Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!lll-lcc!ames!cit-vax!mangler From: mangler@cit-vax.Caltech.Edu (System Mangler) Newsgroups: comp.periphs,comp.unix.wizards Subject: Re: Fujitsu 2361's, SC7003's, 4.3BSD Message-ID: <1421@cit-vax.Caltech.Edu> Date: Sun, 4-Jan-87 01:04:53 EST Article-I.D.: cit-vax.1421 Posted: Sun Jan 4 01:04:53 1987 Date-Received: Sun, 4-Jan-87 04:35:55 EST References: <6387@allegra.UUCP> <1305@cit-vax.Caltech.Edu> <4892@mimsy.UUCP> Organization: California Institute of Technology Lines: 30 Summary: nobody cares about multi-drive throughput Xref: mnetor comp.periphs:92 comp.unix.wizards:490 In article <1305@cit-vax.Caltech.Edu> I write: > The sector search "optimization" code, which is even more wrong in > 4.3bsd, will degrade throughput when several drives are active on > one controller. ... Making it work correctly would require more > cooperation between the hp driver and the mba code, and isn't worth it. In article <4892@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) asks: > Why not? The object of sector searching is to avoid tying up the controller with disk rotational latency. It only gains something if another drive is ready to do a transfer immediately. This is an unlikely event on a low-end VAX, which doesn't have enough CPU power to keep a Massbus controller that busy, and which can ill afford the additional interrupts. For sector searching to work properly, the disk has to repeat the search until it finds the controller ready to transfer immediately. If it were to quit after one search (like in 4.3 BSD), it gets put in a FIFO queue, the same as if it hadn't searched at all. My first attempt at a rewrite did not quit after one search; it searched forever, because it depended on the mindist/sdist/maxdist numbers being sane, and they weren't. 4.3 BSD has all the sdist and maxdist numbers interchanged. That is the great weakness of sector searching: it requires tuning numbers, and obviously, nobody cares about it enough to even make sure the numbers are in the right order! If it's not going to be done right, why do it at all? Don Speck speck@vlsi.caltech.edu {seismo,rutgers,ames}!cit-vax!speck