Date: Mon, 24 Jul 2000 01:14:07 +0200 From: Siegbert Baude <siegbert.baude@gmx.de> To: Mike Smith <msmith@freebsd.org>, "freebsd-questions@freebsd.org" <freebsd-questions@freebsd.org> Subject: Re: No /boot/loader (dangerously dedicated) Message-ID: <397B7C3F.AFE36990@gmx.de> References: <200007232102.OAA17013@mass.osd.bsdi.com>
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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. 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. 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. The geometry of the disk can thus be calculated out of the partition table. The Symbios' ones use this information to adapt their CHS to SCSI-LBA translation according to the disk you will connect to them. So you can use Adaptec formatted disks on Symbios controllers, but not the other way round. To get rid of the information about a special CHS-LBA-translation you have to remove the partition table, that is deleting the MBR (backup first, of course ;-) ). The same can happen to (E)IDE-disks if the BIOSes of different motherboards use different translation modes. Ciao Siegbert To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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