Linux Partition mini-HOWTO
Kristan Koehntopp, kris@koehntopp.de
$Id: howto.sgml,v 2.2 97/07/15 20:07:21 kris Exp Locker: kris $
1. Introduction
1.1. What is this?
This is a Linux Mini-HOWTO text. A Mini-HOWTO is a small text
explaining some business related to Linux installation and maintenance
tutorial style. It's mini, because either the text or the topic it
discusses are to small for a real HOWTO or even a book. A HOWTO is not
a reference: that's what manual pages are for.
1.2. What is in it? and related HOWTO documents
This particular Mini-HOWTO teaches you how to plan and layout disk
space for your Linux system. It talks about disk hardware, partitions,
swap space sizing and positioning considerations, file systems, file
system types and related topics. The intent is to teach some
background knowlegde, so we are talking mainly principles and not
tools in this text.
Ideally, this document should be read before your first installation,
but this is somehow difficult for most people. First timers have
other problems than disk layout optimization, too. So you are probably
someone who just finished a Linux installation and is now thinking
about ways to optimize this installation or how to avoid some nasty
miscalculations in the next one. Well, exspect some desire to tear
down and rebuild your installation when you are finished with this
text. :-)
This Mini-HOWTO limits itself to planning and layouting disk space
most of the time. It does not discuss the usage of fdisk, LILO, mke2fs
or backup programs. There are other HOWTOs that address these
problems. Please see the Linux HOWTO Index for current information on
Linux HOWTOs. There are instructions for obtaining HOWTO documents in
the index, too.
For instructions and considerations regarding disks with more that
1024 cylinders, see "Linux Large Disk mini-HOWTO", Andries Brouwer
<aeb@cwi.nl>.
For instructions on disk spanning and striping, see "Linux Multiple
Disks Layout mini-HOWTO", by Gjoen Stein <gjoen@nyx.net>.
For instructions on limiting disk space usage per user (quotas), see
"Linux Quota mini-HOWTO", by Albert M.C. Tam <bertie@scn.org>
Currently, there is no general document on disk backup, but there are
several documents with pointers to specific backup solutions. See
"Linux ADSM Backup mini-HOWTO", by Thomas Koenig
<Thomas.Koenig@ciw.uni-karlsruhe.de> for instructions on integrating
Linux into an IBM ADSM backup environment. See "Linux Backup with
MSDOS mini-HOWTO", by Christopher Neufeld
<neufeld@physics.utoronto.ca> for information about MS-DOS driven
Linux backups.
For instructions on writing and submitting a HOWTO document, see the
Linux HOWTO Index, by Greg Hankins <gregh@sunsite.unc.edu>.
Browsing through /usr/src/linux/Documentation can be very instructive,
too. See ide.txt and scsi.txt for some background information on the
properties of your disk drivers and have a look at the filesystems/
subdirectory.
2. What is a partition anyway?
When PC hard disks were invented people soon wanted to install
multiple operating systems, even if their system had only one disk.
So a mechanism was needed to divide a single physical disk into
multiple logical disks. So that's what a partition is: A contiguous
section of blocks on your hard disk that is treated like a completely
seperate disk by most operating systems.
It is fairly clear that partitions must not overlap: An operating
system will certainly not be pleased, if another operating system
installed on the same machine were overwriting important information
because of overlapping partitions. There should be no gap between
adjacent partitions, too. While this constellation 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 space at the end of your disk that is not assigned to any of your
installed operating systems, yet. Later, when it is clear which
installation is used by you most of the time, you can partition this
left over space and put a file system on it.
Partitions can not be moved nor can they be resized without destroying
the file system contained in it. So repartitioning usually involves
backup and restore of all file systems touched during the
repartitioning. In fact it is fairly common to mess up things
completely during repartitioning, so you should back up anything on
any disk on that particular machine before even touching things like
fdisk.
Well, some partitions with certain file system types on them actually
can be split in two without losing any data (if you are lucky). For
example there is a program called "fips" for splitting MS-DOS
partitions into two to make room for a Linux installation without
having to reinstall MS-DOS. You are still not going to touch these
things without carefully backing up everything on that machine, aren't
you?
2.1. Backups are important
Tapes are your friend for backups. They are fast, reliable and easy to
use, so you can make backups often, preferably automatically and
without hassle.
Step on soapbox: And I am talking about real tapes, not that disk
controller driven ftape crap. Consider buying SCSI: Linux does support
SCSI natively. You don't need to load ASPI drivers, you are not losing
precious HMA under Linux and once the SCSI host adapter is installed,
you just attach additional disks, tapes and CD-ROMs to it. No more I/O
addresses, IRQ juggling or Master/Slave and PIO-level matching.
Plus: Proper SCSI host adapters give you high I/O performance without
much CPU load. Even under heavy disk activity you will experience good
response times. If you are planning to use a Linux system as a major
USENET news feed or if you are about to enter the ISP business, don't
even think about deploying a system without SCSI. Climb of soapbox.
2.2. Device numbers and device names
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. When it became clear
that people needed more partitions on their systems, logical
partitions were invented. The number of logical partitions is not
limited: Each logical partition contains a pointer to the next logical
partition, so you can have a potentially unlimited chain of partition
entries.
For compatibility reasons, the space occupied by all logical
partitions had to be accounted for. If you are using logical
partitions, one primary partition entry is marked as "extended
partition" and its starting and ending block mark the area occupied by
your logical partitions. This implies that the space assigned to all
logical partitions has to be contiguous.
Linux cannot handle more than a limited number of partitions per
drive. So in Linux you have 4 primary partitions (3 of them useable,
if you are using logical partitions) and at most 15 partitions
altogether on an SCSI disk (63 altogether on an IDE disk).
In Linux, partitions are represented by device files. 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. Unlike other Unices, Linux does not offer "raw" character
versions of disks and their partitions.
The only important thing with a device file are its major and minor
device number, shown instead of the files size:
______________________________________________________________________
$ ls -l /dev/hda
brw-rw---- 1 root disk 3, 0 Jul 18 1994 /dev/hda
^ ^
| minor device number
major device 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
exspect 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.
By convention device files have certain names and many system programs
have knowledge about these names compiled in. They exspect your IDE
disks to be named /dev/hd* and your SCSI disks to be named /dev/sd*.
Disks are numbered a, b, c and so on, so /dev/hda is your first IDE
disk and /dev/sda is your first SCSI disk. Both devices represent
entire disks, starting at block one. Writing to these devices with
the wrong tools will destroy the master boot loader and partition
table on these disks, rendering all data on this disk unusable or
making your system unbootable. Know what you are doing and, again,
back up before you do it.
Primary partitions on a disk are 1, 2, 3 and 4. So /dev/hda1 is the
first primary partition on the first IDE disk and so on. Logical
partitions have numbers 5 and up, so /dev/sdb5 is the first logical
partition on the second SCSI disk.
Each partition entry has a starting and an ending block address
assigned to it and a type. The type is a numerical code (a byte) which
designates a particular partition to a certain type of operating
system. For the benefit of computing consultants partition type codes
are not really unique, so there is always the probability of two
operating systems using the same type code.
Linux reserves the type code 0x82 for swap partitions and 0x83 for
"native" file systems (that's ext2 for almost all of you). The once
popular, now outdated Linux/Minix file system used the type code 0x81
for partitions. OS/2 marks it's 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 proteced FAT partitions, creating a type
clash with Linux/Minix at that time, but neither Linux/Minix nor DR-
DOS are widely used any more. The extended partition which is used as
a container for logical partitions has a type of 0x05, by the way.
Partitions are created and deleted with the fdisk program. Every self
respecting operating system program comes with an fdisk and
traditionally it is even called fdisk (or FDISK.EXE) in almost all
OSes. Some fdisks, noteable the DOS one, are somehow limited when they
have to deal with other operating systems partitions. Such limitations
include the complete inability to deal with anything with a foreign
type code, the inability to deal with cylinder numbers above 1024 and
the inability to create or even understand partitions that do not end
on a cylinder boundary. For example, the MS-DOS fdisk can't delete
NTFS partitions, the OS/2 fdisk has been known to silently "correct"
partitions created by the Linux fdisk that do not end on a cylinder
boundary and both, the DOS and the OS/2 fdisk, have had problems with
disks with more than 1024 cylinders (see the "large-disk" Mini-Howto
for details on such disks).
3. What Partitions do I need?
3.1. How many partitions do I need?
Okay, so what partitions do you need? Well, some operating systems do
not believe into booting from logical partitions for reasons that are
beyond the scope of any sane mind. So you probably want to reserve
your primary partitions as boot partitions for your MS-DOS, OS/2 and
Linux or whatever you are using. Remember that one primary partition
is needed as an extended partition, which acts as a container for the
rest of your disk with logical partitions.
Booting operating systems is a real-mode thing involving BIOSes and
1024 cylinder limitations. So you probably want to put all your boot
partitions into the first 1024 cylinders of your hard disk, just to
avoid problems. Again, read the "large-disk" Mini-Howto for the gory
details.
To install Linux, you will need at least one partition. If the kernel
is loaded from this partition (for example by LILO), this partition
must be readable by your BIOS. If you are using other means to load
your kernel (for example a boot disk or the LOADLIN.EXE MS-DOS based
Linux loader) the partition can be anywhere. In any case this
partition will be of type 0x83 "Linux native".
Your system will need some swap space. Unless you swap to files you
will need a dedicated swap partition. Since this partition is only
accessed by the Linux kernel and the Linux kernel does not suffer from
PC BIOS deficiencies, the swap partition may be positioned anywhere.
I recommed using a logical partition for it (/dev/?d?5 and higher).
Dedicated Linux swap partitions are of type 0x82 "Linux swap".
These are minimal partition requirements. It may be useful to create
more partitions for Linux. Read on.
3.2. 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.
� In Linux, a single swap partition can not be larger than 128 MB.
That is, the partition may be larger than 128 MB, but excess space
is never used. If you want more than 128 MB of swap, you have to
create multiple swap partitions.
� When sizing swap space, keep in mind that too much swap space may
not be useful at all.
Every process has a "working set". This is a set of in-memory pages
which will be referenced by the processor in the very near future.
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 the program has a good
"locality of reference" this assumption will be true and prediction
algorithm will work.
Holding a working set in main memory does only work if there is
enough main memory. If you have too many processes running on a
machine, the kernel is forced to put pages on disk that it will
reference again in the very near future (forcing a page-out of a
page from another working set and then a page-in of the page
referenced). Usually this results in a very heavy increase in
paging activity and in a sustantial drop of performance. A machine
in this state is said to be "thrashing" (For you german readers:
That's "thrashing" ("dreschen", "schlagen", "haemmern") and not
trashing ("muellen")).
On a thrashing machine the processes are essentially running from
disk and not from RAM. Expect performance to drop by approximately
the ratio between memory access speed and disk access speed.
A very old rule of thumb in the days of the PDP and the Vax was
that the size of the 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?).
3.3. 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
this 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.
3.4. 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 DOS file systems.
ext2 has several strategies to avoid external fragmentation. Normally
fragmentation is not a large problem in ext2, not even on heavily used
partitions such as a USENET news spool. While there is a tool for
defragmentation of ext2 file systems, nobody ever uses it and it is
not up to date with the current release of ext2. Use it, but do so on
your own risk.
The MS-DOS file system is well known for its pathological managment of
disk space. 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.
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 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 the 1 KB default block size if possible. You may want
to experiment with a block size of 2 KB for some partitions, but
expect to meet some seldom exercised bugs: Most people use the
default.
3.5. File lifetimes and backup cycles as partitioning criteria
With ext2, Partitioning decisions should be governed by backup
considerations and to avoid external fragmentation from different file
lifetimes.
Files have lifetimes. After a file has been created, it will remain
some time on the system and them be removed. File lifetime varies
greatly throughout the system and is partly dependend 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 exspect 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. An example
4.1. A recommended model for ambitious beginners
A common model creates /, /home and /var partitions as discussed
above. This is simple to install and maintain and differentiates well
enough to avoid adverse effects from different lifetimes. It fits well
into a backup model, too: Almost noone bothers to backup USENET news
spools and only some files in /var are worth backing up
(/var/spool/mail comes to mind). On the other hand, / changes
infrequently and can be backup up on demand (after configuration
changes) and is small enough to fit on most modern backup media as a
full backup (plan 250 to 500 MB depending on the amount of installed
software). /home contains valuable user data and should be backuped up
daily. Some installations have very large /homes and must use
incremental backups.
Some systems put /tmp onto a seperate partition as well, others
symlink it to /var/tmp to achieve the same effect (note that this can
affect single user mode, where /var will be unavailable and the system
will have no /tmp until you create one or mount /var manually) or put
it onto a RAM disk (Solaris does this for example). This keeps /tmp
out of /, a good idea.
This model is convenient for upgrades or reinstallations as well: Save
your configuration files (or the entire /etc) to some /home directory,
scrap your /, reinstall and fetch the old configurations from the save
directory on /home.
5. How I did it on my machine
There was this old ISA bus 386/40 sitting on my shelf that I abandoned
two years ago because it no longer cut it. I was planning to turn it
into a small X-less server for my household LAN.
Here is how I did it: I took that 386 and put 16 MB RAM into it.
Added a cheapo EIDE disk, the smallest I could get (800 MB) and an
ethernet card. Added an old Hercules because I still had a monitor for
it. Installed Linux on it and there I have my local NFS, SMB, HTTP,
LPD/LPR and NNTP server as well as my mail router and POP3 server.
With an additional ISDN card the machine become my TCP/IP router and
firewall, too.
Most of the disk space on this machine went into the /var directories,
/var/spool/mail, /var/spool/news and /var/httpd/html. I put /var on a
separate partition and made this one large. There will be almost no
users on this machine, so I created no home partition and mounted
/home from some other workstation via NFS.
Linux without X plus several locally installed utilities will be fine
with a 250 MB partition as /. The machine has 16 MB of RAM, but it
will be running many servers. 16 MB swap should be in order, 32 MB
should be plenty. We are not short on disk space, so the machine will
get 32 MB. Out of sentimentality a MS-DOS partition of some 20 MB is
kept on it. I decided to import /home from another machine, so the
remaining 500+ MB will end up as /var. This is more than sufficient
for a household USENET news feed.
We get
______________________________________________________________________
Device Mounted on Size
/dev/hda1 /dos_c 25 MB
/dev/hda2 - (Swapspace) 32 MB
/dev/hda3 / 250 MB
/dev/hda4 - (Extended Container) 500 MB
/dev/hda5 /var 500 MB
homeserver:/home /home 1.6 GB
______________________________________________________________________
I am backing up this machine via the network using the tape in
homeserver. Since everything on this machine has been installed from
CD-ROM all I have to save are some configuration files from /etc, my
customized locally installed *.tgz files from /root/Source/Installed
and /var/spool/mail as well as /var/httpd/html. I copy these files
into a dedicated directory /home/backmeup on homeserver every night,
where the regular homeserver backup picks them up.