Date: Thu, 31 Aug 1995 15:21:57 -0700 From: "Justin T. Gibbs" <gibbs@freefall.FreeBSD.org> To: "Rodney W. Grimes" <rgrimes@gndrsh.aac.dev.com> Cc: FreeBSD-hackers@freebsd.org (FreeBSD hackers) Subject: Re: 4GB Drives Message-ID: <199508312221.PAA29434@freefall.FreeBSD.org> In-Reply-To: Your message of "Thu, 31 Aug 95 14:47:50 PDT." <199508312147.OAA12790@gndrsh.aac.dev.com>
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>You hit the nail on the head for what I need to measure. > >> You may also want to double wink the >> led if you get a reconnect so that you can differentiate the case when >> you lose arbitration to a reconnecting target. As you can see, you >> can do a lot with that little led. > >Don't need to, no one can reselect, there is only 1 disk on the >scsi bus in the prototype 2 drive 2 controller base line. I did that >to eliminate all contention situations while studing the rotation offset >problem and zone size effects. I need to clearly see when each bus goes >to data phase so I can understand how the I/O's get processed etc, etc. Right, but with tagged queuing enabled, the drive you want to talk to may reconnect for a prior I/O just as you start selection so it can happen even in a single disk configuration. >> >> So, are you planning on using a 3940W for this type of application? >> > >> >Possibly, prototypes are NCR 825 and aha2940W right now... >> >> Remeber that the 3940 allows up to 255 queued commands as oposed to >> the 16 of the 2940W. As soon as we get better queuing code into >> FreeBSD, this will make a difference. > >Actually I would like to turn off queued commands for the time being, >is there an easy way to do that (I have not been delving into the aic >code like I should be if I need to ask that one, but since I have your >attention any way :-). Just don't enable it. Its a kernel config option (AHC_TAGENABLE). One thing to watch out for is that even without tagged queuing, you can get up to two requests per target sitting on the board. You'll have to blink something when the command that gets taken off the queue is for a target that has an active command pending since it will re-queue the transaction in that case. This will happen when the first transaction to the disk disconnects to do the seek. >I need to fully evaluate very simple systems to understand how things >are going to behave, and complications like having the controller stacking >up queued I/O's on me is going to confuse me looking at timing with respect >to when the host releases the I/O in the driver (right now I have a bit >to watch on the scope when the stripe layer drops the I/O to the driver >layer and then watch the busy bit on the scsi bus, command queueing in >the controller is screwing me correlationg the two events :-(). Understood. > > >-- >Rod Grimes rgrimes@gndrsh.aac.dev.com >Accurate Automation Company Reliable computers for FreeBSD -- Justin T. Gibbs =========================================== Software Developer - Walnut Creek CDROM FreeBSD: Turning PCs into workstations ===========================================
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