Date: Sun, 12 Mar 2000 14:58:26 +0100 (CET) From: Soren Schmidt <sos@freebsd.dk> To: ssiyer@cs.rice.edu (Sitaram Iyer) Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: ATA-disk question Message-ID: <200003121358.OAA73815@freebsd.dk> In-Reply-To: <20000311173548.A1570@cs.rice.edu> from Sitaram Iyer at "Mar 11, 2000 05:35:48 pm"
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It seems Sitaram Iyer wrote: > (kernel 4.0-20000208-CURRENT) > > I have a question about adstrategy() in ata-disk.c: it says > > bufqdisksort() -- which appends to the drive queue using CSCAN, > > and immediately, without a check for adp->active, there is > > ad_start() -- which removes it from there and puts it into the > controller queue, thereby nullifying CSCAN's effect > completely (drive queue length always = 0 or 1) and > blowing up the controller TAILQ to occasional spurts > with tens of requests (I measured this). > > on browsing through cvsweb, I find that such a check used to be there > earlier ( bufqdisksort(&adp->queue, bp); if (!adp->active) ad_start(adp); ) > till "1.14 Fri Jun 25 9:02:59 1999 UTC by sos" (the 9th ATA update), and the > only thing in the commit log that's making sense to me is: > > "The disk driver has been changed a bit to prepare for tagged > queing, which is next on my list." -- sos > > What's the exact rationale behind doing away with CSCAN? is it temporary? > Its not entirely clear to me how "tagged queueing" (whatever that is) would > remedy the fact that requests are taken away from drive queues as soon as > they're put there. This was part of the code for tagged queueing, but since that has been put on hold for now, this is clearly a bug. Tagged queueing is the term for having multiple outstanding requests on the disk, and let the disk sort out in which ord to complete them. -Søren To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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