Date: Fri, 31 Mar 2006 15:48:57 -0500 From: "Bob Johnson" <fbsdlists@gmail.com> To: "Danny MacMillan" <flowers@users.sourceforge.net> Cc: bobo1009@mailtest2.eng.ufl.edu, freebsd-questions@freebsd.org Subject: Re: Can FreeBSD safely use a (un-booted from) drive that is invisible to the BIOS? Message-ID: <54db43990603311248u5be790d5y91490e73bc9cf185@mail.gmail.com> In-Reply-To: <20060331200602.GA68298@aldebaran.local> References: <20060331200602.GA68298@aldebaran.local>
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On 3/31/06, Danny MacMillan <flowers@users.sourceforge.net> wrote: > [,,,] > ad0 is the boot drive. It is recognized by the BIOS, obviously, and > has been in the machine for some years. ad2 is a new drive I just > added to the machine yesterday. It is not visible to the BIOS at all. > If anyone can posit a reason it would not be visible to the BIOS, I > would like to know the answer. The BIOS supports LBA and ad0 is more > than 8GB so it wouldn't appear to be the 8GB limit, and the next limit > I am aware of is comfortably larger than 76GB. If ad2 were operating as the slave drive without a master on that controller, that could explain it, but that doesn't seem to be what's happening here. Are you sure you don't have the second drive disabled in the BIOS somehow? > [...] > At any rate ... it is not visible to the BIOS, but it is visible to > FreeBSD. Since I'm not booting from the drive, I think it shouldn't > matter ... but when I use Fdisk from sysinstall I get the following > familiar error message: > > |WARNING: A geometry of 155061/16/63 for ad2 is incorrect. Using =A6 > =A6a more likely geometry. If this geometry is incorrect or you =A6 > =A6are unsure as to whether or not it's correct, please consult =A6 > =A6the Hardware Guide in the Documentation submenu or use the =A6 > =A6(G)eometry command to change it now. =A6 > =A6 =A6 > =A6Remember: you need to enter whatever your BIOS thinks the =A6 > =A6geometry is! For IDE, it's what you were told in the BIOS =A6 > =A6setup. For SCSI, it's the translation mode your controller is =A6 > =A6using. Do NOT use a ``physical geometry''. | > > Since I don't actually know what the BIOS thinks the geometry is, > I got cold feet and decided to ask the list. I don't =3Dthink=3D it > should matter, since the BIOS shouldn't ever touch the disk, at least > as far as my understanding goes. FreeBSD uses BIOS routines to start the boot process, then uses its own idea of what's on the disk. So, as far as I know, you will only have a problem if they are different enough to either cause the boot process to fail, or on a dual boot system, to cause Windows to think the partitions are in different places than does FreeBSD, or if your BIOS is picky about the partition table. A few years ago I started ignoring that message and it's worked for me. I just let sysinstall do what it wants (I believe I started that practice when a bug in sysinstall gave me no choice). I *think* that with modern block addressed, i/o buffered disks, on which the "physical geometry" is an illusion anyway, the only real problem you can run into is different ideas of the total size of the disk, i.e. where the last usable block is. One "geometry" might give you a few megabytes more than another geometry, but the difference is at the end of the disk. That isn't going to have any effect on booting (assuming the BIOS is willing to start the boot process), and not likely to even be a problem when dual booting. > > I do have one concern. This drive was purchased more or less to act > as an emergency backup of the drive that's already in there. If ad0 > ever fails, ad2 drive will have to be put in a new machine whose BIOS > recognizes it in order to boot. If I accept the mystery geometry for > the drive today, will I later face a problem where the BIOS disagrees > and the drive will be unbootable? > If my understanding is correct, it is unlikely to cause a problem, but it might. The BIOS routines will still be able to read the first few sectors to start the boot process. If your BIOS is so picky that it notices that the partition table claims to use bytes beyond what it thinks is the end of the disk (or some other imagined offense), and refuses to boot, then you might have a problem. I've seen such picky BIOSes, but not for several years. I think (hope) that manufacturers are learning that quibbling over such things doesn't make the system better. If you were to change the geometry settings of a disk after you put a filesystem on it, you would likely trigger other issues, but that's not what you're asking. > Thank you for your kind attention. Good luck, - Bob
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