Date: Thu, 21 Aug 1997 08:55:02 -0600 From: "Justin T. Gibbs" <gibbs@plutotech.com> To: Andreas Klemm <andreas@wup.de> Cc: Stephen McKay <syssgm@dtir.qld.gov.au>, freebsd-hardware@FreeBSD.ORG Subject: Re: Quiet SCSI disk? Message-ID: <199708211455.IAA17197@pluto.plutotech.com> In-Reply-To: Your message of "Thu, 21 Aug 1997 12:02:26 %2B0200." <19970821120226.20829@wup.de>
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>AHC driver uses normally 8 tags. >Or 4 if card doesn't have much SCM space. >And reduces the number of opennings to x, if a queue full race >condition occurs, where x = 3 using the IBM DORS... which is nearly >the same as running without tagged queuing ;-) > This changes with the new CAM code. All controller types can now page up to 255 commands (the aic7770, aic7850, and aic7860 doing this via "indirect paging" which costs an extra byte sized DMA). CAM also is much better at handling the QUEUE FULL and BUSY status conditions making it possible to properly dynamically size the number of transactions to use so long as the drive isn't "broken". On my CAM test system, the number of tags starts out at 64 for all devices, drops to 63 on the Seagate Hawks, drops to 16 on the Quantum Empire 2100, and is fixed (via a quirk entry) at 24 on the two Atlas II drives I have. The Atlas II is fixed at a particular number since it will return QUEUE FULL due to temporary internal resource shortages yet generally it can handle lots of tags. The algorithm that CAM uses is to "freeze" the transaction queue to a device that returns QUEUE FULL until a successful transaction completes, so even if the tag number is not reduced, the drive is not bombarded by requests as is the case in the current system. The increased number of tags that I'm able to use now makes the system much more responsive. I don't think that 4 is the optimum number for seek intensive workloads like a news expire. >-- >Wiechers & Partner Datentechnik GmbH phone: +49 2173 3964 161 >Support Unix - Andreas Klemm fax: +49 2173 3964 222 >An der alten Ziegelei 2 mail1: andreas.klemm@wup.de >D-40789 Monheim mail2: andreas@FreeBSD.ORG > -- Justin T. Gibbs =========================================== FreeBSD: Turning PCs into workstations ===========================================
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