Partitions are areas on your hard drive that are reserved for file systems. Let's look at their relationship to your hard drive at large.

Hard drives are composed of one or more layers called platters. Older SCSI drives, in particular, often house multiple platters. Please see Figure 3.2.

Figure 3.2. Hard drives can have one platter or several.

Each platter's surface vaguely resembles the surface of a vinyl record. Please see Figure 3.3.

As depicted in Figure 3.3, platters are covered by groove-like structures, circles that get increasingly smaller as they get closer to the center. The spaces between these circles are tracks. Tracks are divided into smaller units called sectors, which contain even smaller units that record data bits.

Figure 3.3. Your hard drive's tracks, sectors, and data.

The total number of tracks that occupy the same region on all platters form a cylinder. Please see Figure 3.4.

Partitions are composed of a user-specified range of contiguous cylinders. With DOS and Windows 3.11 in days of old (or even Windows 95's early release), users needed only one partition. This occupied virtually the entire disk and contained system files, user files, and swap files. Please see Figure 3.5.

Figure 3.4. All tracks occupying an identical area form a cylinder.

Figure 3.5. The DOS partition occupies almost the entire disk.

In Linux, it's more common to have multiple partitions, primarily to maintain strict control over where data ends up. Normally, when you use only a single partition (as you would with DOS), your operating system writes data arbitrarily wherever it finds suitable space, and so do users. Eventually, your data becomes spread out, unmanageable, and disorganized.

In contrast, things are a bit more orderly when you create multiple partitions. For example, you can separate swap files from your live file system. Each partition exclusively owns a specific disk area. Figure 3.6 depicts a fairly common partitioning scenario.

Figure 3.6. Here, the disk has two swap partitions and one native file Linux partition.

Another common scenario is when you install two or more operating systems on the same disk drive but different partitions, and they can coexist problem-free.

Linux supports a wide range of partition types. Table 3.1 lists a few of the more interesting ones.

Linux supports more partitions than those listed here. For a complete list, go to http://linuxclub.mnf.nu/lsa/lsg18.htm#E69E130. Also, for a complete list of all PC partition types (including those Linux does not support), go to http://www.win.tue.nl/math/dw/personalpages/aeb/linux/partitions/partition_types-1.html.

Many folks install both DOS/Windows and Linux on the same hard drive, on separate partitions. This offers them latitude and flexibility. They can learn Linux while still relying on Windows, and enjoy at least one-way compatibility. Please see Figure 3.7.

Figure 3.7. Linux and DOS/Windows can coexist, but only Linux offers compatibility.

Although DOS and Windows cannot access the Linux partition, Linux can access the DOS partition, thereby allowing you to copy files back and forth across file systems.

Linux newcomers often use the configurations depicted in Figures 3.6 and 3.7 because they're easy to implement. Many new Linux users are satisfied if they can just complete the installation with no problems, so they're apt to avoid more complicated partitioning schemes. Moreover, few installation routines highlight the relationship between partitioning and security, and give no hint that such configurations are risky. In fact, the scenarios depicted in Figures 3.6 and 3.7 expose your system to attack and hinder your ability to exercise effective system administration.

First, you should never put root and user file systems on the same Linux partition. If you do so, you increase the chance that attackers can exploit SUID programs to access restricted areas.

Additionally, lumping Linux into a single native partition makes your life as a system administrator difficult. For example, it may hinder your ability to incisively update or back up individual packages or file systems. And when the full Linux system occupies one partition, even limited file corruption can cause systemic problems (meaning that one corrupted directory hierarchy can affect others). You may even have to reinstall.

To avoid these problems, create a separate partition for each major file system. Figure 3.8 depicts one possible configuration.

This enhances security and makes backups and recovery manageable. You can specify different backup schedules for different partitions, system files are separated from data files, and so on. This approach also allows you to exercise more stringent control over each file system and how it is mounted.

Figure 3.8. All major file systems are on separate partitions.

At system startup, Linux mounts all available file systems per the specifications set forth in /etc/fstab. You can use /etc/fstab to incisively control how users and the system access your partitions. Let's quickly cover /etc/fstab now.

/dev/hda4 /dos msdos defaults 1 1

So, multiple partitions offer you at least four advantages:

There are other advantages. One is that the multi-partition approach prevents accidental denial of service and shields your root file system from overflow. For example, /var stores logging information. If you have just a single partition containing root, /usr, /var, and /tmp, logs in /var can potentially flood your entire file system (and users can too).

As noted, new users sometimes shy away from multiple partitions (beyond swap and root). That's because creating multiple partitions forces you to make some hard choices. For example, just how large should each partition be? Unfortunately, there's no definite answer to this question except when you're dealing with swap and root partitions. Swap partitions should be 128MB or less, and root should have 64MB minimum, (although I allocate 100MB).

In respect to other file systems, you'll make your choices depending on different factors. One factor is what you intend to do with your Linux box. On a multi-user system, you'll want to give your users at least 20MB each (and probably more). Hence, for 10 users, you'll need a /home partition of at least 210MB.

Some of these values are interdependent. For example, if you're accommodating many users and providing mail and news services, your /var and /home partitions will need to be substantial. Unless, of course, users use third-party mail and news solutions. In that case, their messages will be stored in their /home/user directory, e.g. /home/user/.netscape/.

If you run a firewall, you'll need a large log directory hierarchy, and this should have its own partition. In fact, you may be forced to put this partition on a separate disk drive. That way you'll avoid losing valuable audit information if the primary file system is corrupted.

However, in most cases your largest partitions will house the /usr and /home directories. Let's look at a conservative example. Here's a df report from a 1.6MB IDE hard drive with a 128MB swap partition that isn't visible from the df query:

Filesystem         1024-blocks  Used Available Capacity Mounted on

/dev/hda2              66365   17160    45778     27%   /
/dev/hda5             373695    1549   352845      0%   /home
/dev/hda6             703417  344725   322356     52%   /usr
/dev/hda7             127816   21235    99981     18%   /var
/dev/hda8             123919      22   117498      0%   /tmp

Here's the fstab information immediately after installation:

/dev/hda2 / ext2 defaults 0 1
/proc /proc proc defaults 0 0
/dev/hda1 none swap defaults 0 0
/dev/hda5 /home ext2 defaults 0 2
/dev/hda6 /usr ext2 defaults 0 2
/dev/hda7 /var ext2 defaults 0 2
/dev/hda8 /tmp ext2 defaults 0 2
#
/dev/fd0 /mnt/floppy ext2 defaults,noauto 0 0
#
/dev/hdb /mnt/cdrom iso9660 ro,noauto 0 0

Note partitions 5, 6, 7, and 8. These are logical partitions. You're allowed only four primary partitions in the Intel world, or three primary partitions, one extended partition, and multiple logical partitions. To create additional partitions, first establish an extended partition and then slice this into logical partitions using either fdisk or, if you have Red Hat, Disk Druid.

Although you've probably used fdisk already, some folks that have purchased this book may not have installed Linux yet. For their benefit, I'll briefly address fdisk here.

Using /dev/hda as default device!

The number of cylinders for this disk is set to 1579.
This is larger than 1024, and may 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)

Command (m for help):

Command action

   a   toggle a bootable flag
   d   delete a partition
   l   list known partition types
   m   print this menu
   n   add a new partition
   p   print the partition table
   q   quit without saving changes
   t   change a partition's system id
   u   change display/entry units
   v   verify the partition table
   w   write table to disk and exit
   x   extra functionality (experts only)

Disk /dev/hda: 32 heads, 63 sectors, 1579 cylinders
Units = cylinders of 2016 * 512 bytes

   Device Boot    Start      End   Blocks   Id  System

Command (m for help):

First, you'll need to create your swap and root partitions. In this example, I'll assume that you're installing to a new hard drive, with no other existing file systems previously installed.

To create a new partition, type n and press Enter. In response, fdisk will ask you what style of partition you want. Type p and press Enter for primary:

Command  Action

e  extended
p  primary partition (1-4)
p

fdisk will then ask you to number the new partition. This is your first primary partition and will house your swap file, so choose 1:

Partition Number (1-4): 1

Next, fdisk will ask you to specify where the partition starts. This is your first partition and you want to write it from the first cylinder onward, so choose 1:

First cylinder: (1-1579) 1

Finally, to complete the cycle, fdisk will ask you to size the partition. Swap file size is a matter of personal preference. In past years, Linux tutorials prescribed a ratio approach: "If you have 8MB of RAM, you'll need a 16MB swap file, minimum." Today, RAM is so inexpensive and folks have so much of it that this is unnecessary and, in many cases, impossible. Currently, Linux supports swap files of 128MB or less.

As per the preceding example, choose 128MB:

Last cylinder or +size or +sizeM or +sizek (1-1579): +128M

After you create each partition, reexamine the fdisk partition table. This way, if you make typographical errors, you can catch them before writing changes to disk. Here's what the updated table will look like after you create the first partition:

Command (m for help): p

Disk /dev/hda: 32 heads, 63 sectors, 1579 cylinders
Units = cylinders of 2016 * 512 bytes

   Device Boot    Start      End   Blocks   Id  System
/dev/hda1             1      130   131008+  83  Linux native

Note that the partition is type 83 (Linux native). You need to change this. This partition is a swap partition, and you must manually designate it as such. To do so, type t and press Enter:

Command (m for help): t

In response, fdisk will prompt you for the partition number. Choose 1:

Partition number (1-4):1

Finally, fdisk will ask which partition type you want. Choose 82 to convert the partition to a Linux swap:

Hex Code (L to list): 82

When you reexamine the partition table, fdisk will reflect the changes:

Command (m for help): p

Disk /dev/hda: 32 heads, 63 sectors, 1579 cylinders
Units = cylinders of 2016 * 512 bytes

   Device Boot    Start      End   Blocks   Id  System
/dev/hda1             1      130   131008+  82  Linux swap

Next, create the root partition. Here again, size is a matter of personal preference. You should allocate at least 32MB to root, although I've seen people make this partition as large as 100MB. In any case, the procedure is precisely the same. You start by creating a new partition. Type n and press Enter. Then fdisk will ask what style of partition you'd like. Again, type p and press Enter for primary:

Command  Action

e  extended
p  primary partition (1-4)
p

Then fdisk will ask you to number the new partition. This will be your second primary partition, so choose 2:

Partition Number (1-4): 2

In response, fdisk will ask you to specify where the partition starts:

First cylinder: (131-1579)

Note that the first valid starting cylinder is now 131. That's because your swap partition occupies cylinders 1 through 130. Therefore, you'll start your root partition at cylinder 131:

First cylinder: (1-1560) 131

And finally, fdisk will ask you to size the partition. For this example, allocate 64MB:

Last cylinder or +size or +sizeM or +sizek (131-1579):+64M

The results show a Linux (type 82) swap partition and a root (type 83) partition:

Command (m for help): p

Disk /dev/hda: 32 heads, 63 sectors, 1579 cylinders
Units = cylinders of 2016 * 512 bytes

   Device Boot    Start      End   Blocks   Id  System
/dev/hda1             1      130   131008+  82  Linux swap
/dev/hda2           131      198    68544   83  Linux native

The next step is to create an extended partition that will occupy the remaining disk space. To create an extended partition, type n and press Enter (new), and then choose e for extended:

Command  Action

e  extended
p  primary partition (1-4)
e

Here, fdisk will ask you to specify the extended partition's first cylinder. In this case, the first available cylinder is 199, so choose that:

First cylinder: (199-1579):199

Finally, fdisk will ask you to specify the extended partition's last cylinder. In general, you should go with the very last cylinder. That way, the extended partition occupies the remaining disk space. However, you choose to leave some space at the end of the disk, so specify cylinder 1560:

Last cylinder or +size or +sizeM or +sizek (199-1579): 1560

Here are the results:

Command (m for help): p

Disk /dev/hda: 32 heads, 63 sectors, 1579 cylinders
Units = cylinders of 2016 * 512 bytes

   Device Boot    Start      End   Blocks   Id  System
/dev/hda1             1      130   131008+  82  Linux swap
/dev/hda2           131      198    68544   83  Linux native
/dev/hda3           199     1560  1372896    5  Extended

The table now shows one Linux swap, one Linux native, and one Linux extended partition. Your remaining task is to allocate several logical partitions.

Now that fdisk is aware of an extended partition, the fdisk menu will change. To create your first logical partition (for /home), type n and press Enter. In response, fdisk offers a new menu. Here, choose l for logical:

Command  Action

l   logical (5 or over)
p   primary partition (1-4)
l

Then fdisk will ask you to specify the new logical partition's first cylinder. Note that the first available cylinder is 199, which is the same first available cylinder that you specified for the extended partition. That's because your logical partitions will lie on top of the extended partition. So choose 199:

First cylinder: (199-1579):199

Finally, fdisk will ask you to specify this logical partition's last cylinder. To give /home 370MB, choose 581:

Last cylinder or +size or +sizeM or +sizek (199-1579): 581

Here are the results so far:

Command (m for help): p

Disk /dev/hda: 32 heads, 63 sectors, 1579 cylinders
Units = cylinders of 2016 * 512 bytes

   Device Boot    Start      End   Blocks   Id  System
/dev/hda1             1      130   131008+  82  Linux swap
/dev/hda2           131      198    68544   83  Linux native
/dev/hda3           199     1560  1372896    5  Extended
/dev/hda5           199      581   386032+  83  Linux native

You add the remaining partitions, /usr, /var, and /tmp, in the same fashion. Here's the sequence for /usr:

Command  Action

l   logical (5 or over)
p   primary partition (1-4)
l

First cylinder: (582-1579):582
Last cylinder or +size or +sizeM or +sizek (581-1579): 1302

Here's the sequence for /var:

Command  Action

l   logical (5 or over)
p   primary partition (1-4)
l
First cylinder: (1303-1579):1303
Last cylinder or +size or +sizeM or +sizek (1303-1579): 1433

And finally, the sequence for /tmp:

Command  Action

l   logical (5 or over)
p   primary partition (1-4)
l
First cylinder: (1433-1579):1303
Last cylinder or +size or +sizeM or +sizek (1433-1579): 1560

When you view the final results, fdisk will reflect the following changes:

Command (m for help): p

Disk /dev/hda: 32 heads, 63 sectors, 1579 cylinders
Units = cylinders of 2016 * 512 bytes

   Device Boot    Start      End   Blocks   Id  System
/dev/hda1             1      130   131008+  82  Linux swap
/dev/hda2           131      198    68544   83  Linux native
/dev/hda3           199     1560  1372896    5  Extended
/dev/hda5           199      581   386032+  83  Linux native
/dev/hda6           582     1302   726736+  83  Linux native
/dev/hda7          1303     1433   132016+  83  Linux native
/dev/hda8          1434     1560   127984+  83  Linux native

Once you've achieved and verified your desired results, choose w. This will exit fdisk and permanently commit these changes to disk. Linux will then return you to the main installation program.

Not every Linux installation program directs you to fdisk for partitioning. Instead, you may end up working with cfdisk or Disk Druid. These tools are much easier to use.

cfdisk

cfdisk is a Curses-based partition manipulator for Linux.

Note

Curses is a development package for creating menu-based programs on UNIX terminals. Curses applications vaguely resemble old DOS programs, in that you can navigate menu choices by using arrow keys. Traditional Curses applications have a black background and white foreground. Menu choices appear in white until highlighted with a white bar, at which point the highlighted text turns black. Learn more about Curses programming at http://aotech1.tuwien.ac.at/~dusty/ncurses-intro.html.

cfdisk presents a comfortable and easy-to-navigate interface. Please see Figure 3.9.

Figure 3.9. Partitions viewed in cfdisk's Curses environment.

For the most part, you'll have no trouble navigating cfdisk using arrow keys—the program provides ample help along the way. However, I've provided a summary of important cfdisk keystrokes and their functions in Table 3.2. This is in the event that on your first installation, you're forced to use cfdisk but have little or no accompanying documentation—a common problem.

Table 3.2. Keystroke Commands in cfdisk

Key	Function
?	Get help.
b	Set (or unset) the highlighted partition as bootable.
d	Delete the highlighted partition.
g	Enter an expert mode where you can alter the disk's listed geometry. Warning: Use this function with caution. This is much like specifying your own disk drive settings (heads, cylinders, blocks) in your BIOS. Chances are that cfdisk's auto-detected values are correct. If you specify erroneous values, your Linux system may not boot.
h	Get help.
n	Create a new partition.
p	Obtain and print the current partition table information.
q	Quit cfdisk.
t	Change the file system type (much like tworks in fdisk).
W	Write changes to disk. (You must issue the W command in uppercase.)

Disk Druid

Disk Druid, common to Red Hat installation as a fdisk alternative, is even easier to use. The application is entirely graphical. Please see Figure 3.10.

Figure 3.10. Disk Druid's opening screen.

To add your partitions, highlight the Add button and press Enter. In response, Disk Druid displays a dialog box with all the options you'll ever need. Please see Figure 3.11.

Figure 3.11. Disk Druid's partition editing screen.

Because partitioning has a strong bearing on your system security, you should carefully weigh your options before installation. Making your final decisions will never be easy.

Balancing disk load is probably the most challenging aspect of partitioning, particularly with smaller disks. By creating multiple partitions, you limit each file system's ability to grow. In certain instances, of course, that's exactly what you want. However, it's irritating to later discover that you failed to allocate adequate disk space.

One thing that can help is to know each major file system's purpose. Here they are, in short order:

Disk balancing also gets easier if you develop policies for a consistent application set. For example, perhaps you limit third-party software to Netscape Communicator, StarOffice, and Corel WordPerfect. This eliminates the need for a large /var partition and gives you a ballpark figure on how large /opt has to be.

Of course, there's no law mandating that you create a dozen partitions. The partition parameters in the preceding examples are for demonstration purposes only. You can get along nicely with just three partitions, especially if only a few trusted users have access to your Linux system. Only you can accurately assess how many partitions you'll need and which file systems to segregate.

Here are some closing tips:

Finally, here are some resources for more information on partitioning:

After installation, your /etc/lilo.conf will contain values for boot images, target drives, and the root partition. Here's the /etc/lilo.conffrom the drive partitioned in the preceding example:

#
# general section
#
boot = /dev/hda
install = /boot/boot.b
message = /boot/message
prompt

# wait 20 seconds (200 10ths) for user to select the entry to load
timeout = 200
#
# default entry
#

image = /vmlinuz
    label = linux
    root = /dev/hda2
    read-only

#
# additional entries
#

Let's quickly familiarize you with /etc/lilo.conf and its contents. This way, when you edit it, you'll feel confident that you're making the right changes. Table 3.3 lists some commonly used options for /etc/lilo.conf.

password=123456

You may later decide not to use LILO. After all, it's not the only boot manager out there. Consult your boot loader documentation to see whether it also supports password protection. Every layer counts.

And finally, note that the /etc/lilo.conf password option does not prevent attackers from booting with a floppy. If your BIOS/PROM offers an option to disable floppy diskette boots, use it.