VIA Chipsets slow down PCI cards

SCSI-RAIDs and PCI

The timing checks we did explain, why ATA/133 cards perform so bad with VIA chipsets. These new high speed adapters max out the bandwidth of the PCI bus and VIAs solution can not keep up with that.

This effect can also be found with other PCI cards. We also checked the PCI transfers taking place using Adaptecs SCSI-RAID-2100S. This card is designed for Ultra160 SCSI and easily outperforms PCI.

For a real world test we set up four pieces of Fujtsus Ultra160 hard drive MAM3184MP as a RAID-0 stripe set. These drives spin their platters at 15.000 rpm and offer sequential transfers from the disc media at 54 MBytes/s. They are among the fastest available hard drives at the moment.

In a RAID-0 configuration the SCSI adapter combines the physical hard drives to one logical drive. Data is distributed across the physical drives in consecutive blocks. This enables parallel reads and writes which in the end boosts throughput. In theory the maximum transfer rate of every physical drive nicely adds up, but you still loose a little performance due to the overhead included in managing the RAID itself.

With Adaptecs SCSI-RAID-2100S one can adjust the block size of the RAID. 128 KBytes are set per default, the minimum size is 8 KBytes. Taking the maximum of 128 KBytes, a file of 512 KBytes is distributed among all four hard drives in our test configuration. This speeds up transfers significantly, for every drive reads or writes one segment simultaneously.

This performance boost is most noticeable when working with large consistent files, as every drive is working with sequential reads or writes. When working with a lot of small files, access time of the hard drives used becomes the bottleneck. In this situation even a RAID-0 configuration shows the performance of single drive at best.