Date: Wed, 3 Apr 1996 10:12:34 +0200 (MET DST) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: msmith@atrad.adelaide.edu.au (Michael Smith) Cc: bde@zeta.org.au, msmith@atrad.adelaide.edu.au, davidg@Root.COM, dutchman@spase.nl, freebsd-hackers@freebsd.org Subject: Re: HDD cpu usage (IDE vs. SCSI). Message-ID: <199604030812.KAA01334@labinfo.iet.unipi.it> In-Reply-To: <199604030310.MAA17901@genesis.atrad.adelaide.edu.au> from "Michael Smith" at Apr 3, 96 12:40:03 pm
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> > I would prefer lower latency to lower overhead in most cases. IDE disks > > have natural advantages in this area (no complicated SCSI protocol to > > interpreted by the slow i/o processor on the controller). > > Presuming you only have one application making requests in a linear fashion, > that's fine. Tagged queueing and disconnect rapidly improves things > once you start to get busy though. Disconnections are only useful to avoid locking the IO (SCSI) bus during (implicit) seeks -- it's nothing different from getting an interrupt when the transfer is complete. Queueing requests _within_ the disk is useful if the OS does not know the actual geometry of the disk, otherwise the OS can probably do a more sensible work of reordering requests. On a busy system with a single disk the only difference is really the PIO overhead vs. the SCSI controller overhead (and many of them are much slower than your main processor). Unless you want to say that SCSI supports many devices on the same bus, which is certainly a big advantage. Luigi ==================================================================== Luigi Rizzo Dip. di Ingegneria dell'Informazione email: luigi@iet.unipi.it Universita' di Pisa tel: +39-50-568533 via Diotisalvi 2, 56126 PISA (Italy) fax: +39-50-568522 http://www.iet.unipi.it/~luigi/ ====================================================================
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