PMAC-FDISK(8)	    Linux Programmer's Manual	    PMAC-FDISK(8)



NAME
       pmac-fdisk - Partition table manipulator for Linux

SYNOPSIS
       pmac-fdisk [-u] device

       pmac-fdisk -l [-u] device ...

       pmac-fdisk -s partition ...

       pmac-fdisk -v

DESCRIPTION
       Hard  disks  can be divided into one or more logical disks
       called partitions.  This division is described in the par_
       tition table found in sector 0 of the disk.

       In  the	BSD  world  one  talks	about `disk slices' and a
       `disklabel'.

       Linux needs at least one partition, namely  for	its  root
       file  system.   It  can	use swap files and/or swap parti
       tions, but the latter are more efficient. So, usually  one
       will  want a second Linux partition dedicated as swap par
       tition.	On Intel compatible hardware, the BIOS that boots
       the  system can often only access the first 1024 cylinders
       of the disk.  For this  reason  people  with  large  disks
       often create a third partition, just a few MB large, typi
       cally mounted on /boot, to store the kernel  image  and	a
       few  auxiliary  files  needed  at boot time, so as to make
       sure that this stuff is accessible to the BIOS.	There may
       be reasons of security, ease of administration and backup,
       or testing, to use more than the minimum number of  parti
       tions.

       pmac-fdisk  (in	the  first  form of invocation) is a menu
       driven program for creation and manipulation of	partition
       tables.	 It understands DOS type partition tables and BSD
       or SUN type disklabels.

       The device is usually one of the following:
	      /dev/hda
	      /dev/hdb
	      /dev/sda
	      /dev/sdb
       (/dev/hd[a-h] for IDE disks, /dev/sd[a-p] for SCSI disks).
       A device name refers to the entire disk.

       The  partition  is  a  device name followed by a partition
       number.	For example, /dev/hda1 is the first partition  on
       the first IDE hard disk in the system.  IDE disks can have
       up to 63 partitions,  SCSI  disks  up  to  15.	See  also
       /usr/src/linux/Documentation/devices.txt.

       A  BSD/SUN  type  disklabel can describe 8 partitions, the
       third of which should be a `whole disk' partition.  Do not
       start  a  partition  that  actually  uses its first sector
       (like a swap partition) at cylinder  0,	since  that  will
       destroy the disklabel.

       An IRIX/SGI type disklabel can describe 16 partitions, the
       eleventh of which should be an entire `volume'  partition,
       while  the  ninth  should be labeled `volume header'.  The
       volume header will also cover the partition  table,  i.e.,
       it  starts  at block zero and extends by default over five
       cylinders.  The remaining space in the volume  header  may
       be  used  by  header directory entries.	No partitions may
       overlap with the volume header.	Also do  not  change  its
       type  and make some file system on it, since you will lose
       the partition table.  Use this type  of	label  only  when
       working	with Linux on IRIX/SGI machines or IRIX/SGI disks
       under Linux.

       A DOS type partition table can describe an unlimited  num
       ber  of	partitions.  In  sector  0  there is room for the
       description of 4 partitions  (called  `primary').  One  of
       these  may be an extended partition; this is a box holding
       logical partitions, with descriptors  found  in	a  linked
       list  of sectors, each preceding the corresponding logical
       partitions.  The four primary partitions, present or  not,
       get  numbers 1-4.  Logical partitions start numbering from
       5.

       In a DOS type partition table the starting offset and  the
       size  of each partition is stored in two ways: as an abso
       lute number of sectors (given in 32 bits) and as a  Cylin
       ders/Heads/Sectors triple (given in 10+8+6 bits). The for
       mer is OK - with 512-byte sectors this will work up  to	2
       TB.  The  latter has two different problems. First of all,
       these C/H/S fields can be filled only when the  number  of
       heads  and the number of sectors per track are known. Sec
       ondly, even if we know what these numbers should  be,  the
       24 bits that are available do not suffice.  DOS uses C/H/S
       only, Windows uses both, Linux never uses C/H/S.

       If possible, pmac-fdisk	will  obtain  the  disk  geometry
       automatically.	This is not necessarily the physical disk
       geometry (indeed, modern disks do not really have anything
       like a physical geometry, certainly not something that can
       be described in simplistic Cylinders/Heads/Sectors  form),
       but  is	the disk geometry that MS-DOS uses for the parti
       tion table.

       Usually all goes well by default, and there are	no  prob
       lems  if Linux is the only system on the disk. However, if
       the disk has to be shared with other operating systems, it
       is  often a good idea to let an fdisk from another operat
       ing system make at least one partition. When  Linux  boots
       it  looks at the partition table, and tries to deduce what
       (fake) geometry is  required  for  good	cooperation  with
       other systems.

       Whenever  a  partition table is printed out, a consistency
       check is performed on the partition table  entries.   This
       check verifies that the physical and logical start and end
       points are identical, and that the  partition  starts  and
       ends  on  a cylinder boundary (except for the first parti
       tion).

       Some versions of MS-DOS create  a  first  partition  which
       does  not begin on a cylinder boundary, but on sector 2 of
       the first cylinder.  Partitions beginning  in  cylinder	1
       cannot  begin on a cylinder boundary, but this is unlikely
       to cause difficulty unless you have OS/2 on your  machine.

       A  sync()  and a BLKRRPART ioctl() (reread partition table
       from disk) are performed before exiting when the partition
       table  has been updated.  Long ago it used to be necessary
       to reboot after the use of pmac-fdisk.	I  do  not  think
       this  is  the case anymore - indeed, rebooting too quickly
       might cause loss of not-yet-written data. Note  that  both
       the kernel and the disk hardware may buffer data.


DOS 6.x WARNING
       The  DOS  6.x FORMAT command looks for some information in
       the first sector of the data area of  the  partition,  and
       treats this information as more reliable than the informa
       tion in the partition table.  DOS FORMAT expects DOS FDISK
       to  clear the first 512 bytes of the data area of a parti
       tion whenever a size change occurs.  DOS FORMAT will  look
       at  this extra information even if the /U flag is given --
       we consider this a bug in DOS FORMAT and DOS FDISK.

       The bottom line is that if you use  pmac-fdisk  to  change
       the  size  of  a  DOS partition table entry, then you must
       also use dd to zero the first 512 bytes of that	partition
       before  using  DOS  FORMAT  to  format the partition.  For
       example, if you were using pmac-fdisk to make a DOS parti
       tion  table entry for /dev/hda1, then (after exiting pmac-
       fdisk and rebooting Linux  so  that  the  partition  table
       information  is	valid)	you  would  use  the  command "dd
       if=/dev/zero of=/dev/hda1  bs=512  count=1"  to	zero  the
       first 512 bytes of the partition.

       BE  EXTREMELY  CAREFUL  if you use the dd command, since a
       small typo can make all of the data on your disk  useless.

       For  best  results,  you  should always use an OS-specific
       partition table program.  For example, you should make DOS
       partitions with the DOS FDISK program and Linux partitions
       with the Linux pmac-fdisk program.


OPTIONS
       -v     Print version  number  of  pmac-fdisk  program  and
	      exit.

       -l     List   the   partition   tables  for  /dev/hd[a-d],
	      /dev/sd[a-h], and then exit.

       -u     When listing partition tables, give sizes  in  sec
	      tors instead of cylinders.

       -s partition
	      The size of the partition (in blocks) is printed on
	      the standard output.

BUGS
       There are several *fdisk programs around.   Each  has  its
       problems  and  strengths.   Try	them in the order cfdisk,
       pmac-fdisk, sfdisk.  (Indeed, cfdisk is a  beautiful  pro
       gram  that has strict requirements on the partition tables
       it accepts, and produces high  quality  partition  tables.
       Use  it	if  you  can.  pmac-fdisk is a buggy program that
       does fuzzy things - usually it happens to produce  reason
       able  results.  Its  single  advantage is that it has some
       support for BSD disk labels and	other  non-DOS	partition
       tables.	 Avoid it if you can.  sfdisk is for hackers only
       - the user interface is terrible, but it is  more  correct
       than pmac-fdisk and more powerful than both pmac-fdisk and
       cfdisk.	Moreover, it can be used noninteractively.)

       The IRIX/SGI type disklabel is currently not supported  by
       the kernel.  Moreover, IRIX/SGI header directories are not
       fully supported yet.

       The option `dump partition table to file' is missing.



Linux 2.0		   11 June 1998 	    PMAC-FDISK(8)