Date: Tue, 06 Aug 1996 13:19:39 -0400 From: Gary Chrysler <tcg@ime.net> To: Chuck Robey <chuckr@glue.umd.edu> Cc: Annelise Anderson <andrsn@andrsn.stanford.edu>, Steve Marmer <ids@interlog.com>, questions@FreeBSD.org Subject: Re: Panic mounting root Message-ID: <32077EAB.563D@ime.net> References: <Pine.OSF.3.95.960806105423.26504A-100000@fiber.eng.umd.edu>
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Chuck Robey wrote: > > On Tue, 6 Aug 1996, Gary Chrysler wrote: > > > > > > But bootable partitions *must* be under cylinder 1024. (504M) > > > > > This is a limitation of the PC System BIOS boot code! > > > > > > > > I think this refers only to IDE/EIDE drives. I have a 2.1G Conner > > > > fast scsi 2 installed as a second hard drive on an Adaptec 1542CF, > > > > and FreeBSD is running in the *last* 660 mb. on that drive (booting > > > > from the OS/2 boot manager). > > > > > > Actually, it applies to both, but scsi drives all support more flexible > > > remapping of sectors to make them all fit under 1024 cylinders, so they > > > don't feel the pinch. BTW, the whole partition DOES NOT have to be under > > > 1024 cylinders, just the boot sector of that partition. > > > > > > > Chuck, While we're on the subject I'd like to clear this up in > > my mind. > > > > A FreeBSD slice is: Sections of a partition?? > > So properly stated: Only the root slice needs to be under 1024. > > (If needed :) > > Is that about how it works.. I never fully understood the slice. > > The need to fully understand hasn't came upon me yet.. :) > > > > I'm not a SCSI guru, But it is my understanding that SCSI's access > > in a LBA method. > > ie: c:0 h:0 s:0 = LBA 0 > > c:0 h:0 s:1 = LBA 1 > > etc ... > > etc ... > > > > It is also my understanding that most OS's (Advanced OS's) also use > > a LBA method. > > So if your drive uses CHS, The bios has to translate OS calls > > to/from CHS to write/read. > > > > But like I said, I'm no guru on the internals of disk IO. > > I welcome the knowledge though. > > I don't use IDE drives, and LBA refers ONLY to IDE drive remapping, so I'm > just a little hazy on it. SCSI systems have no LBA as such. SCSI > controllers have 2 options on remapping, depending on the size of the > drive, and that essentially removes any consideration, because the > remappings available move the entire drive inside 1024 cylinders. > Ok, I used LBA method for a reason, I was under the impression that SCSI disks didn't use a CHS scheme, So I used LBA method as a alternative for my question.. But the way I see your explanation below they do. > Understand that the word 'partition' has two meanings here, which is the > base cause of all the confusion. Dos partitions are areas of the disk set > aside and noted in partition tables. The location of the partition tables > is set in stone, a standard. For Unixes, on computers OTHER THAN pc > compatibles, this is also true. The trouble comes in the way the standard > unix disk partitioning has been adapted to work with PC disks. > > FOR PC DISKS, FreeBSD makes 'partitions' or slices (same meaning here) by > taking a dos partition and splitting it up. Thus, multiple FreeBSD > partitions (or slices, I like slices much better for the word here) sit > inside a dos partition. On an earlier machine of mine, I had 4 dos > partitions, where I used partition 1 for a small dos segment, and > partition 2 for FreeBSD. FreeBSD then took that partition 2, and split it > into three of what it calls partitions, which has nothing in common with > the dos partition table at all. > > FreeBSD keeps track of it's partitions using the disklabel, which might be > loosely called it's equivalent of a dos partition label. The disklabel > resides inside one dos partition, and defines the slices (or partitioning) > that FreeBSD uses of it's dos partition. On that older system of mine, I > had three FreeBSD slices, all of which sat in the second dos partition. > > To get a new partition, on a new disk or something, to be useful to > FreeBSD, first you use your dos tool (or FreeBSD's sysinstall) to define > the dos partition. That gives you an area on that disk that you can use > for FreeBSD. Then, after you decide how you're going to use the FreeBSD > part (how many chunks are you going to cut it into, what size, what > filesystems you'll put there) then you disklabel the FreeBSD partition, > whic tells FreeBSD how to use it's partition. If you're going to have any > of the FreeBSD slices bootable, then make sure that slice is below 1024 > cylinders. That means, of course, that at least part of the dos partition > that FreeBSD is using is also below 1024. > > After you've disklabeled the FreeBSD slices, then you can newfs the slices > so that they can be used by FreeBSD. You'll probably want to modify your > /etc/fstab so that FreeBSD will use the partition the way you want, either > for swapping, or for some mountable filesystem. > > Have I completely confused you yet? Nope, In fact I got a grip on it.. Thanks! > > This problem wouldn't exist on, say, a DEC system, because the Unix > partitioning is the base system for the disk, but for PCs, they stick the > structure inside the dos partition structure, and then called it with > names guaranteed to cause confusion. > Confusion, Yup, Thats a fact.. :) Thanks for the clearity. -Enjoy Gary ~~~~~~~~~~~~~~~~ Improve America's Knowledge... Share yours The Borg... Where minds meet (207) 929-3848
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