Linux Partition HOWTO
Tony Harris
Kristian Koehntopp
Revision History
Revision 3.2 1 September 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 [1]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 [2]here.
_________________________________________________________________
Table of Contents
1. [3]Introduction
1.1. [4]What is a partition?
1.2. [5]Constraints
1.3. [6]Other Partitioning Software:
1.4. [7]Related HOWTOs
1.5. [8]Additional information on your system:
2. [9]Devices
2.1. [10]Device names
2.2. [11]Device numbers
3. [12]Partition Types
3.1. [13]Partition Types
3.2. [14]Foreign Partition Types
3.3. [15]Primary Partitions
3.4. [16]Logical Partitions
3.5. [17]Swap Partitions
4. [18]Partitioning requirements
4.1. [19]What Partitions do I need?
4.2. [20]Discussion:
4.3. [21]File Systems
4.4. [22]Swap Partitions
5. [23]Partitioning with fdisk
5.1. [24]Partitioning with fdisk
6. [25]Recovering a Deleted Partition Table
7. [26]Formating Partitions
7.1. [27]Activating Swap Space
7.2. [28]Mounting Partitions
7.3. [29]Some facts about file systems and fragmentation
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.
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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
* [30]parted: Gnu partition editor
* [31]Partition Magic: a commercial utility to create, resize, merge
and convert partitions, without destroying data.
* [32]Disk Drake: a Perl/Gtk program to create, rsize, and delete
partitions
_________________________________________________________________
1.4. Related HOWTOs
Table 1. Related HOWTOs
Title Author Description
[33]Linux Multiple Disk System Tuning [34]Gjoen Stein How to estimate
the various size and speed requirements for different parts of the
filesystem.
[35]Linux Large Disk [36]Andries Brouwer Instructions and
considerations regarding disks with more than 1024 cylinders
[37]Linux Quota [38]Albert M.C. Tam Instructions on limiting disk
space usage per user (quotas)
[39]Partition-Rescue mini-HOWTO [40]Jean-Daniel Dodin How to restore
linux partitions after they have been deleted by a Windows install.
Does not appear to preserve data.
[41]Linux ADSM Backup [42]Thomas Koenig Instructions on integrating
Linux into an IBM ADSM backup environment.
[43]Linux Backup with MSDOS [44]Christopher Neufeld Information about
MS-DOS driven Linux backups.
Linux HOWTO Index [45]Tim Bynum Instructions on writing and submitting
a HOWTO document
_________________________________________________________________
1.5. Additional information on your system:
* [46]/usr/src/linux/Documentation
+ [47]ide.txt: Info about your IDE drivers
+ [48]scsi.txt: Info about your SCSI drivers
_________________________________________________________________
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 ([49]Section 3.3) on a disk are 1, 2, 3 and 4.
Logical partitions ([50]Section 3.4) have numbers 5 and up, for
reasons explained later ([51]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 1994 /dev/hda
permissions owner group major device number minor device number date
device name
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
[52]/usr/src/linux/Documentation/ide.txt. For SCSI disks, one would
expect such documentation in
[53]/usr/src/linux/Documentation/scsi.txt, but it isn't there. One has
to look at the driver source to be sure
([54]/usr/src/linux/driver/scsi/sd.c:184-196). Fortunately, there is
Peter Anvin's list of device numbers and names in
[55]/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.
_________________________________________________________________
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 [56]Section 4.4.1 for tips on determining the size of swap
space you need.
_________________________________________________________________
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 [57]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 [58]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:
[59]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: [60]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 [61]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 ([62]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 [63]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.
_________________________________________________________________
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 ([64]Section 5.1.2)
2. Mixed primary and logical partitions ([65]Section 5.1.3)
_________________________________________________________________
5.1.1. Notes about fdisk:
fdisk is started by typing (as root) fdisk device at the command
prompt. "device" ([66]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
[67]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 ([68]Section 4.4.1) of your swap space
and where ([69]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 [70]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):<RETURN>
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):<RETURN>
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:
* [71]Section 7.1
* [72]Section 7
* [73]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
[74]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 ([75]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): <RETURN>
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): <RETURN>
Using default value 9775
Last cylinder or +size or +sizeM or +sizeK (9775-22800, default 22800): <RETURN
>
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): <RETURN>
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 ([76]Section 7)
each partition and then mount ([77]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.
[78]Valkor
_________________________________________________________________
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): <RETURN>
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.
_________________________________________________________________
7. Formating Partitions
At the shell prompt, I begin making the file systems on my partitions.
Continuing with the [79]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.
References
1. mailto:tony@nmr.mgh.harvard.edu
2. http://surfer.nmr.mgh.harvard.edu/partition/Partition.html
3. Partition.html#INTRO
4. Partition.html#EXPLANATION
5. Partition.html#CONSTRAINTS
6. Partition.html#AEN57
7. Partition.html#HOWTOS
8. Partition.html#AEN135
9. Partition.html#PARTITION-2
10. Partition.html#NAMES
11. Partition.html#NUMBERS
12. Partition.html#PARTITION-3
13. Partition.html#TYPES
14. Partition.html#AEN228
15. Partition.html#PRIMARY
16. Partition.html#LOGICAL
17. Partition.html#SWAP-PARTITIONS
18. Partition.html#PARTITION-4
19. Partition.html#NUMBER
20. Partition.html#AEN266
21. Partition.html#AEN291
22. Partition.html#AEN343
23. Partition.html#PARTITION-5
24. Partition.html#AEN384
25. Partition.html#RECOVERING
26. Partition.html#FORMATING
27. Partition.html#SWAP
28. Partition.html#MOUNTING
29. Partition.html#FRAGMENTATION
30. http://www.gnu.org/software/parted/parted.html
31. http://www.powerquest.com/partitionmagic/index.html
32. http://www.linux-mandrake.com/diskdrake
33. http://www.nyx.net/~sgjoen/disk.html
34. mailto:sgjoen@mail.nyx.net
35. http://metalab.unc.edu/mdw/HOWTO/Large-Disk-HOWTO.html
36. mailto:aeb@cwi.nl
37. http://metalab.unc.edu/mdw/HOWTO/mini/Quota.html
38. mailto:bertie@scn.org
39. http://metalab.unc.edu/mdw/HOWTO/mini/Partition-Rescue-mini-HOWTO.htmlPartition-Rescue
40. mailto:jdanield@dodin.net
41. http://metalab.unc.edu/mdw/HOWTO/mini/ADSM-Backup.html
42. mailto:Thomas.Koenig@ciw.uni-karlsruhe.de
43. http://metalab.unc.edu/mdw/HOWTO/mini/Backup-With-MSDOS.html
44. mailto:neufeld@physics.utoronto.ca
45. mailto:linux-howto@sunsite.unc.edu
46. file://localhost/usr/src/linux/Documentation
47. file://localhost/usr/src/linux/Documentation/ide.txt
48. file://localhost/usr/src/linux/Documentation/scsi.txt
49. Partition.html#PRIMARY
50. Partition.html#LOGICAL
51. Partition.html#MIXED
52. file://localhost/usr/src/linux/Documentation/ide.txt
53. file://localhost/usr/src/linux/Documentation/scsi.txt
54. file://localhost/usr/src/linux/driver/scsi/sd.c
55. file://localhost/usr/src/linux/Documentation/devices.txt
56. Partition.html#SWAPSIZE
57. http://metalab.unc.edu/mdw/HOWTO/mini/LILO.html
58. http://metalab.unc.edu/mdw/HOWTO/Large-Disk-HOWTO.html
59. Partition.html#SWAPPLACEMENT
60. Partition.html#SWAPSIZE
61. http://metalab.unc.edu/mdw/HOWTO/mini/LILO.html
62. Partition.html#FRAGMENTATION
63. partition-3.html#swap
64. Partition.html#PRIMARY-EXAMPLE
65. Partition.html#MIXED
66. Partition.html#NAMINGCONVENTION
67. Partition.html#BLOCKSIZE
68. Partition.html#SWAPSIZE
69. Partition.html#SWAPPLACEMENT
70. Partition.html#NAMES
71. Partition.html#SWAP
72. Partition.html#FORMATING
73. Partition.html#MOUNTING
74. Partition.html#NAMES
75. Partition.html#PARTITION-4
76. Partition.html#FORMATING
77. Partition.html#MOUNTING
78. mailto:valkor@qx.net
79. Partition.html#MIXED