Most if not all ATA RAID adapters advertise prominently that their controllers are either PCI 64 bit, 66MHz or both. A 32 bit 33 MHz PCI adapter has a theoretical maximum transfer speed of 133 MB/s. A 32 bit 66 MHz PCI adapter is 266 MB/s and a 64 bit 66 MHz PCI adapter is 533 MB/s maximum transfer speed. Of course your motherboard needs 64 bit and/or 66 MHz PCI slots to take advantage of all this speed.
When a platform choice was being made for ATA RAID 2003, dual Athlons were chosen for speed and the fact the AMD-768 Southbridge supported 64 bit 66 MHz PCI slots. For all of the original benchmarks the ATA RAID adapters were tested in a 64 bit 66 MHz PCI slot. The Promise SX4000 is a 32 bit 66 MHz adapter and the 3ware Escalade 7500-8 is a 64 bit 33 MHz adapter.
Common sense dictates that performance should be increased across the board for these adapters when used in a 64 bit 66 MHz PCI slot, but in the world of computers common sense doesn’t always play. To see if this holds true these two adapters were benchmarked on the exact same hardware, but using a 32 bit 33 MHz PCI slot.
Open the benchmarks page and then peruse the charts ( bottom of this page ): what shows is startling to say the least. Out of twelve benchmarks run, the cards perform best using the 32 bit 33 MHz PCI slot on six of them. Staggering, to say the least. To be fair on two of those tests the SX4000 on a 64 bit slot outperformed it’s 32 bit doppelganger, and the reverse held for the 7500-8.
The first reaction was to blame the motherboard and their implementation of two PCI buses. Checking the AMB 760 MPX chipset documentation showed the 64 bit 66 MHz PCI bus was the primary, and the 32 bit 33 MHz PCI the secondary. As well there were quite a few contradictory statements regarding the PCI slots, so I went to Asus to see what their official statement was. Asus states the A7M266 has two 64 bit 66 MHz PCI slots. This is reiterated in the manual, so it’s safe to say the slots on the motherboard are 64 bit 66 MHz and capable of handling the various itinerations required for these ATA RAID adapters.
Results don’t lie: a clear conclusion is difficult based on these benchmarks. I would say users don’t have to worry about a performance loss from only having 32 bit 33 MHz PCI slots. Of course ATA RAID 5 adapters would generally see usage in dual CPU motherboards, and most have at least some form of advanced PCI slot.
For now the results are limited to the one motherboard. Look for more information after a Tyan MPX board is added to the eBabble stables.
32 vs 64 Benchmark Graphs
August 7th, 2003
Update
With the new generation of ATA RAID controllers benchmarked on eBabble, the 3ware Escalade 8500 and Promise FastTrak 150 SX4 perform equal or faster in a 64 bit PCI slot than in a 32 bit PCI slot. Looks like the players are on the right track..
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
