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
partition 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 partitions, but the latter are more efficient. So,
usually one will want a second Linux partition dedicated as swap partition. 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, typically 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 partitions.
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 number 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 absolute number of sectors (given in 32 bits) and
as a Cylinders/Heads/Sectors triple (given in 10+8+6 bits). The former 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. Secondly, 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 partition table.
Usually all goes well by default, and there are no problems 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 operating 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 partition).
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 information in the partition table. DOS FORMAT expects DOS FDISK to clear
the first 512 bytes of the data area of a partition 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 partition 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 sectors
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 program 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 reasonable 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.