SCSI and IDE(hardware cache)
The harddrivers always have cache for buffer the data. The cache is very fast than both harddrivers and Host Bus, so it is impossible to form continous transfer stream in the Host Bus, because it acts like middlestone between harddriver and host bus. I think the following example would indicate the factor more clearly.
Just image, in an operating system, copy data from harddriver “A” to harddriver “B”.
SCSI: because the cache is more faster device, it always has to wait for harddisk to full it. when the cache is full, A will send all the data in cache to host, in this time, the cache is receiving data again, and the host send the data to B through host bus! Yes, it saves time and increase the transfer rate greatly.
IDE: A will hold the channel single until it writes all data to host memory, even there’s other way which is free when A’s cache is fulling. This mode waste a large machine time and has low rate.
Absolutely, SCSI is more powerful than IDE.