Date: Sun, 8 Oct 1995 09:07:19 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, gibbs@freefall.FreeBSD.org Cc: dennis@etinc.com, hackers@freebsd.org Subject: Re: VLB Disk Controllers Message-ID: <199510072307.JAA05772@godzilla.zeta.org.au>
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>>That's not saying much. An IDE controller has a much lower command >>overhead than the buslogic: >It also has much higher CPU overhead, but we've been down that road >before. :) :-). >>A high command overhead causes slow file system operations for >>everything except large i/o's. >Yup, but your whole machine is affected by a PIO device. That's mainly because IDE PIO is slow, not because it's PIO. The overhead for SCSI DMA is not insignificant. For the BT455C, which has a DMA speed of up to 40MB/sec (I doubt that my bus can keep up with this), the overhead for reading from a Grand Prix at 5MB/sec seems to be about 27%. This is hard to measure accurately. I measured it by running a hog process in the background: on an idle system it took 7.58 sec (real) and while transferring it took 10.42 sec. 7.58/10.42 = 73%. Bruce
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