SUBJECT This file documents fdisk 0.92 - the version on the root disk is 0.96, plus some other small modifications. The documentation should be quite close, however. fdisk 0.93 was written by A.V. LeBlanc (A.V.LeBlanc@mcc.ac.uk) modifications for -l made by Jim Winstead Jr. (jwinstea@jarthur.claremont.edu) TABLE OF CONTENTS: WARNINGS Introduction Disks and how they are described Dividing up your disk Running fdisk Deleting and adding partitions Active flags and system types Extra commands for experts Messages, errors, and warnings WARNINGS: In general, you should not use this 'fdisk' program to create partitions for other operating systems, only for Linux. Dr. DOS 5.0 and 6.0 has been reported to have problems cooperating with Linux, and with this version of fdisk in particular. This fdisk sets the system type to hexadecimal 81. Dr. DOS seems to confuse this with hexadecimal 1, a DOS code. If you use Dr. DOS, use the fdisk command 't' to change the system code of any Linux partitions to some number less than hexadecimal 80; I suggest 41 and 42 for the moment. PARTITIONING a hard disk may DESTROY data which is on that disk if you are not careful. Go slowly, write down a description of what you started out with, and always verify before you write. See the explanations below for more more information. INTRODUCTION In this document we look at the utility 'fdisk', and we try to cover enough of its general background to help anyone use it. 'fdisk' is Linux's partition table editor. DISKS AND HOW THEY ARE DESCRIBED A typical disk consists physically of one or more circular objects called 'platters', which rotate about a central axis. Devices called 'heads' move to specified places on the disk surface to read or write information. There is typically one head on each side of every platter, and all of these heads are attached to a comb-like controller arm which moves all of them at the same time, either closer to the centre of the disk, or closer to the outer edge. Suppose the arm is in one position, putting an area of the disk surface within reach of one or another of the heads. This total area, everything that is accessible without moving the arm, is called a 'cylinder'. A cylinder is a barrel-shaped cross section of a disk, consisting of a circular strip from each side of each platter. Each of these strips is the part of a cylinder that one head can read or write without moving, and it is called a 'track'. Each track is divided into a number of pie-shaped slices called 'sectors', which are the smallest parts of the disk which can be read or written at a time. The sectors on one disk are usually all the same size. (If you want to be accurate, there are not always two heads to every platter, there are some disks which do not have the same amount of data in every cylinder, and there may be disks which do not have the same amount of data in every sector. I hope you are not dealing with such disks, for both our sakes.) The numbers which describe the 'geometry' of a disk are (a) the number of cylinders it contains, (b) the number of tracks per cylinder, which is the number of heads, (c) the number of sectors per track, and (d) the size of each sector. Some of these numbers will vary, but a typical PC disk might have about 1000 cylinders, about half a dozen heads, and 15 or 20 sectors per track, with each sector containing 512 bytes or characters; such a disk contains 40 to 60 megabytes of data. A 'double density' floppy disk contains 40 cylinders, with 2 heads (2 tracks per cylinder), and with 9 sectors per track; such a disk contains 360 kilobytes, or 360 * 1024 characters. A 'high density' 3.5 inch floppy contains 80 cylinders, with 2 heads and 18 sectors per track, or 1.44 megabytes, or 1440 * 1024 characters. The exact size of a track or cylinder in bytes varies from one disk to another. This fdisk for Linux, at least in its present form, deals mainly with 'sectors', which are 512 bytes each. fdisk also reports partition sizes in 'blocks' of 1024 bytes, or 2 sectors, since you need the size in blocks in order to make a file system using mkfs. A block is the smallest amount of space which can be set aside for a file in the current file system. An operating system, such as Linux or MS-DOS or OS/2, may use a disk in any way that it wishes, but if two operating systems share the same disk, they must agree on who owns what, or else one will interfere with the other (that is, by damaging the other's files). A 'partition' is a section of a hard disk which is handled as a unit by all operating systems which can access the disk. The standard way to define partitions (for the moment) is the 'partition table', a list of information which is stored in parts of the disk that don't belong to any of the systems using the disk. The beginning of the partition table is stored in the disk's primary boot sector, and the rest is stored in a chain of sectors scattered throughout the disk. The first sector on the disk is called the 'primary boot block' or 'primary boot sector' because (1) it comes first, before other, similar sectors; (2) it tells where the other, similar sectors are found, so that it is logically 'prior' to them; and (3) it usually contains code which is executed when the system boots up. This sector contains a table describing at most four partitions. These areas are called 'primary partitions'. The partition table in the primary boot sector may also describe at most one 'extended partition'. This is a large area of the disk, usually containing all the space which is not in any primary partition. Within this space we can set aside other areas which are called 'logical partitions', because they look almost exactly like primary partitions. In fact, the main difference between them is that we can boot from primary partitions, while we cannot boot from logical partitions. This happens because the address of primary partitions is in a fixed place, whereas the address of secondary partitions is not, and so we require a more complicated process to discover it, one which is too difficult for most primary boot programs. DIVIDING UP YOUR DISK It is a good idea to plan ahead before you start creating partitions on your disk. If you set aside a partition for some purpose, it is not easy to change its size: you've got to copy all the data off of the partition, whether to floppies, to another partition, to another hard disk, or somewhere else; then you must edit the table which describes this partition, so changing its size; then you must reboot and initialise the new partition, formatting it, for example, under MS-DOS, or running 'mkfs' under Linux; finally you can copy all of the data back. It is possible, if you have several partitions, to copy data back and forth between them while you change their sizes, but this is a bit risky and time consuming. It is better to plan well what you will need, since it is hard to change it afterwards. Many people with large disks and recent versions of MS-DOS have their entire file system on one large partition. They always ask, 'Isn't there any way I can reformat my disk without copying everything off?' There is no way to do it using standard DOS utilities, and there is no truly safe way to do it using commercial software, because if you make a mistake, you will lose the entire contents of your disk. So if you are going to back up your disk anyway, you might as well copy the data back safely. MS-DOS and Linux both allow you to access several partitions on a single disk; on MS-DOS these are treated as if they were separate disks or drives, and under Linux they are treated as different 'devices'. You can have up to 60 partitions on a single disk, at least as far as Linux is concerned; in practice you will rarely want so many. The maximum size of a Linux file system on a single partition is presently 64 megabytes. You may, in other words, have all of your Linux files in a single partition, or you may have two, three, or more Linux file systems, and similarly with MS-DOS. If you have several small partitions, you run much less risk of losing all your files if your disk gets corrupted. On the other hand, you may run out of space on a small partition more easily. Under MS-DOS, you must refer to each partition by a separate drive letter, but all partitions are automatically accessible. Under Linux only the root partition is automatically accessible, but once we 'mount' another partition, it is indistinguishable from the rest of the file system. Disks are usually mounted by a command in one of the system startup files, /etc/rc, so that you need not worry about having to do it yourself whenever you boot the system. Linux requires at least one partition, which is the 'root' of the file system. You may prefer to have a separate partition for /usr, which contains most of the executable files, or for /home, which contains most of your private files. You may also wish to set aside a partition to use for swap space, depending on the amount of memory your PC has. You will certainly need swap space if you have less than 4 megabytes of RAM and wish to compile anything substantial. You can reserve swap space in a file, but you need a partition big enough to hold it, and this will probably be less efficient than having a partition devoted to swap. If you wish to play with Linux, without compiling C programs or using any large packages (like Emacs, TeX, or the X window system), you need 4 to 5 megabytes of disk space, plus 2 megabytes of RAM. If you install the C and C++ compilers, you will need 8 to 10 megabytes of disk space, plus at least 4 megabytes of RAM: you can compile with 2 megabytes of RAM plus some swap space, but performance will be poor. Also, remember that /tmp, which many programs (compilers and editors) use for scratch files: you'll need to have a couple of free megabytes on the partition containing this directory. No matter how much space you have, it is never enough. Are you going to boot Linux up from the hard disk, or will you boot from a floppy? If you boot Linux from a hard disk, then the root of the file system must be one of the four primary partitions. If you have an extended partition with logical partitions in it, you can have only three primary partitions containing data. RUNNING fdisk Every operating system, whether MS-DOS, OS/2, or Linux, should provide its own utility for editing hard disk partition tables. At least four of these utilities have been called 'fdisk', for 'Fixed DISK setup program', where 'fixed' means 'not removable'. I believe the first program named fdisk came from Microsoft in about 1985; before that time disks were too small to divide into separate sections. Every operating system has its own peculiarities. Normally you should set up a partition for the use of one operating system by using its own fdisk program. Do not use the Linux fdisk to create partitions for MS-DOS or for any system besides Linux. Otherwise you may have problems. An fdisk program performs two functions: it reports how the disk is configured, and it changes that configuration by adding or deleting partitions. Most fdisks can also change other information in partition tables. This fdisk for Linux operates on one hard disk at a time. If you give the command fdisk it reports on, and is able to change, /dev/hda, the first hard disk. To look at or change the second hard disk, /dev/hdb, give the command fdisk /dev/hdb To look at or change the first SCSI disk, give the command fdisk /dev/sda efdisk responds by printing a list of partitions n the selected drive, and asks for a command: Command (m for help): _ Each fdisk command consists of a single letter, which must be followed by before it is obeyed. Upper and lower case are not distinguished. Anything you type after the first character is ignored. Give the command 'm', and you should see this menu: Command action a toggle a bootable flag d delete a partition l list known partition types m print this menu n add a new partition p print the partition table q quit without saving changes t change a partition's system id v verify the partition table w write table to disk and exit x extra functionality The simplest commands are Print, Verify, and Quit. On a small disk, the Print command might produce a display like this one: Disk /dev/hda: 5 heads, 17 sectors, 977 cylinders Device Boot Begin Start End Blocks Id System /dev/hda1 * 1 17 20059 10021+ 1 DOS 12-bit FAT /dev/hda2 71060 71060 83044 5992+ 5 Extended /dev/hda3 * 20060 20060 71059 25500 81 Linux/MINIX /dev/hda5 71061 71061 79559 4249+ 82 Linux swap /dev/hda6 80002 80018 83044 1734 1 DOS 12-bit FAT There are 5 partitions reported; /dev/hda4 does not appear because it is not allocated. Partitions 1 and 3 are flagged as bootable. Note that the start of data in both DOS partitions is 16 sectors after the beginning of the partition: this is one reason why you should use DOS's own fdisk to create DOS partitions. The size of each partition is reported in 1 kilobyte blocks; hence the primary Linux partition, partition 3, is 25 1/2 megabytes in size. The '+' after three of the sizes warns that these partitions contain an odd number of sectors: Linux normally allocates filespace in 1 kilobyte blocks, so the extra sector in partition 5 is wasted. Id numbers are reported in hexadecimal and explained in English. The Verify command is useful because (1) It warns you if anything is wrong. ALWAYS give a Verify command before writing any changes to disk. See the section on Messages, Errors, and Warnings. (2) It reports how many unallocated sectors there are on the disk. The Quit command is also useful. fdisk does not actually change any data on your disk unless you give a Write command. If you are unhappy about any changes you may have made, give the Quit command, and your disk will remain as it was before you ran fdisk. You can also interrupt fdisk with CTRL-C. DELETING AND ADDING PARTITIONS Deleting a partition is simple. Give the Delete command. fdisk asks: Partition number (1-6): _ Once you get this far, you must either delete a partition or interrupt the program with CTRL-C (or whatever your current interrupt character is). Note: (1) You may delete a nonexistent partition. You will get a warning message. (2) You may delete an extended partition. This has the side effect of deleting all partitions greater than or equal to 5. (3) You may delete a logical partition. In that case, all partitions above it are renumbered at once. For example, if you delete partition 5, then partition 6 becomes known as partition 5, and partition 7 as partition 6. Adding a partition is just a bit more complicated. Give the New command. fdisk allows you to (1) create a primary partition, if there is a free slot in the primary partition table; (2) create an extended partition if there is a free slot in the primary partition table, and if there is no extended partition; (3) create a logical partition if an extended partition exists. If more than one of these actions is possible, you will be asked to select Primary, Extended, or Logical, depending on what is currently permissible. Before you create a primary or an extended partition, you are asked what slot it is to have in the table (1-4). You may not add a primary or an extended partition if the selected slot in the primary partition table is already occupied: in that case you simply return to the main menu. You are not allowed to add a new primary partition unless there are sectors available outside of the extended partition. You are not allowed to add a new logical partition unless there are sectors available inside the extended partition. If space is available, you are prompted for the first sector: First sector (20060-71059): _ The limits are the lowest and the highest free sectors available in the appropriate part of the disk. Not all numbers in this range are necessarily available: they may fall inside an existing partition. If you select a sector which is already in use, you are told off and prompted again for the first sector. After selecting the first sector, you are prompted again: Last sector (20060-71059): _ The limits are the sector you have chosen as the first sector, and the highest available sector which is legitimate as the other boundary of the partition. In other words, all numbers in the given range are legitimate, unlike those in the first sector range. Adding or deleting partitions has no effect unless you subsequently give the Write command. Please remember to give the Verify command first, just before giving the Write command: this is a safety precaution. After giving the Write command, you will see this message: The partition table has been altered. Please reboot before doing anything else. Here are some important things to note: (1) Before you reboot, you MAY run fdisk again, either to manage another disk, or to make additional changes to the same disk, or just to check that the changes have been made as you expected. (2) NEVER run any of the programs mkfs, mkswap, mount, or swapon before you reboot. It is dangerous to run any programs, but these in particular may cause serious damage to any data on your disk, including the partition tables themselves. ACTIVE FLAGS AND SYSTEM TYPES The active flag is a bit in the partition table entry which marks a partition as bootable. This is important to some primary boot sector programs, which will not boot from an unflagged partition. Other such programs do not allow more than one partition to be flagged. Some, like shoelace, ignore the flags completely. I prefer to flag all bootable partitions as active so that they stand out on the menu shoelace displays. fdisk prints a star after the name of a partition's device file if its active flag is set. The Active command changes, or toggles, a partitions active flag. Give the Active command, and select a partition by number. If it was marked inactive, it will be flagged as active; if it was flagged as active, it will be marked inactive. You may not set the active flag on an extended partition, but you may set it on a logical partition, though the meaning of such a flag is by no means clear. The Type command changes the ID number which describes what type a partition is. fdisk currently recognises 24 system ID's, in the sense that it prints a string for each of them, but it allows you to change any system ID to any other, with the following exceptions: you may not change any partition to or from the type Extended, and you may not change a partition whose type is Empty (0) to any other type. You may, however, change the type of any data partition to 0, which is equivalent to deleting it. The new system ID or type code is a hexadecimal number. There are two ways of listing the numbers which fdisk recognises: use the List command, which prints the list, or use the Type command, which, when it prompts you for the code, says Hex code (type L to list codes): _ where the upper case 'L' is used for clarity. The codes printed are 1 DOS 12-bit FAT 7 OS/2 HPFS 51 Novell? 81 Linux/MINIX 2 XENIX 8 AIX 52 CP/M? 82 Linux swap 3 Old XENIX 9 AIX bootable 63 386/IX 93 Amoeba 4 DOS 16-bit <32mb a OPUS? 64 Novell 94 Amoeba BBT 5 Extended 10 OPUS? 75 PCIX db CP/M 6 DOS 16-b >=32mb 40 Venix 80 Old MINIX ff BBT Some of these numbers are a trifle uncertain. By default (for the moment) fdisk uses a type of 81, the type used by the present MINIX fdisk. When our new file system is introduced, I shall change the default to a new value. The number 81 in any case gives problems to Dr. DOS 5.0 and 6.0; if you use this software, change the 81 to any other value (except 0, 5, 80, or 85). The value of 82 for Linux swap partitions is my own invention, and is intended to give some recognisable distinctiveness to the partitions when the values are displayed in hexadecimal, as they are by shoelace. New active flags and new system type codes are not written to the disk until you exit from fdisk with the Write command, as described above, in the section on deleting and adding partitions. EXTRA COMMANDS FOR EXPERTS The eXtra command 'x' puts fdisk into 'expert' mode, in which a slightly different set of commands is available. The Active, Delete, List, New, Type, Verify, and 'eXpert' commands are not available in expert mode. The commands Write and Quit are available as in ordinary mode, the Print command is available, but produces output in a slightly different format, and of course the Menu command prints the expert menu. There are several new commands. (1) The Return command brings you back to the main menu. (2) The Extended command prints the list of table entries which point to other tables. Ordinary users do not need this information. The data is shown as it is stored. The same format is used for the expert Print command. (3) The dangerous Begin command allows you to move the start of data in a partition away from its beginning. I believe shoelace has difficulties with partitions changed in this way. But other systems create partitions with this format, and it is sometimes useful to be able to reproduce it. (4) The slightly dangerous Cylinders command allows you to change the available number of cylinders. This will allow you to end a partition at the end of a cylinder, if you wish. (5) The extremely dangerous Heads and Sectors commands allow you to change the number of heads and sectors. It should not be necessary to use these commands, but I understand that there are disks whose geometry generic Linux is unable to determine, so I have provided the ability to cope with such cases. WARNING: If you set either of these numbers to a false value, you may lose all data on your disk. Always, after giving any of the commands Begin, Cylinder, Heads, or Sectors, you should Return to the main menu and give the Verify command.