Johnny Hughes mailing-lists@hughesjr.com wrote:
You only want to do that as a last resort, it makes your disk IO very slow. You would change it using hdparm ... the option would be: hdparm -d0 You can make it happen every reboot by editing the file: /etc/sysconfig/harddisks set: USE_DMA=0 and remove the # in front of it. AGAIN ... only if absolutely necessary, because it greatly slows down disk I/O.
Yep. Programmed I/O (PIO) means you now involve your CPU and system interconnect, instead of Direct Memory Access (DMA) which transfers directly from drive to memory.
With DMA, the limit is your PCI (peripheral) interconnect, typically 75-100MBps realistically for a legacy 32-bit @ 33MHz PCI with no contention (much higher for PCI64/66, PCI-X or PCIe).
With PIO, you'll quickly find that not only will your CPU usage spike as the CPU gets bogged down with just controlling the I/O channel, but pushing more than 15MBps over a legacy 32-bit @ 33MHz PCI bus is virtually impossible.