Date: Sun, 23 Jul 2000 16:40:30 -0700 From: Mike Smith <msmith@freebsd.org> To: Siegbert Baude <siegbert.baude@gmx.de> Cc: "freebsd-questions@freebsd.org" <freebsd-questions@freebsd.org> Subject: Re: No /boot/loader (dangerously dedicated) Message-ID: <200007232340.QAA17987@mass.osd.bsdi.com> In-Reply-To: Your message of "Mon, 24 Jul 2000 01:14:07 %2B0200." <397B7C3F.AFE36990@gmx.de>
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> Mike Smith wrote: > > > > > > > > I explicitly installed _all_ production machines "dangerously dedicated" > > > _to_get_around_ geometry problems. So I could swap SCSI controllers > > > of different brands with different translations. > > > > That was a silly idea. > > > > > So I just wouldn't need to care if the disk was > 1GB or not. > > > Try what happens if you install a "proper" partition table with > > > an Adaptec controller and swap it for an NCR one - boom! > > > > Actually, no. Both of these controllers' firmware will read the on-disk > > geometry and prefer that. > > That's not true for all SCSI controllers. It's true for the vast majority of those available today. You're welcome to go look at their BIOS code and check it out yourself. > SCSI-disks internally use LBA. If you call the SCSI Controller with > CHS-addressing, the BIOS of the SCSI-controller has to translate this. > For disks with a number of cylinders more than 1024 (>1GB) Adaptec > decided to introduce the geometry 255 heads 63 sectors. Symbios' ones > choose the number of heads and sectors depending of the disk capacity, > so that the number of cylinders will result in less than 1024. Yes, we know all this, only you've left out most of the interesting bits. Adaptec allow you a configuration option which mutates the selection of translation modes (there are actually three of them). Both Adaptec and Symbios/LSI will read the first block off the disk and prefer the geoemtry there if they find a valid MBR that matches the disk. > Symbios' adapters happened to be smarter than Adaptecs' (don't know if > this is still true for nowadays controllers) in the following manner. > Ends of partitions (slices) are always on the end of a cylinder (without > special reason, but all tools do it this way.) So any end address of a > partition can be used to figure out the maximum head and sector numbers. You can't expect this to work with a "dangerously dedicated" disk, which is one reason not to use it. > The same can happen to (E)IDE-disks if the BIOSes of different > motherboards use different translation modes. This is less of an issue with the EDD/EDD3 standard defining the traslation algorithms. -- ... every activity meets with opposition, everyone who acts has his rivals and unfortunately opponents also. But not because people want to be opponents, rather because the tasks and relationships force people to take different points of view. [Dr. Fritz Todt] To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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