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Name

fdisk - Partition table manipulator for Linux

SYNOPSIS

fdisk [-u] [-b sectorsize] [-C cyls] [-H heads] [-S sects] device

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

fdisk -s partition ...

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.

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, /dev/ed[a-d] for ESDI disks, /dev/xd[ab] for XT 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. Disks can have up to 15 partitions. See also /usr/src/linux/Documentation/devices.txt.

A BSD/SUN type disk label 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 disk label.

An IRIX/SGI type disk label 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, 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 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 cfdisk or 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 cfdisk to make a DOS partition table entry for /dev/hda1, then (after exiting fdisk or cfdisk 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 fdisk or Linux cfdisk program.

OPTIONS

-b sectorsize

Specify the sector size of the disk. Valid values are 512, 1024, or 2048. (Recent kernels know the sector size. Use this only on old kernels or to override the kernel's ideas.)

-C cyls

Specify the number of cylinders of the disk. I have no idea why anybody would want to do so.

-H heads

Specify the number of heads of the disk. (Not the physical number, of course, but the number used for partition tables.) Reasonable values are 255 and 16.

-S sects

Specify the number of sectors per track of the disk. (Not the physical number, of course, but the number used for partition tables.) A reasonable value is 63.

-l

List the partition tables for the specified devices and then exit. If no devices are given, those mentioned in /proc/partitions (if that exists) are used.

-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.

-v

Print version number of fdisk program and exit.

How to install a WD Advanced Format Drive on a non-Windows Operating System

It is recommended to read the WD Advanced Format Technology White Paper as an introduction to the technology. The key requirement for Advanced Format Drives to give their best performance is the alignment of partitions to match the 4KB boundaries of the drive's internal sectors.

Windows:

For information about using Advanced Format Drives under Windows, refer to About WD Advanced Format Technology.

Mac OSX:

Mac OSX version 10.4 onwards (Tiger, Leopard and Snow Leopard) can use the GUID Partitioning Table scheme (GPT) which will align an Advanced Format drive correctly. Mac OS versions using Apple Partition Manager (APM) will not align partitions correctly and will not provide optimum performance.

Linux

Linux as an operating system is a combination of a kernel, user space utilities and customized packaging provided within the "distributions" such as Ubuntu, SuSE and Redhat.

The Linux kernel has had specific support for the alternate sector sizes and offsets used by WD Advanced Format disk drives since version 2.6.31. However, distributions based on Linux 2.6.34, the latest stable version of Linux, will provide the most thorough support. Advanced Format parameters are available in the sysfs file system from this kernel version onwards. Kernel versions older than 2.6.31 will not specifically detect Advanced Format Drives, but with proper alignment the system performance will be maintained. To ensure the best performance, it is important to:

·         Align partitions to the internal 4KB sector boundaries of the drive.

·         Make sure that data writes are aligned to 4KB sector boundaries.

Partitions are created by a number of possible utilities under Linux with "fdisk" and "parted" being typical command line tools. Parted is one of the better tools and from version 2.1 onwards it includes support for aligning Advanced Format drives. Fdisk and earlier versions of Parted won't automatically align partitions but can be used manually to set up the correct partition boundaries.

If older versions of parted (pre 2.1) or fdisk are used to create partitions, a manual calculation of partition boundaries will ensure optimum performance of Advanced Format disk drives. The partitioning tool needs to be set t work in units of sectors rather than cylinders or Megabytes for example. Under parted, enter:

unit s

For fdisk, simply entering "u" will toggle between cylinder and sector mode. Make sure that all partitions start on a multiple of 8 sectors (8x 512B = 4KB) and that partition sizes are multiples of 8 sectors. Make sure that there is space left at the start of partitions as required. For example on a boot drive, do not start at sector 0 as there needs to be space for the boot code. Sector 64 is a good start point or even 2048 which would be a 1MB boundary. Also extended partitions will need a gap between their start point and the first logical partition contained within them.

The file system defaults on most Linux systems are to use blocks of 4KB data. Typically no specific actions are needed to configure block sizes for WD Advanced Format drives. For options on block sizes for the mkfs tools, consult the "man" pages. Note that for the e2fstools used for file system creation, version 1.41.10 onwards will warn the user that they are attempting to make a file system on an unaligned partition. The user is advised to repartition appropriately.

The installation defaults for Linux distributions create partitions based on pre-defined rule sets which may not allow for Advanced Format drive geometry optimisation. Current distributions will probably require manual setup of partitions as described above. Before running the installation process, access a command line terminal and create the required partitions using parted or fdisk. Once created, use the installer's option to install on existing partitions rather than create a new or modified partition table. Assume that distributions released in 2009 or earlier will not take account of Advanced Format drives.

Western Digital has been working with the Linux kernel development teams and with the vendors of the some of the more popular Linux distributions. Starting with Linux Kernel Version 2.6.34, the latest stable kernel release, the default installations will align partitions to the most appropriate boundaries on Advanced Format disk drives. For example, the following distributions will default to good alignment for Advanced Format disk drives: Ubuntu 10.04, Fedora 13, Redhat 6 and derived products. It is also expected that by release time Debian 6 (Squeeze) and open SUSE 11.3 will have completed the changes required for the default installation to be well aligned.