RE: Drive Geometry -- confusion.

From: Jim (jhorvath_at_frabill.com)
Date: 10/29/03

  • Next message: Lowell Gilbert: "Re: installing freebsd on another disk" 
    To: "nw1" <network101@covad.net>, <freebsd-questions@freebsd.org>
Date: Wed, 29 Oct 2003 07:46:18 -0600

    > -----Original Message-----
    > From: owner-freebsd-questions@freebsd.org
    > [mailto:owner-freebsd-questions@freebsd.org]On Behalf Of nw1
    > Sent: Wednesday, October 29, 2003 5:51 AM
    > To: freebsd-questions@freebsd.org
    > Subject: Drive Geometry -- confusion.
    >
    >
    > FreeBSD 4.8-RELEASE-p13
    > This document can also be viewed @
    > https://69.3.136.141/freebsd/installation/geometry
    >
    > Install 4.8 from CD-rom
    >
    > Drive Geometry confusion.
    > My BIOS allows me a very limited amount of settings when it comes
    > to an HDD.
    > The following are the only settings offered by my BIOS:
    >
    > Read Prefetch - [Disabled | Enabled] - set to (Disabled)
    > Disk Bios Translation - [LBA | CHS | Disabled] - set to (LBA)
    > Local Bus IDE Mode - [High Performance | Compatible] - set to
    > (High Performance)
    >
    > Size - 8455 MB --Auto-detected-- This is actually an 80GB HDD
    > model: WD800JB.
    > (Unable to change the 'Size' setting.)
    >
    > The only other setting that could remotely be associated with any HDD's
    > installed would be the 'boot order' of the devices.
    >
    > ... sysinstall main menu | Custom | Partition :
    >
    > Here's where I'm confused; The following dialog message is printed:
    >
    > WARNING: A geometry of 155061/16/63 for ad0 is incorrect. Using
    > a more likely
    > geometry. If this geometry is incorrect or you are unsure as to
    > whether or not
    > it's correct, Please consult the Hardware Guide in the
    > Documentation submenu or
    > use the (G)eometry command to change it now.
    >
    > Remember: you need to enter whatever your BIOS thinks the
    > geometry is! For IDE,
    > it's what you were told in the BIOS setup. For SCSI, its the
    > translation mode
    > your controller is using. Do NOT use a "physical Geometry".
    >
    > ## With the above two (2) paragraphs having been said, All I
    > have at this point
    > is the physical geometry given by the WD800JB specs --which the above
    > paragraphs are currently fobbing me to use.
    >
    > Lets press <enter> for OK and move further into this...
    >
    > We're in the FDISK Partition Editor where it states on the second
    > line from the
    > top: DISK Geometry: 9729 cyls/255 heads/63 sectors = 156296385
    > sectors (76316MB)
    >
    > If I understand correctly, the physical geometry of this WD800JB is:
    >
    > Cylinders = 16383
    > Heads = 16
    > Sectors/Track = 63
    >
    > As seen on/at: The Westerndigital.com site.
    > http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/std_adp.php?p_
    faqid=699&p_cr
    eated=1037217622

    I'm inclined to use the above three (3) values, however, The above FreeBSD
    installation 'dialog' expressly *forbids the use of the physical geometry.
    Is this not confusing to you as well?

    Why does the installation forbid us to use the 'physical geometry? --I'm
    sure
    there's a valid reason, I'd like to hear it.

    Can someone explain what to do and, for future reference, <-- *why do that?

    As stated above; I'm confused.

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    This was a message from a little while ago that might shed some light:

    On Tue, 9 Sep 2003, at 12:17:54, Mark Terribile wrote:
    >>>>
    > Well, I've got my 120G disk up and running and
    > now I'd like to understand something odd.
    >
    > I went into /stand/sysinstall to put down the
    > partition and slice tables and the FS and found that
    > it determined the geometry (cyl/head/sectors) as
    > 15107/255/63 . Since it warned that the values must
    > match the BIOS values, I set them to the 59131/16/255
    > that the BIOS reports. It rejected this entry and
    > went back to its own. I also tried the values
    > reported
    > by the ata driver on startup; these too were rejected.
    >
    > Since it's working (with about 117G -- that's
    > Gi_b_abytes) it must be OK (famous last words!) but
    > what's going on here? Why does it demand the values
    > reported by the BIOS if it will refuse them, and why
    > does the driver come up with another set of values,
    > also unacceptible to the disklabel/fdisk machinery?
    >>>>

    I expect that most people reading this are aware of some of the
    limitations of "BIOS" disk addressing, including the physical disk
    addresses in the disk partition table in the MBR (master bootstrap
    record) of bootable PC disk drives which allocates only 10 bits for
    the cylinder number, 8 bits for the head number, and 6 bits for the
    sector number. There is also a Fixed Disk "Drive Parameter Table"
    available through the traditional BIOS. The table format allocates
    16 bits for the number of cylinders, 8 bits for the number of heads,
    and 8 bits for the number of sectors per track. Since BIOS sector
    numbers begin at one (not zero), the BIOS addressing limitations are
    usually given as 1024 cylinders, 255 heads, and 63 sectors per track.
    (Note: 1024*255*63 sectors is just under 8GB.)

    The ATA/IDE specifications limit you to 65535 cylinders, 16 heads,
    and 255 sectors per track (see http://www.t13.org). If you accept
    both sets of limitations at once, you get a maximum of 1024 cylinders,
    16 heads and 63 sectors, resulting in the infamous "528" MB hard
    disk size limitation.

    In an effort to work around this unacceptable limitation, your
    BIOS may pretend that your disk drive has some other geometry,
    typically using 255 heads and 63 sectors per track though other
    fake geometries are sometimes used. When a program uses the
    traditional BIOS system call to do disk I/O (machine instruction
    "int 13" with a small function code < 0x16 in the AH register),
    it must use the same disk geometry that is used by the BIOS for
    interpreting disk addresses. Otherwise the BIOS might translate
    the specified physical disk address into the wrong actual disk
    address.

    (Note: there are also "Enhanced Disk Drive Services" (EDD) using
    AH register function codes >=0x40 which have bigger limitations and
    might support Logical Block Addresses (LBA) instead of physical disk
    addresses. EDD is also described at http://www.t13.org. I don't
    know which BIOS/bootstrap programs support/use EDD. Modern BIOS
    presumably provides EDD services. It doesn't do you any good if
    your BIOS supports EDD but your bootstrap program doesn't use it.
    The FreeBSD minimal bootstrap that just boots the active partition,
    /usr/src/sys/boot/i386/mbr/mbr.s, appears to use EDD services when
    available. The "booteasy" program that allows you to specify a
    desired partition via a function key, which may be the program
    .../tools/srcs/bteasy/BOOT.ASM in your FreeBSD distribution, does
    not seem to use EDD services. (I am not certain that this is the
    bootstrap installed by /stand/sysinstall.) The GRUB bootstrap
    program (in /usr/ports/sysutils/grub) appears to use both EDD
    services and LBA when available.)

    The only way to discover the geometry expected by the BIOS is to
    ask the BIOS. This is no problem for bootstrap programs and some
    Microsoft "operating systems" that use the BIOS disk services for
    all disk I/O, but the BIOS services cannot be used by a "real"
    operating system after it takes over the interrupt and virtual
    memory management hardware and possibly trashes the data structures
    built by the BIOS during POST (Power On Self Test). A BIOS service
    routine is likely to do things unacceptable to a "real" OS, such
    as screwing with system hardware configuration registers or going
    into indefinite spin loops waiting for device register bits to
    change.

    The FreeBSD /stand/sysinstall and fdisk programs need to use the
    BIOS disk geometry when constructing MBR partition tables. It would
    be nice if the kernel could get this information from the BIOS early
    in the bootstrap process and stash it somewhere for further reference,
    but this seems not to be a FreeBSD kernel feature. The fdisk and
    /stand/sysinstall programs apparently try to guess the BIOS disk
    geometry and then ask the user for the "correct" geometry. It would
    only be fair if the man pages for those programs told you *how*
    to determine the correct geometry.

    Some BIOS self-configuration programs (what you boot when holding
    down the DEL key) may tell you about some ATA/IDE drives, but I am
    not sure I trust them. One BIOS gives me a choice of three or four
    different geometries for each drive, some of which are suspect
    because they specify more than 63 sectors per track. Keep in mind
    that your BIOS may support EDD services but your bootstrap program
    might not use them.

    An old hack for determining BIOS disk geometry is to clear the disk
    MBR (zero the first sector), boot DOS, create a new partition table
    with a moderately sized DOS partition using the DOS FDISK program,
    and then use the FreeBSD fdisk program to examine the result. Each
    partition in the MBR partition table is described both by beginning
    and ending physical disk addresses and by a 32 bit starting block
    number (zero origin) and a 32 bit size in blocks. The DOS partition
    should begin after the first track. Its starting block number should
    be the number of sectors per track. From that and the partition size
    and the physical address of the last block in the partition, you can
    compute the number of heads.

    You might try booting a GRUB floppy and use the "geometry" command
    to display the disk geometry, but I am not sure where GRUB gets its
    information.

    When I recently used the FreeBSD 5.1 /stand/sysinstall program on a
    120GB serial ATA disk drive, the program program first said that the
    drive had 16 heads and 255 sectors per track and some ridiculous
    number of cylinders. Then the program said that this geometry was
    incorrect and that it was going to use 255 heads and 63 sectors per
    track. Then it strongly advised me to tell it the actual geometry
    used by the BIOS. I doubt it got the initial geometry from the BIOS
    (which ought to know better than to use more than 63 sectors per track).
    Perhaps it got the geometry from an ATA disk controller function.
    This seems more likely and it explains the geometry. I would expect
    the BIOS to use 255 heads and 63 sectors per track. This is what the
    GRUB geometry command reports for the drive. So I let /stand/sysinstall
    use that geometry.

    Since I don't boot off that drive and I don't access it using any
    operating system that uses BIOS disk I/O, it won't make any difference
    if the physical disk addresses are wrong. I am pretty sure that
    FreeBSD uses the partition starting block numbers and sizes in the
    partition table instead of the physical C/H/S disk addresses.

    For an alternative discussion of BIOS geometry,
    see http://www.ata-atapi.com.

    Dan Strick
    strick@covad.net
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