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Path: euryale.cc.adfa.oz.au!newshost.carno.net.au!harbinger.cc.monash.edu.au!munnari.OZ.AU!news.ecn.uoknor.edu!feed1.news.erols.com!howland.erols.net!surfnet.nl!newsfeed.sunet.se!news01.sunet.se!sunic!mn6.swip.net!seunet!news2.swip.net!nike.volvo.se!cyklop.volvo.se!peter From: peter@cyklop.volvo.se (peter hakanson) Newsgroups: comp.os.linux.misc,comp.os.linux.setup,comp.unix.bsd.bsdi.misc,comp.unix.bsd.misc Subject: Re: IDE vs SCSI (was Re: Linux vs BSD) Followup-To: comp.os.linux.misc,comp.os.linux.setup,comp.unix.bsd.bsdi.misc,comp.unix.bsd.misc Date: 27 Jan 1997 15:42:36 GMT Organization: Volvo Corp. Lines: 42 Message-ID: <5ciidc$51l@nike.volvo.se> References: <32DFFEAB.7704@usa.net> <5c155c$p6u@raven.eva.net> <87k9p4rckd.fsf_-_@murkwood.gaffaneys.com> <5c8a39$7tn@usenet.srv.cis.pitt.edu> <5ce3cu$vs2$1@brokaw.wa.com> NNTP-Posting-Host: cyklop.volvo.se X-Newsreader: TIN [version 1.2 PL2] Xref: euryale.cc.adfa.oz.au comp.os.linux.misc:152569 comp.os.linux.setup:92345 comp.unix.bsd.bsdi.misc:5616 comp.unix.bsd.misc:1927 Tim, this put the hammer on one importent point, one should be able to modify i/o strategy , disk by disk. (since a mix of disks is typical,and the same strategy will not fit all types of disks) Still remembers how this could be done on rsx11 on some RP type disks... Anyone doing some work in this area ? peter h Tim Smith (tzs@coho.halcyon.com) wrote: : Mark Hahn <hahn@neurocog.lrdc.pitt.edu> wrote: : >: With the new tag-command-queuing (or whatever its officially called), : >: each drive can queue up to 32 commands. The most obvious advantage : >: this gives is optimizing head accesses, optimally servicing all 32 : >: commands with one sweep across the platters. : > : >this is pretty iffy. while its certainly true that the OS has little : >hope of optimally scheduling head motion (because of zoned recording), : >blocks _are_ sorted, and so the gross motions are already in a good : >order. where tagged queueing would give SCSI an advantage is those : >places where naive elevator sorting (by block number) doesn't work: : >lower-numbered blocks within the current cylinder and bad-block : >remapping. : What you are both overlooking is the fact that in modern mid to high-end : drives, rotational latency is more of a problem than head movement time. : This completely changes the way you need to sort your requests. : Also, command overhead is a big factor in many disk operations. Command : queuing allows you to bury much of that overhead in what otherwise would : be dead time. : --Tim Smith -- -- <peter.devnull@cyklop.volvo.se> (remove ".devnull" before use!) Peter Hakanson VolvoData Dep 2580 phone +46 31 66 74 27