Date: Tue, 6 Aug 1996 11:14:36 -0400 (EDT) From: Chuck Robey <chuckr@glue.umd.edu> To: Gary Chrysler <tcg@ime.net> Cc: Annelise Anderson <andrsn@andrsn.stanford.edu>, Steve Marmer <ids@interlog.com>, questions@FreeBSD.org Subject: Re: Panic mounting root Message-ID: <Pine.OSF.3.95.960806105423.26504A-100000@fiber.eng.umd.edu> In-Reply-To: <32074437.67E@ime.net>
next in thread | previous in thread | raw e-mail | index | archive | help
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. 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? 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. ----------------------------+----------------------------------------------- Chuck Robey | Interests include any kind of voice or data chuckr@eng.umd.edu | communications topic, C programming, and Unix. 9120 Edmonston Ct #302 | Greenbelt, MD 20770 | I run Journey2 and n3lxx, both FreeBSD (301) 220-2114 | version 2.2 current -- and great FUN! ----------------------------+-----------------------------------------------
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.OSF.3.95.960806105423.26504A-100000>