSI bus doubles. In Wide-Fast SCSI, the throughput reaches 20 MB/s; and in WideUltra SCSI, the throughput has a maximum of 40 MB/s. ECG044.0399