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GPART(8)							      GPART(8)

NAME
       gpart - guess PC-type hard disk partitions

SYNOPSIS
       gpart [options] device

       Options:	    [-b	    <backup	MBR>][-C    c,h,s][-c][-d][-E][-e][-f]
       [-g][-h][-i][-K	<last-sector>][-k  <#  of  sectors>]  [-L]  [-l	  <log
       file>][-n    <increment>]    [-q][-s    <sector-size>]	[-t   <module-
       name>][-V][-v] [-W <device>][-w <module-name, weight>]

DESCRIPTION
       gpart tries to guess which partitions are on a hard disk.  If the  pri‐
       mary partition table has been lost, overwritten or destroyed the parti‐
       tions still exist on the disk but the operating	system	cannot	access
       them.

       gpart  ignores  the primary partition table and scans the disk (or disk
       image, file)  sector  after  sector  for	 several  filesystem/partition
       types.  It  does	 so by "asking" filesystem recognition modules if they
       think a given sequence of sectors resembles the beginning of a filesys‐
       tem  or	partition  type.  Currently the following filesystem types are
       known to gpart (listed by module names) :

       beos   BeOS filesystem type.

       bsddl  FreeBSD/NetBSD/386BSD disklabel sub-partitioning scheme used  on
	      Intel platforms.

       ext2   Linux second extended filesystem.

       fat    MS-DOS FAT12/16/32 "filesystems".

       hpfs   IBM OS/2 High Performance filesystem.

       hmlvm  Linux LVM physical volumes (LVM by Heinz Mauelshagen).

       lswap  Linux swap partitions (versions 0 and 1).

       minix  The Minix operating system filesystem type.

       ntfs   MS Windows NT/2000 filesystem.

       qnx4   QNX 4.x filesystem.

       rfs    The Reiser filesystem (version 3.5.X, X > 11).

       s86dl  Sun Solaris on Intel platforms uses a sub-partitioning scheme on
	      PC hard disks similar to the BSD disklabels.

       xfs    Silicon Graphic's journalling filesystem for Linux.

       More filesystem guessing modules can be added at runtime	 (see  the  -t
       option). Please consult the gpart README file for detailed explanations
       on how to create guessing modules. All modules  are  accompanied	 by  a
       guessing	 weight	 factor which denotes how "educated" their guesses are
       compared to other modules. This weight can be changed if a certain mod‐
       ule keeps on mis-identifying a partition.

       Naturally  only partitions which have been formatted in some way can be
       recognized. If the type of a partition entry in the  primary  partition
       table  is  changed from x to y while the filesystem is still of type x,
       gpart will also still guess a type x.

       No checks are performed whether a found filesystem  is  clean  or  even
       consistent/mountable,  so  it is quite possible that gpart may identify
       partitions which existed prior to the current  partitioning  scheme  of
       the disk. Especially on large disks old file system headers/superblocks
       may be present a long time until they are finally overwritten with user
       data.

       It  should  be  stressed	 that  gpart  does a very heuristic job, never
       believe its output without any plausability checks. It  can  be	easily
       right in its guesswork but it can also be terribly wrong. You have been
       warned.

       After having found a list of possible  partition	 types,	 the  list  is
       checked	for  consistency. For example, a partition which overlaps with
       the previous one	 will  be  discarded.  All  remaining  partitions  are
       labelled	 with  one  of the following attributes: "primary", "logical",
       "orphaned" or "invalid".

       A partition labelled "orphaned" is a logical partition which  seems  ok
       but  is	missed in the chain of logical partitions. This may occur if a
       logical partition is deleted from the extended partition table  without
       overwriting the actual disk space.

       An  "invalid" partition is one that cannot be accepted because of vari‐
       ous reasons. If a consistent primary partition  table  was  created  in
       this process it is printed and can be written to a file or device.

EXTENDED PARTITIONS
       If  the	disk/file  to be examined consists of primary partitions only,
       gpart has quite a good chance to identify them. Extended partitions  on
       the other hand can result in a lot of problems.

       Extended partitions are realized as a linked list of extended partition
       tables, each of which include an entry pointing to a logical partition.
       The  size  of  an  extended partition depends on where the last logical
       partition  ends.	 This  means  that  extended  partitions  may  include
       "holes",	 unallocated disk space which should only be assigned to logi‐
       cal, not primary partitions.

       gpart tries to do its best to check a found chain of logical partitions
       but  there  are	very  many possible points of failure. If "good" fdisk
       programs are used to create extended partitions, the  resulting	tables
       consist of a zeroed boot record and the four partition entries of which
       at least two should be marked unused. Unfortunately e.g. the fdisk pro‐
       gram  shipped with Windows NT does not seem to zero out the boot record
       area so gpart has to be overly tolerant in recognizing extended	parti‐
       tion tables. This tolerance may result in quite stupid guesses.

DISK TRANSFERS
       If  you	want  to  investigate hard disks from other systems you should
       note down the geometry (number of cylinders,  heads  per	 cylinder  and
       sectors	per head) used for that disk, and tell gpart about this geome‐
       try.

       Investigating disks from machines with a different endianness than  the
       scanning	 one  has  not been tested at all, and is currently not recom‐
       mended.

LARGE DISKS
       gpart relies on the OS reporting the correct disk  geometry.   Unfortu‐
       nately  sometimes  the  OS may report a geometry smaller the the actual
       one (e.g. disks with more than 1024 or 16384 cylinder).

       gpart checks if guessed partitions extend  beyond  the  disk  size  and
       marks  those  "invalid",	 but  may be mistaken in case the disk size is
       calculated from an incorrect geometry. For instance if a disk with  the
       geometry	 1028/255/63 should be scanned, and the OS reports 1024/255/63
       gpart should be called like

	      gpart -C 1028,255,63 <other options> <device>

PRECAUTIONS
       gpart may be of some help when the primary partition table was lost  or
       destroyed  but it can under no circumstances replace proper disk/parti‐
       tion table backups.  To save the master boot record (MBR) including the
       primary partition table to a file type

	      dd if=/dev/hda of=mbr bs=512 count=1

       exchanging /dev/hda with the block device name of the disk in question.
       This should be done for all disks in the system. To restore the primary
       partition table without overwriting the MBR type

	      dd if=mbr of=/dev/hda bs=1 count=64 skip=446 seek=446

       Warning:	 make sure that all parameters are typed as shown and that the
       disk device is correct. Failing to do so may result in severe  filesys‐
       tem  corruption. The saved file should be stored in a safe place like a
       floppy disk.

OPTIONS
       -b backupfile
	      If the guessed primary  partition	 table	seems  consistent  and
	      should  be  written  (see	 the -W option) backup the current MBR
	      into the specified file.

       -C c,h,s
	      Set the disk geometry (cylinders, heads, sectors) for the	 scan.
	      This is useful if a disk should be scanned which was partitioned
	      using a different geometry, if the device is a disk-image or  if
	      the  disk	 geometry cannot be retrieved through the PCs BIOS. No
	      spaces are allowed between the numbers,  unless  all  three  are
	      enclosed in quotes.

       -c     Check/compare mode (implies the -q quiet option). After the scan
	      is done, the resulting primary partition table  is  compared  to
	      the  existing  one.  The	return code of gpart then contains the
	      number of differences (0 if they are identical  except  for  the
	      boot/active  flag	 which	cannot be guessed). This option has no
	      effect if -d is given on the command line.

       -d     Do not start the guessing loop. Useful if	 the  partition	 table
	      should  be  printed  (in combination with the -v option) without
	      actually scanning for partitions.

       -E     Do not try to identify extended partition tables. If  there  are
	      extended	partitions  on	the  given device gpart will most cer‐
	      tainly complain about too many primary partitions because	 there
	      can be only four primary partitions. Existing logical partitions
	      will be listed as primary ones.

       -e     Do not skip disk read errors. If this option is given, and short
	      disk  reads  or  general disk read errors (EIO) are encountered,
	      gpart will exit. If not given, the program tries to continue.

       -f     Full scan. When a possible partition is  found,  gpart  normally
	      skips  all  sectors this entry seems to occupy and continues the
	      scan from the end of the last possible partition. The disk  scan
	      can take quite a while if this option is given, be patient.

       -g     Do  not  try to get the disk geometry from the OS. If the device
	      is no block or character device but a  plain  file  this	option
	      should  be  supplied. If the file to be scanned is an image of a
	      disk, the geometry can be given by the -C option.

       -h     Show some help.

       -i     Run interactively. Each time a possible partition is  identified
	      the user is asked for confirmation.

       -K last sector
	      Scan  only  up  to  the  given  sector or the end of the file or
	      device whichever comes first.

       -k sectors
	      Skip given number of sectors before the scan. Potentially useful
	      if a partition is looked for at the end of a large disk.

       -L     List  available  filesystem/partition  type  modules  and	 their
	      weights, then exit.

       -l logfile
	      Log output to the given file (even if -q was supplied).

       -n increment
	      Scan increment: number of	 sectors  or  "s"  for	single	sector
	      increment,  "h" for an increment of sectors per head (depends on
	      geometry) or "c" for cylinder increment.

	      The increment also influences the condition where extended  par‐
	      tition  tables  are searched: if the scan increment is "s" (i.e.
	      1) extended partition tables are required to be on a head bound‐
	      ary, otherwise they must be on a cylinder boundary.

	      If  the disk geometry could not be retrieved and no geometry was
	      given on the command line, the default increment is one sector.

       -q     Quiet/no output mode. However if a logfile was specified (see -l
	      option)  all  output  is written to that file. This option over‐
	      rides the -i interactive mode.

       -s sector size
	      Preset medium sector size.  gpart tries to find out  the	sector
	      size  but	 may  fail  in	doing so. Probed sector sizes are 2^i,
	      i=9..14 (512 to 16384 bytes). The default medium sector size  is
	      512 bytes.

       -t module name
	      Plug  in	another	 guessing module. The module to be dynamically
	      linked in must be a shared object file named "gm_<modname>.so".

       -V     Show version number.

       -v     Be verbose. This option can be given more than once resulting in
	      quite a lot of information.

       -W device
	      Write  partition	table. If a consistent primary partition table
	      has been guessed it can be written  to  the  specified  file  or
	      device. The supplied device can be the same as the scanned one.

	      Additionally  the	 guessed  partition  entries can be edited. No
	      checks are performed on the entered values, thus	the  resulting
	      table is allowed to be highly inconsistent. Please beware.

	      Warning:	The  guessed  partition	 table	should be checked very
	      carefully before writing it  back.  You  can  always  write  the
	      guessed  partition  table	 into a plain file and write this into
	      sector 0 using dd(1) (see section PRECAUTIONS above).

       -w module name,weight
	      Put the given module at the head of the module chain and	assign
	      a	 new  weight  to that module. All modules are given an initial
	      weight of 1.0. Again no spaces are allowed.

       Default settings are "-n h".

EXAMPLES
       - To scan the first IDE hard disk under Linux  using  default  settings
       type

	      gpart /dev/hda

       - To  print  the primary partition table of the third IDE drive without
       starting the scan loop in FreeBSD type

	      gpart -vvd /dev/wd2

       - If lilo(8) was installed in the master boot record (MBR)  of  a  hard
       disk  it	 saves	the  contents  of  the	first  sector in a file called
       /boot/boot.<major/minor>. To list the partitions contained  in  such  a
       file type e.g.

	      gpart -vdg /boot/boot.0300

       If  the partition table contains an extended partition, gpart will com‐
       plain about invalid extended  partition	tables	because	 the  extended
       entry points to sectors not within that file.

       - Usually  the  first  primary  partition starts on the second head. If
       gpart cannot identify the first partition properly this may not be  the
       case.   gpart can be told to start the scan directly from sector one of
       the disk, using the sector-wise scan mode:

	      gpart -k 1 -n s /dev/hdb

       - Suppose gpart identifies an NTFS partition as FAT on a certain	 disk.
       In  this situation the "ntfs" module should be made the first module to
       be probed and given a weight higher than the usual weight of 1.0:

	      gpart -w ntfs,1.5 /dev/hdb

       To list the available modules and their weights use the -L option.

       - After having checked the output of gpart at least thrice, the primary
       partition table can be written back to the device this way:

	      gpart -W /dev/sdc /dev/sdc

       This  of	 course	 may  be extremely dangerous to your health and social
       security, so beware.

       - A hard disk with 63 sectors per head is scanned in steps of  63  sec‐
       tors. To perform the scan on every second head while skipping the first
       1008 sectors type

	      gpart -k 1008 -n 126 /dev/sda

       - If you want to see how easily gpart can  be  mislead,	and  how  many
       probable	 partition  starts are on a disk, search the whole disk really
       sector by sector, writing all output to a logfile:

	      gpart -vvfn s -ql /tmp/gpart.log /dev/sd2 &

       Usually gpart will not be able to produce an educated guess of the pri‐
       mary  partition	table  in  this	 mode. The logfile however may contain
       enough hints to manually reconstruct the partition table.

FILES
       /dev/*
	      Hard disk block devices. The naming scheme of  hard  disk	 block
	      devices  is  OS  dependent, consult your system manuals for more
	      information.

DIAGNOSTICS
       There are many error message types, all of them should be self-explana‐
       tory. Complain if they are not.

BUGS
       gpart is beta software, so expect buggy behaviour.

       -   gpart only accepts extended partition links with one logical parti‐
       tion. There may be fdisk variants out there creating links with	up  to
       three logical partition entries but these are not accepted.

TO DO
       - Support big-endian architectures.
       - Test on 64-bit architectures.
       - Look for boot manager partitions (e.g. OS/2 BM).
       - Think about reconstructing logical partition chains.

AUTHOR
       Please send bug reports, suggestions, comments etc. to

	      Michail Brzitwa <michail@brzitwa.de>

SEE ALSO
       fdisk(8).

Administration Tools		 January 2001			      GPART(8)
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