Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!lll-winken!uunet!cyclops!csense!bote From: bote@csense.UUCP (John Boteler) Newsgroups: comp.unix.i386 Subject: Re: Mylex SCSI Controller Message-ID: <372@csense.UUCP> Date: 27 Sep 89 17:46:44 GMT References: <19245@gatech.edu> Organization: Common Sense Computing, McLean Va Lines: 31 From article <19245@gatech.edu>, by ken@gatech.edu (Ken Seefried III): > In article <369@csense.UUCP> bote@csense.UUCP (John Boteler) writes: >>Curious as to how a SCSI host controller can easily perform >>cache operations? > Well...they have a microprocessor and a hoard of memory on board. > Think about it...or at least go review the recent discussion of > the DPT controller for some excellent commentary, pro and con, on > the issues involved in caching disk controllers... I don't need to. I talked to one of the engineers from DPT at Comdex. He explained to me the difficulty of caching a SCSI host controller: since the controller handles up to seven devices on the SCSI bus, putting the cache system on the controller makes for little improvement, if any. The best solution, and one which has been implemented by at least one drive manufacturer, is to put the caching components on the SCSI drive. When a SCSI request is sent to the drive by the controller, the controller is free to service one of the other six devices on the bus and the drive can check its own cache for the presence of the data requested and respond accordingly. The previous discussion of caching controllers pertained primarily to RLL and ESDI formats. -- Bote Old & Improved path!: uunet!comsea!csense!bote New & Improved path!: {zardoz|uunet!tgate|cos!}ka3ovk!media!cyclops!csense!bote