GPART(8) System Manager's Manual 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 <mod-
ule-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 filesystem 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.
reiserfs
The Reiser filesystem (version 3.5.X, X > 11, 3.6.X).
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 op-
tion). Please consult the gpart README file for detailed explanations on
how to create guessing modules. All modules are accompanied by a guess-
ing weight factor which denotes how "educated" their guesses are com-
pared to other modules. This weight can be changed if a certain module
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 con-
sistent/mountable, so it is quite possible that gpart may identify par-
titions 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 be-
lieve 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 la-
belled with one of the following attributes: "primary", "logical", "or-
phaned" 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 various
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 par-
tition ends. This means that extended partitions may include "holes",
unallocated disk space which should only be assigned to logical, 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 pro-
grams are used to create extended partitions, the resulting tables con-
sist of a zeroed boot record and the four partition entries of which at
least two should be marked unused. Unfortunately e.g. the fdisk program
shipped with Windows NT does not seem to zero out the boot record area
so gpart has to be overly tolerant in recognizing extended partition ta-
bles. 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 geometry.
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 filesystem
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 en-
closed 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/ac-
tive 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 ac-
tually scanning for partitions.
-E Do not try to identify extended partition tables. If there are
extended partitions on the given device gpart will most certainly
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 incre-
ment, "h" for an increment of sectors per head (depends on geome-
try) or "c" for cylinder increment.
The increment also influences the condition where extended parti-
tion tables are searched: if the scan increment is "s" (i.e. 1)
extended partition tables are required to be on a head boundary,
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 overrides
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.
-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 de-
vice. 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 care-
fully 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 se-
curity, 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 prob-
able 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 de-
vices is OS dependent, consult your system manuals for more in-
formation.
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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