New HOWTO: Linux Partition HOWTO
By: Linux HOWTOs
Thursday, April 5, 2001 11:01:31 AM EST
URL: http://www.linuxplanet.com/linuxplanet/tutorials/3174/1/
Table of Contents
Linux Partition HOWTO
Tony Harris
Kristian Koehntopp
Revision History
Revision 3.4 25 March 2001
Rewrote device names (Section 2) to include examples.
Revision 3.3 23 Feb 2001
Kristian contributes discussion on cylinder mapping Included example from
Karsten Self in Section 5 Added making a generic ramdisk Dan Scott is
credited for converting version 3.2 to SGML
Revision 3.2 1 September 2000
Dan Scott provides sgml conversion 2 Oct. 2000. Rewrote Introduction. Rewrote
discussion on device names in Logical Devices. Reorganized Partition Types.
Edited Partition Requirements. Added Recovering a deleted partition table.
Revision 3.1 12 June 2000
Corrected swap size limitation in Partition Requirements, updated various
links in Introduction, added submitted example in How to Partition with
fdisk, added file system discussion in Partition Requirements.
Revision 3.0 1 May 2000
First revision by [mailto:tony@nmr.mgh.harvard.edu] Tony Harris based on
Linux Partition HOWTO by Kristian Koehntopp.
Revision 2.4 3 November 1997
Last revision by Kristian Koehntopp.
This Linux Mini-HOWTO teaches you how to plan and create partitions on IDE
and SCSI hard drives. It discusses partitioning terminology and considers
size and location issues. Use of the fdisk partitioning utility for creating
and recovering of partition tables is covered. The most recent version of
this document is [http://surfer.nmr.mgh.harvard.edu/partition/Partition.html]
here.
-----------------------------------------------------------------------------
Table of Contents
1. Introduction
1.1. What is a partition?
1.2. Constraints
1.3. Other Partitioning Software:
1.4. Related HOWTOs
1.5. Additional information on your system:
2. Devices
2.1. Device names
2.1.1. Naming Convention
2.1.2. Name Assignment
2.2. Device numbers
3. Partition Types
3.1. Partition Types
3.2. Foreign Partition Types
3.3. Primary Partitions
3.4. Logical Partitions
3.5. Swap Partitions
4. Partitioning requirements
4.1. What Partitions do I need?
4.2. Discussion:
4.3. File Systems
4.3.1. Which file systems need their own partitions?
4.3.2. File lifetimes and backup cycles as partitioning criteria
4.4. Swap Partitions
4.4.1. How large should my swap space be?
4.4.2. Where should I put my swap space?
5. Partitioning with fdisk
5.1. Partitioning with fdisk
5.1.1. Notes about fdisk:
5.1.2. Four primary partitions
5.1.3. Mixed primary and logical partitions
5.1.4. Submitted Examples
6. Recovering a Deleted Partition Table
7. Formating Partitions
7.1. Activating Swap Space
7.2. Mounting Partitions
7.3. Some facts about file systems and fragmentation
Section 1: Introduction
1. Introduction
1.1. What is a partition?
Partitioning is a means to divide a single hard drive into many logical
drives. A partition is a contiguous set of blocks on a drive that are treated
as an independant disk. A partition table (the creation of which is the topic
of this HOWTO) is an index that relates sections of the hard drive to
partitions.
Why have multiple partitions?
* Encapsulate your data. Since file system corruption is local to a
partition, you stand to lose only some of your data if an accident
occurs.
* Increase disk space efficiency. You can format partitions with varying
block sizes, depending on your usage. If your data is in a large number
of small files (less than 1k) and your partition uses 4k sized blocks,
you are wasting 3k for every file. In general, you waste on average one
half of a block for every file, so matching block size to the average
size of your files is important if you have many files.
* Limit data growth. Runaway processes or maniacal users can consume so
much disk space that the operating system no longer has room on the hard
drive for its bookkeeping operations. This will lead to disaster. By
segregating space, you ensure that things other than the operating system
die when allocated disk space is exhausted.
-----------------------------------------------------------------------------
1.2. Constraints
* Partitions must not overlap. This will cause data corruption and other
spooky stuff.
* There ought to be be no gap between adjacent partitions. While this is
not harmful, you are wasting precious disk space by leaving space between
partitions.
* A disk need not be partitioned completely. You may decide to leave some
unpartitioned space at the end of your disk and partition it later.
* Partitions cannot be moved but they can be resized and copied using
special software. This HOWTO only covers the use of the fdisk utility,
which does not permit any of these operations.
-----------------------------------------------------------------------------
1.3. Other Partitioning Software:
* sfdisk: a command-line version of fdisk
* cfdisk: a curses-based version of fdisk
* parted: Gnu partition editor
* Partition Magic: a commercial utility to create, resize, merge and
convert partitions, without destroying data.
* Disk Drake: a Perl/Gtk program to create, rsize, and delete partitions
-----------------------------------------------------------------------------
1.4. Related HOWTOs
Table 1. Related HOWTOs
+--------------------------------+----------------------------+------------------------+
|Title |Author |Description |
+--------------------------------+----------------------------+------------------------+
|Linux Multiple Disk System |[mailto:sgjoen@mail.nyx.net]|How to estimate the |
|Tuning |Gjoen Stein |various size and speed |
| | |requirements for |
| | |different parts of the |
| | |filesystem. |
+--------------------------------+----------------------------+------------------------+
|Linux Large Disk |[mailto:aeb@cwi.nl] Andries |Instructions and |
| |Brouwer |considerations regarding|
| | |disks with more than |
| | |1024 cylinders |
+--------------------------------+----------------------------+------------------------+
|Linux Quota |[mailto:bertie@scn.org] |Instructions on limiting|
| |Albert M.C. Tam |disk space usage per |
| | |user (quotas) |
+--------------------------------+----------------------------+------------------------+
|[http://metalab.unc.edu/mdw/ |[mailto:jdanield@dodin.net] |How to restore linux |
|HOWTO/mini/ |Jean-Daniel Dodin |partitions after they |
|Partition-Rescue-mini-HOWTO.html| |have been deleted by a |
|Partition-Rescue] | |Windows install. Does |
|Partition-Rescue mini-HOWTO | |not appear to preserve |
| | |data. |
+--------------------------------+----------------------------+------------------------+
|Linux ADSM Backup |Thomas Koenig |Instructions on |
| | |integrating Linux into |
| | |an IBM ADSM backup |
| | |environment. |
+--------------------------------+----------------------------+------------------------+
|Linux Backup with MSDOS |Christopher Neufeld |Information about MS-DOS|
| | |driven Linux backups. |
+--------------------------------+----------------------------+------------------------+
|Linux HOWTO Index |[mailto: |Instructions on writing |
| |linux-howto@sunsite.unc.edu]|and submitting a HOWTO |
| |Tim Bynum |document |
+--------------------------------+----------------------------+------------------------+
-----------------------------------------------------------------------------
1.5. Additional information on your system:
* /usr/src/linux/Documentation
+ ide.txt: Info about your IDE drivers
+ scsi.txt: Info about your SCSI drivers
-----------------------------------------------------------------------------
Section 2: Devices
2. Devices There is a special nomenclature that linux uses to refer to hard drive partitions that must be understood in order to follow the discussion on the following pages. In Linux, partitions are represented by device files. These are phoney files located in /dev. Here are a few entries: brw-rw---- 1 root disk 3, 0 May 5 1998 hda brw-rw---- 1 root disk 8, 0 May 5 1998 sda crw------- 1 root tty 4, 64 May 5 1998 ttyS0 A device file is a file with type c ( for "character" devices, devices that do not use the buffer cache) or b (for "block" devices, which go through the buffer cache). In Linux, all disks are represented as block devices only. ----------------------------------------------------------------------------- 2.1. Device names 2.1.1. Naming Convention By convention, IDE drives will be given device names /dev/hda to /dev/hdd. The first drive is 'a' the second drive 'b' and so on. For example, /dev/hda is the first drive on the first IDE controller and /dev/hdd is the second drive on the second controller (the fourth IDE drive in the computer). You can write to these devices directly (using cat or dd). However, since these devices represent the entire disk, starting at the first block, you can mistakenly overwrite the master boot record and the partition table, which will render the drive unusable. Once a drive has been partitioned, the partitions will represented as numbers on the end of the names. For example, the second partition on the second drive will be /dev/hdb2. SCSI drives follow a similar pattern; They are represented by 'sd' instead of 'hd'. The first partition of the second SCSI drive would therefore be /dev/sdb1. Primary partitions (Section 3.3) on a disk are 1, 2, 3 and 4. Logical partitions (Section 3.4) have numbers 5 and up, for reasons explained later ( Section 5.1.3). ----------------------------------------------------------------------------- 2.1.2. Name Assignment Under (Sun) Solaris and (SGI) IRIX, the device name given to a SCSI drive has some relationship to where you plug it in. Under linux, there is only wailing and gnashing of teeth. Lower SCSI ID numbers are assigned lower-order letters, so if you remove one drive from the chain, the names of the higher ID number drives will change. If you have two SCSI controllers in your linux box, you will need to examine the output of /bin/dmesg in order to see what name each drive was assigned. If you remove one of two controllers, the remaining controller might have all its drives renamed. Grrr... This is all you have to know to deal with linux disk devices. For the sake of completeness, see Kristian's discussion of device numbers below. ----------------------------------------------------------------------------- 2.2. Device numbers The only important thing with a device file are its major and minor device numbers, which are shown instead of the file size: $ ls -l /dev/hda Table 2. Device file attributes +-----------+--------+--------+--------+---------+---------+--------+--------+ |brw-rw---- | 1 | root | disk | 3, | 0 | Jul 18 |/dev/hda| | | | | | | | 1994 | | +-----------+--------+--------+--------+---------+---------+--------+--------+ |permissions| | owner | group | major | minor | date | device | | | | | | device | device | | name | | | | | | number | number | | | +-----------+--------+--------+--------+---------+---------+--------+--------+ When accessing a device file, the major number selects which device driver is being called to perform the input/output operation. This call is being done with the minor number as a parameter and it is entirely up to the driver how the minor number is being interpreted. The driver documentation usually describes how the driver uses minor numbers. For IDE disks, this documentation is in /usr/src/linux/Documentation/ide.txt. For SCSI disks, one would expect such documentation in /usr/src/linux/Documentation/scsi.txt, but it isn't there. One has to look at the driver source to be sure (/usr/src/ linux/driver/scsi/sd.c:184-196). Fortunately, there is Peter Anvin's list of device numbers and names in /usr/src/linux/Documentation/devices.txt; see the entries for block devices, major 3, 22, 33, 34 for IDE and major 8 for SCSI disks. The major and minor numbers are a byte each and that is why the number of partitions per disk is limited. -----------------------------------------------------------------------------
Section 3: Partition Types
3. Partition Types 3.1. Partition Types A partition is labeled to host a certain kind of file system. Such a file system could be the linux standard ext2 file system or linux swap space, or even foreign file systems like (Microsoft) NTFS or (Sun) UFS. There is a numerical code associated with each partition type. For example, the code for ext2 is 0x83 and linux swap is 0x82. ----------------------------------------------------------------------------- 3.2. Foreign Partition Types The partition type codes have been arbitrarily chosen (you can't figure out what they should be) and they are particular to a given operating system. Therefore, it is theoretically possible that if you use two operating systems with the same hard drive, the same code might be used to designate two different partition types. OS/2 marks its partitions with a 0x07 type and so does Windows NT's NTFS. MS-DOS allocates several type codes for its various flavors of FAT file systems: 0x01, 0x04 and 0x06 are known. DR-DOS used 0x81 to indicate protected FAT partitions, creating a type clash with Linux/Minix at that time, but neither Linux/Minix nor DR-DOS are widely used any more. ----------------------------------------------------------------------------- 3.3. Primary Partitions The number of partitions on an Intel-based system was limited from the very beginning: The original partition table was installed as part of the boot sector and held space for only four partition entries. These partitions are now called primary partitions. ----------------------------------------------------------------------------- 3.4. Logical Partitions One primary partition of a hard drive may be subpartitioned. These are logical partitions. This effectively allows us to skirt the historical four partition limitation. The primary partition used to house the logical partitions is called an extended partition and it has its own file system type (0x05). Unlike primary partitions, logical partitions must be contiguous. Each logical partition contains a pointer to the next logical partition, which implies that the number of logical partitions is unlimited. However, linux imposes limits on the total number of any type of prtition on a drive, so this effectively limits the number of logical partitions. This is at most 15 partitions total on an SCSI disk and 63 total on an IDE disk. ----------------------------------------------------------------------------- 3.5. Swap Partitions Every process running on your computer is allocated a number of blocks of RAM. These blocks are called pages. The set of in-memory pages which will be referenced by the processor in the very near future is called a "working set." Linux tries to predict these memory accesses (assuming that recently used pages will be used again in the near future) and keeps these pages in RAM if possible. If you have too many processes running on a machine, the kernel will try to free up RAM by writing pages to disk. This is what swap space is for. It effectively increases the amount of memory you have available. However, disk I/O is very slow compared to reading from and writing to RAM. If memory becomes so scarce that the kernel pages out from the working set of one process in order to page in for another, the machine is said to be thrashing. Expect performance to drop by approximately the ratio between memory access speed and disk access speed. Swap space is something you need to have, but it is no substitute for sufficient RAM. See Section 4.4.1 for tips on determining the size of swap space you need. -----------------------------------------------------------------------------
Section 4. Partitioning requirements
4. Partitioning requirements
4.1. What Partitions do I need?
Boot Drive: If you want to boot your operating system from the drive you are
about to partition, you will need:
* A primary partition
* One or more swap partitions
* Zero or more primary/logical partitions
Any other drive:
* One or more primary/logical partitions
* Zero or more swap partitions
-----------------------------------------------------------------------------
4.2. Discussion:
Boot Partition:
Your boot partition ought to be a primary partition, not a logical
partition. This will ease recovery in case of disaster, but it is not
technically necessary. It must be of type 0x83 "Linux native". If you are
using lilo, your boot partition must be contained within the first 1024
cylinders of the drive. (Typically, the boot partition need only contain
the kernel image.)
If you have more than one boot partition (from other OSs, for example,)
keep them all in the first 1024 cylinders (All DOS partitions must be
within the first 1024). If you are using a means other than lilo loading
your kernel (for example, a boot disk or the LOADLIN.EXE MS-DOS based
Linux loader), the partition can be anywhere. See the [http://
metalab.unc.edu/mdw/HOWTO/Large-Disk-HOWTO.html] Large-disk HOWTO for
details.
Swap Partition:
Unless you swap to files you will need a dedicated swap partition. It
must be of type 0x82 "Linux swap". It may be positioned anywhere on the
disk (but see notes on placement: Section 4.4.2). Either a primary or
logical partition can be used for swap. More than one swap partition can
exist on a drive. 8 total (across drives) are permitted. See notes on
swap size: Section 4.4.1.
Logical Partition:
A single primary partition must be used as a container (extended
partition) for the logical partitions. The extended partition can go
anywhere on the disk. The logical partitions must be contiguous, but
needn't fill the extended partition.
-----------------------------------------------------------------------------
4.3. File Systems
4.3.1. Which file systems need their own partitions?
Everything in your linux file system can go in the same (single) partition.
However, there are circumstances when you may want to restrict the growth of
certain file systems. For example, if your mail spool was in the same
partition as your root fs and it filled the remaining space in the partition,
your computer would basically hang.
/var
This fs contains spool directories such as those for mail and printing.
In addition, it contains the error log directory. If your machine is a
server and develops a chronic error, those msgs can fill the partition.
Server computers ought to have /var in a different partition than /.
/usr
This is where most executable binaries go. In addition, the kernel source
tree goes here, and much documentation.
/tmp
Some programs write temporary data files here. Usually, they are quite
small. However, if you run computationally intensive jobs, like science
or engineering applications, hundreds of megabytes could be required for
brief periods of time. In this case, keep /tmp in a different partition
than /.
/home
This is where users home directories go. If you do not impose quotas on
your users, this ought to be in its own partition.
/boot
This is where your kernel images go. If you use MSDOS, which must go in
the first 1024 cylinders, you need to at least get this partition in
there in order to ensure that [http://metalab.unc.edu/mdw/HOWTO/mini/
LILO.html] lilo can see it. If you have a drive larger than 1024
cylinders, making this your first partition guarantees that it will be
visible to lilo.
-----------------------------------------------------------------------------
4.3.2. File lifetimes and backup cycles as partitioning criteria
With ext2, partitioning decisions should be governed by backup considerations
and to avoid external fragmentation (Section 7.3) from different file
lifetimes.
Files have lifetimes. After a file has been created, it will remain some time
on the system and then be removed. File lifetime varies greatly throughout
the system and is partly dependent on the pathname of the file. For example,
files in /bin, /sbin, /usr/sbin, /usr/bin and similar directories are likely
to have a very long lifetime: many months and above. Files in /home are
likely to have a medium lifetime: several weeks or so. File in /var are
usually short lived: Almost no file in /var/spool/news will remain longer
than a few days, files in /var/spool/lpd measure their lifetime in minutes or
less.
For backup it is useful if the amount of daily backup is smaller than the
capacity of a single backup medium. A daily backup can be a complete backup
or an incremental backup.
You can decide to keep your partition sizes small enough that they fit
completely onto one backup medium (choose daily full backups). In any case a
partition should be small enough that its daily delta (all modified files)
fits onto one backup medium (choose incremental backup and expect to change
backup media for the weekly/monthly full dump - no unattended operation
possible).
Your backup strategy depends on that decision.
When planning and buying disk space, remember to set aside a sufficient
amount of money for backup! Unbackuped data is worthless! Data reproduction
costs are much higher than backup costs for virtually everyone!
For performance it is useful to keep files of different lifetimes on
different partitions. This way the short lived files on the news partition
may be fragmented very heavily. This has no impact on the performance of the
/ or /home partition.
-----------------------------------------------------------------------------
4.4. Swap Partitions
4.4.1. How large should my swap space be?
If you have decided to use a dedicated swap partition, which is generally a
Good Idea [tm], follow these guidelines for estimating its size:
* In Linux RAM and swap space add up (This is not true for all Unices). For
example, if you have 8 MB of RAM and 12 MB swap space, you have a total
of about 20 MB virtual memory.
* When sizing your swap space, you should have at least 16 MB of total
virtual memory. So for 4 MB of RAM consider at least 12 MB of swap, for 8
MB of RAM consider at least 8 MB of swap.
* Currently, the maximum size of a swap partition is
architecture-dependent. For i386 and PowerPC, it is approximately 2Gb. It
is 128Gb on alpha, 1Gb on sparc, and 3Tb on sparc64. For linux kernels
2.1 and earlier, the limit is 128Mb. The partition may be larger than 128
MB, but excess space is never used. If you want more than 128 MB of swap
for a 2.1 and earlier kernel, you have to create multiple swap
partitions. See the man page for mkswap for details.
* When sizing swap space, keep in mind that too much swap space may not be
useful at all.
A very old rule of thumb in the days of the PDP and the Vax was that the size
of the [partition-3.html#swap] working set of a program is about 25% of its
virtual size. Thus it is probably useless to provide more swap than three
times your RAM.
But keep in mind that this is just a rule of thumb. It is easily possible to
create scenarios where programs have extremely large or extremely small
working sets. For example, a simulation program with a large data set that is
accessed in a very random fashion would have almost no noticeable locality of
reference in its data segment, so its working set would be quite large.
On the other hand, an xv with many simultaneously opened JPEGs, all but one
iconified, would have a very large data segment. But image transformations
are all done on one single image, most of the memory occupied by xv is never
touched. The same is true for an editor with many editor windows where only
one window is being modified at a time. These programs have - if they are
designed properly - a very high locality of reference and large parts of them
can be kept swapped out without too severe performance impact.
One could suspect that the 25% number from the age of the command line is no
longer true for modern GUI programs editing multiple documents, but I know of
no newer papers that try to verify these numbers.
So for a configuration with 16 MB RAM, no swap is needed for a minimal
configuration and more than 48 MB of swap are probably useless. The exact
amount of memory needed depends on the application mix on the machine (what
did you expect?).
-----------------------------------------------------------------------------
4.4.2. Where should I put my swap space?
* Mechanics are slow, electronics are fast.
Modern hard disks have many heads. Switching between heads of the same
track is fast, since it is purely electronic. Switching between tracks is
slow, since it involves moving real world matter.
So if you have a disk with many heads and one with less heads and both
are identical in other parameters, the disk with many heads will be
faster.
Splitting swap and putting it on both disks will be even faster, though.
* Older disks have the same number of sectors on all tracks. With these
disks it will be fastest to put your swap in the middle of the disks,
assuming that your disk head will move from a random track towards the
swap area.
* Newer disks use ZBR (zone bit recording). They have more sectors on the
outer tracks. With a constant number of rpms, this yields a far greater
performance on the outer tracks than on the inner ones. Put your swap on
the fast tracks.
* Of course your disk head will not move randomly. If you have swap space
in the middle of a disk between a constantly busy home partition and an
almost unused archive partition, you would be better of if your swap were
in the middle of the home partition for even shorter head movements. You
would be even better off, if you had your swap on another otherwise
unused disk, though.
Summary: Put your swap on a fast disk with many heads that is not busy doing
other things. If you have multiple disks: Split swap and scatter it over all
your disks or even different controllers.
Even better: Buy more RAM.
-----------------------------------------------------------------------------
Section 5: Partitioning with fdisk
5. Partitioning with fdisk
5.1. Partitioning with fdisk
This section shows you how to actually partition your hard drive with the
fdisk utility. Linux allows only 4 primary partitions. You can have a much
larger number of logical partitions by sub-dividing one of the primary
partitions. Only one of the primary partitions can be sub-divided.
Examples:
1. Four primary partitions (Section 5.1.2)
2. Mixed primary and logical partitions (Section 5.1.3)
-----------------------------------------------------------------------------
5.1.1. Notes about fdisk:
fdisk is started by typing (as root) fdisk device at the command prompt.
"device" (Section 2.1.1) might be something like /dev/hda or /dev/sda. The
basic fdisk commands you need are:
p
print the partition table
n
create a new partition
d
delete a partition
q
quit without saving changes
w
write the new partition table and exit
Changes you make to the partition table do not take effect until you issue
the write (w) command. Here is a sample partition table:
Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 184 370912+ 83 Linux
/dev/hdb2 185 368 370944 83 Linux
/dev/hdb3 369 552 370944 83 Linux
/dev/hdb4 553 621 139104 82 Linux swap
The first line shows the geometry of your hard drive. It may not be
physically accurate, but you can accept it as though it were. The hard drive
in this example is made of 32 double-sided platters with one head on each
side (probably not true). Each platter has 621 concentric tracks. A
3-dimensional track (the same track on all disks) is called a cylinder. Each
track is divided into 63 sectors. Each sector contains 512 bytes of data.
Therefore the block size in the partition table is 64 heads * 63 sectors *
512 bytes er...divided by 1024. (See 4 for discussion on problems with this
calculation.)
The start and end values are cylinders. The first cylinder (0) is reserved
for information about the drive layout.
-----------------------------------------------------------------------------
5.1.2. Four primary partitions
The overview:Decide on the size (Section 4.4.1) of your swap space and where
(Section 4.4.2) it ought to go. Divide up the remaining space for the three
other partitions.
Example:
I start fdisk from the shell prompt:
# fdisk /dev/hdb
which indicates that I am using the second drive on my IDE controller. (See
Section 2.1.) Now I need to plan my layout. In this example, I want to use
only primary partitions for my linux partitions and my swap space. When I
print the (empty) partition table, I just get configuration information.
Command (m for help): p
Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
I knew that I had a 1.2Gb drive, but now I really know: 64 * 63 * 512 * 621 =
1281982464 bytes. I decide to reserve 128Mb of that space for swap, leaving
1153982464. If I use one of my primary partitions for swap, that means I have
three left for ext2 partitions. Divided equally, that makes for 384Mb per
partition. Now I get to work.
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-621, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-621, default 621): +384M
Next, I set up the partition I want to use for swap:
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 2
First cylinder (197-621, default 197):
Using default value 197
Last cylinder or +size or +sizeM or +sizeK (197-621, default 621): +128M
Now the partition table looks like this:
Device Boot Start End Blocks Id System
/dev/hdb1 1 196 395104 83 Linux
/dev/hdb2 197 262 133056 83 Linux
I set up the remaining two partitions the same way I did the first. Finally,
I make the first partition bootable:
Command (m for help): a
Partition number (1-4): 1
And I make the second partition of type swap:
Command (m for help): t
Partition number (1-4): 2
Hex code (type L to list codes): 82
Changed system type of partition 2 to 82 (Linux swap)
Command (m for help): p
The end result:
Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 196 395104+ 83 Linux
/dev/hdb2 197 262 133056 82 Linux swap
/dev/hdb3 263 458 395136 83 Linux
/dev/hdb4 459 621 328608 83 Linux
Finally, I issue the write command (w) to write the table on the disk.
For more information, see:
* Section 7.1
* Section 7
* Section 7.2
-----------------------------------------------------------------------------
5.1.3. Mixed primary and logical partitions
The overview: create one use one of the primary partitions to house all the
extra partitions. Then create logical partitions within it. Create the other
primary partitions before or after creating the logical partitions.
Example:
I start fdisk from the shell prompt:
# fdisk /dev/sda
which indicates that I am using the first drive on my SCSI chain. (See
Section 2.1.)
First I figure out how many partitions I want. I know my drive has a 183Gb
capacity and I want 26Gb partitions (because I happen to have back-up tapes
that are about that size).
183Gb / 26Gb = ~7
so I will need 7 partitions. Even though fdisk accepts partition sizes
expressed in Mb and Kb, I decide to calculate the number of cylinders that
will end up in each partition because fdisk reports start and stop points in
cylinders. I see when I enter fdisk that I have 22800 cylinders.
> The number of cylinders for this disk is set to 22800. There is
> nothing wrong with that, but this is larger than 1024, and could in
> certain setups cause problems with: 1) software that runs at boot
> time (e.g., LILO) 2) booting and partitioning software from other
> OSs (e.g., DOS FDISK, OS/2 FDISK)
So, 22800 total cylinders divided by seven partitions is 3258 cylinders. Each
partition will be about 3258 cylinders long. I ignore the warning msg because
this is not my boot drive (Section 4).
Since I have 4 primary partitions, 3 of them can be 3258 long. The extended
partition will have to be (4 * 3258), or 13032, cylinders long in order to
contain the 4 logical partitions.
I enter the following commands to set up the first of the 3 primary
partitions (stuff I type is bold ):
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-22800, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-22800, default 22800): 13032
The last partition is the extended partition:
Partition number (1-4): 4
First cylinder (9775-22800, default 9775):
Using default value 9775
Last cylinder or +size or +sizeM or +sizeK (9775-22800, default 22800):
Using default value 22800
The result, when I issue the print table command is:
/dev/sda1 1 3258 26169853+ 83 Linux
/dev/sda2 3259 6516 26169885 83 Linux
/dev/sda3 6517 9774 26169885 83 Linux
/dev/sda4 9775 22800 104631345 5 Extended
Next I segment the extended partition into 4 logical partitions, starting
with the first logical partition, into 3258-cylinder segments. The logical
partitions automatically start from /dev/sda5.
Command (m for help): n
First cylinder (9775-22800, default 9775):
Using default value 9775
Last cylinder or +size or +sizeM or +sizeK (9775-22800, default 22800): 13032
The end result is:
Device Boot Start End Blocks Id System
/dev/sda1 1 3258 26169853+ 83 Linux
/dev/sda2 3259 6516 26169885 83 Linux
/dev/sda3 6517 9774 26169885 83 Linux
/dev/sda4 9775 22800 104631345 5 Extended
/dev/sda5 9775 13032 26169853+ 83 Linux
/dev/sda6 13033 16290 26169853+ 83 Linux
/dev/sda7 16291 19584 26459023+ 83 Linux
/dev/sda8 19585 22800 25832488+ 83 Linux
Finally, I issue the write command (w) to write the table on the disk. To
make the partitions usable, I will have to format (Section 7) each partition
and then mount (Section 7.2) it.
-----------------------------------------------------------------------------
5.1.4. Submitted Examples
I'd like to submit my partition layout, because it works well with any
distribution of Linux (even big RPM based ones). I have one hard drive that
... is 10 gigs, exactly. Windows can't see above 9.3 gigs of it, but Linux
can see it all, and use it all. It also has much more than 1024 cylenders.
Table 3. Partition layout example
+-------------------+--------------------+-------------------+
|Partition |Mount point |Size |
+-------------------+--------------------+-------------------+
|/dev/hda1 |/boot |(15 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda2 |windows 98 partition|(2 gigs) |
+-------------------+--------------------+-------------------+
|/dev/hda3 |extended |(N/A) |
+-------------------+--------------------+-------------------+
|/dev/hda5 |swap space |(64 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda6 |/tmp |(50 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda7 |/ |(150 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda8 |/usr |(1.5 gigs) |
+-------------------+--------------------+-------------------+
|/dev/hda9 |/home |(rest of drive) |
+-------------------+--------------------+-------------------+
I test new kernels for the USB mass storage, so that explains the large /boot
partition. I install LILO into the MBR, and by default I boot windows (I'm
not the only one to use this computer).
I also noticed that you don't have any REAL examples of partition tables, and
for newbies I HIGHLY suggest putting quite a few up. I'm freshly out of the
newbie stage, and partitioning was what messed me up the most.
[mailto:valkor@qx.net] Valkor
-----------------------------------------------------------------------------
Section 6: Recovering a Deleted Partition Table
6. Recovering a Deleted Partition Table
1. Make a partition that is at least as big as your first partition was. You
can make it larger than the original partition by any amount. If you
underestimate, there will be much wailing and gnashing of teeth.
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-23361, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-22800, default 22800): 13032
Command (m for help): w
2. Run dumpe2fs on the first partition and grep out the block count.
Example:
% dumpe2fs /dev/sda1 | grep "Block count:"
Block count: 41270953
If you are uncertain about this value, repeat Step 1 with a bigger
partition size. If the block count changes, then you underestimated the
size of the original partition. Repeat Step 1 until you get a stable
block count.
3. Remove the partition you just created
Command (m for help): d
Partition number (1-4): 1
4. Make a new partition with the exact size you got from the block count.
Since you cannot enter block size in fdisk, you need to figure out how
many cylinders to request. Here is the formula:
(number of needed cylinders) = (number of blocks) / (block size)
(block size) = (unit size) / 1024
(unit size) = (number of cylinders) * (number of heads) * (number of sectors/cylinder) * (number of bytes/sector)
In theory! In practice, it's rather more complicated. fdisk tries to end
its allocation for a partition on a cylinder boundary, so it can be hard
to figure out the relationship of cylinders to blocks.
Here is an example of the problem. Below, I have formatted a drive with
partitions with 1, 2, 4, and 8 cylinders.
disk /dev/sda: 16 heads, 63 sectors, 23361 cylinders
Units = cylinders of 1008 * 512 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 2 976+ 83 Linux
/dev/sda2 3 5 1512 83 Linux
/dev/sda3 6 10 2520 83 Linux
/dev/sda4 11 19 4536 83 Linux
If I divide each partition by the number of cylinders, I ought to get the
block size (16 heads * 63 sectors * 512 bytes/sector divided by 1024 =
504), right? Not true!
allocated #of block
blocks cyl size
976 / 1 = 976
1512 / 2 = 756
2520 / 4 = 630
4536 / 8 = 567
8568 / 16 = 535
16632 / 32 = 519
32760 / 64 = 512
64984 / 128 = 507
129528 / 256 = 505
258552 / 512 = 504
516600 / 1024 = 504
1032664 / 2048 = 504
Notice that as the number of cylinders grows, the closer to the real
block size the calculated value for the allocated blocks becomes.
You will have to make guestimates and converge on the true number of
cylinders to use. You will ultimately get an exact match because the
block count from dumpe2fs came from a well-formed partition.
5. Run e2fsck on it to verify that you can read the new partition.
6. Repeat Steps 1-5 on remaining partitions.
Remount your partitions. Amazingly, all of your data will be there.
Credit goes to:
* Mike Vevea, jedi sys admin and MGH's finest, for giving me these tips.
-----------------------------------------------------------------------------
Section 7: Formating Partitions
7. Formating Partitions
At the shell prompt, I begin making the file systems on my partitions.
Continuing with the Section 5.1.3, this is:
# mke2fs /dev/sda1
I need to do this for each of my partitions, but not for /dev/sda4 (my
extended partition). Linux supports types of file systems other than ext2.
You can find out what kinds your kernel supports by looking in: /usr/src/
linux/include/linux/fs.h
The most common file systems can be made with programs in /sbin that start
with "mk" like mkfs.msdos and mke2fs.
-----------------------------------------------------------------------------
7.1. Activating Swap Space
To set up a swap partition:
# mkswap -f /dev/hda5
To activate the swap area:
# swapon /dev/hda5
Normally, the swap area is activated by the initialization scripts at boot
time.
-----------------------------------------------------------------------------
7.2. Mounting Partitions
Mounting a partition means attaching it to the linux file system. To mount a
linux partition:
# mount -t ext2 /dev/sda1 /opt
-t ext2
File system type. Other types you are likely to use are:
+ msdos (DOS)
+ hfs (mac)
+ iso9660 (CDROM)
+ nfs (network file system)
/dev/sda1
Device name. Other device names you are likely to use:
+ /dev/hdb2 (second partition in second IDE drive)
+ /dev/fd0 (floppy drive A)
+ /dev/cdrom (CDROM)
/opt
mount point. This is where you want to "see" your partition. When you
type ls /opt, you can see what is in /dev/sda1. If there are already some
directories and/or files under /opt, they will be invisible after this
mount command.
-----------------------------------------------------------------------------
7.3. Some facts about file systems and fragmentation
Disk space is administered by the operating system in units of blocks and
fragments of blocks. In ext2, fragments and blocks have to be of the same
size, so we can limit our discussion to blocks.
Files come in any size. They don't end on block boundaries. So with every
file a part of the last block of every file is wasted. Assuming that file
sizes are random, there is approximately a half block of waste for each file
on your disk. Tanenbaum calls this "internal fragmentation" in his book
"Operating Systems".
You can guess the number of files on your disk by the number of allocated
inodes on a disk. On my disk
# df -i
Filesystem Inodes IUsed IFree %IUsed Mounted on
/dev/hda3 64256 12234 52022 19% /
/dev/hda5 96000 43058 52942 45% /var
there are about 12000 files on / and about 44000 files on /var. At a block
size of 1 KB, about 6+22 = 28 MB of disk space are lost in the tail blocks of
files. Had I chosen a block size of 4 KB, I had lost 4 times this space.
Data transfer is faster for large contiguous chunks of data, though. That's
why ext2 tries to preallocate space in units of 8 contigous blocks for
growing files. Unused preallocation is released when the file is closed, so
no space is wasted.
Noncontiguous placement of blocks in a file is bad for performance, since
files are often accessed in a sequential manner. It forces the operating
system to split a disk access and the disk to move the head. This is called
"external fragmentation" or simply "fragmentation" and is a common problem
with MS-DOS file systems. In conjunction with the abysmal buffer cache used
by MS-DOS, the effects of file fragmentation on performance are very
noticeable. DOS users are accustomed to defragging their disks every few
weeks and some have even developed some ritualistic beliefs regarding
defragmentation.
None of these habits should be carried over to Linux and ext2. Linux native
file systems do not need defragmentation under normal use and this includes
any condition with at least 5% of free space on a disk. There is a
defragmentation tool for ext2 called defrag, but users are cautioned against
casual use. A power outage during such an operation can trash your file
system. Since you need to back up your data anyway, simply writing back from
your copy will do the job.
The MS-DOS file system is also known to lose large amounts of disk space due
to internal fragmentation. For partitions larger than 256 MB, DOS block sizes
grow so large that they are no longer useful (This has been corrected to some
extent with FAT32). Ext2 does not force you to choose large blocks for large
file systems, except for very large file systems in the 0.5 TB range (that's
terabytes with 1 TB equaling 1024 GB) and above, where small block sizes
become inefficient. So unlike DOS there is no need to split up large disks
into multiple partitions to keep block size down.
Use a 1Kb block size if you have many small files. For large partitions, 4Kb
blocks are fine.