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4. Preparing the Disks


4.a. Introduction to Block Devices

Block Devices

We'll take a good look at disk-oriented aspects of Gentoo Linux and Linux in general, including Linux filesystems, partitions and block devices. Then, once you're familiar with the ins and outs of disks and filesystems, you'll be guided through the process of setting up partitions and filesystems for your Gentoo Linux installation.

To begin, we'll introduce block devices. The most famous block device is probably the one that represents the first SCSI HD in a Linux system, namely /dev/sda.

The block devices above represent an abstract interface to the disk. User programs can use these block devices to interact with your disk without worrying about whether your drives are IDE, SCSI or something else. The program can simply address the storage on the disk as a bunch of contiguous, randomly-accessible 512-byte blocks.


Although it is theoretically possible to use a full disk to house your Linux system, this is almost never done in practice. Instead, full disk block devices are split up in smaller, more manageable block devices. On Alpha systems, these are called slices.

4.b. Designing a Partitioning Scheme

Default Partitioning Scheme

As an example we use the following slice layout:

Slice Description
/dev/sda1 Swap slice
/dev/sda2 Root slice
/dev/sda3 Full disk (required)

If you are interested in knowing how big a partition should be, or even how many partitions (or volumes) you need, read on. Otherwise continue now with Using fdisk to Partition your Disk.

How Many and How Big?

The number of partitions is highly dependent on your environment. For instance, if you have lots of users, you will most likely want to have your /home separate as it increases security and makes backups easier. If you are installing Gentoo to perform as a mailserver, your /var should be separate as all mails are stored inside /var. A good choice of filesystem will then maximise your performance. Gameservers will have a separate /opt as most gaming servers are installed there. The reason is similar for /home: security and backups.

As you can see, it very much depends on what you want to achieve. Separate partitions or volumes have the following advantages:

  • You can choose the best performing filesystem for each partition or volume
  • Your entire system cannot run out of free space if one defunct tool is continuously writing files to a partition or volume
  • If necessary, file system checks are reduced in time, as multiple checks can be done in parallel (although this advantage is more with multiple disks than it is with multiple partitions)
  • Security can be enhanced by mounting some partitions or volumes read-only, nosuid (setuid bits are ignored), noexec (executable bits are ignored) etc.

However, multiple partitions have one big disadvantage: if not configured properly, you might result in having a system with lots of free space on one partition and none on another.

4.c. Using fdisk on Alpha to Partition your Disk

The following parts explain how to create the example slice layout described previously, namely:

Slice Description
/dev/sda1 Swap slice
/dev/sda2 Root slice
/dev/sda3 Full disk (required)

Change your slice layout according to your own preference.

Identifying Available Disks

To figure out what disks you have running, use the following commands:

Code Listing 3.1: Identifying available disks

# dmesg | grep 'drive$'        (For IDE disks)
# dmesg | grep 'scsi'          (For SCSI disks)

From this output you should be able to see what disks were detected and their respective /dev entry. In the following parts we assume that the disk is a SCSI disk on /dev/sda.

Now fire up fdisk:

Code Listing 3.2: Starting fdisk

# fdisk /dev/sda

Deleting All Slices

If your hard drive is completely blank, then you'll have to first create a BSD disklabel.

Code Listing 3.3: Creating BSD disklabel

Command (m for help): b
/dev/sda contains no disklabel.
Do you want to create a disklabel? (y/n) y
A bunch of drive-specific info will show here
3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  c:        1      5290*     5289*    unused        0     0

We start with deleting all slices except the 'c'-slice. The following shows how to delete a slice (in the example we use 'a'). Repeat the process to delete all other slices (again, except the 'c'-slice).

Use p to view all existing slices. d is used to delete a slice.

Code Listing 3.4: Deleting a slice

BSD disklabel command (m for help): p

8 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  a:        1       235*      234*    4.2BSD     1024  8192    16
  b:      235*      469*      234*      swap
  c:        1      5290*     5289*    unused        0     0
  d:      469*     2076*     1607*    unused        0     0
  e:     2076*     3683*     1607*    unused        0     0
  f:     3683*     5290*     1607*    unused        0     0
  g:      469*     1749*     1280     4.2BSD     1024  8192    16
  h:     1749*     5290*     3541*    unused        0     0

BSD disklabel command (m for help): d
Partition (a-h): a

After repeating this process for all slices, a listing should show you something similar to this:

Code Listing 3.5: Viewing an empty scheme

BSD disklabel command (m for help): p

3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  c:        1      5290*     5289*    unused        0     0

Creating the Swap Slice

On Alpha based systems you don't need a separate boot partition. However, the first cylinder cannot be used as the aboot image will be placed there.

We will create a swap slice starting at the third cylinder, with a total size of 1 Gbyte. Use n to create a new slice. After creating the slice, we will change its type to 1 (one), meaning swap.

Code Listing 3.6: Creating the swap slice

BSD disklabel command (m for help): n
Partition (a-p): a
First cylinder (1-5290, default 1): 3
Last cylinder or +size or +sizeM or +sizeK (3-5290, default 5290): +1024M

BSD disklabel command (m for help): t
Partition (a-c): a
Hex code (type L to list codes): 1

After these steps you should see a layout similar to the following:

Code Listing 3.7: Slice layout after creating the swap slice

BSD disklabel command (m for help): p

3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  a:        3      1003      1001       swap
  c:        1      5290*     5289*    unused        0     0

Create the Root Slice

We will now create the root slice, starting from the first cylinder after the swap slice. Use the p command to view where the swap slice ends. In our example, this is at 1003, making the root partition start at 1004.

Another problem is that there is currently a bug in fdisk making it think the number of available cylinders is one above the real number of cylinders. In other words, when you are asked for the last cylinder, decrease the cylinder number (in this example: 5290) with one.

When the partition is created, we change the type to 8, for ext2.

Code Listing 3.8: Creating the root slice

D disklabel command (m for help): n
Partition (a-p): b
First cylinder (1-5290, default 1): 1004
Last cylinder or +size or +sizeM or +sizeK (1004-5290, default 5290): 5289

BSD disklabel command (m for help): t
Partition (a-c): b
Hex code (type L to list codes): 8

Your slice layout should now be similar to this:

Code Listing 3.9: Viewing the slice layout

BSD disklabel command (m for help): p

3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  a:        3      1003      1001       swap
  b:     1004      5289      4286       ext2
  c:        1      5290*     5289*    unused        0     0

Save the Slice Layout and Exit

Save fdisk by typing w. This will also save your slice layout.

Code Listing 3.10: Save and exit fdisk

Command (m for help): w

Now that your slices are created, you can now continue with Creating Filesystems.

4.d. Creating Filesystems


Now that your partitions are created, it is time to place a filesystem on them. If you don't care about what filesystem to choose and are happy with what we use as default in this handbook, continue with Applying a Filesystem to a Partition. Otherwise read on to learn about the available filesystems...


Several filesystems are available. Most of them are found stable on the Alpha architecture.

ext2 is the tried and true Linux filesystem but doesn't have metadata journaling, which means that routine ext2 filesystem checks at startup time can be quite time-consuming. There is now quite a selection of newer-generation journaled filesystems that can be checked for consistency very quickly and are thus generally preferred over their non-journaled counterparts. Journaled filesystems prevent long delays when you boot your system and your filesystem happens to be in an inconsistent state.

ext3 is the journaled version of the ext2 filesystem, providing metadata journaling for fast recovery in addition to other enhanced journaling modes like full data and ordered data journaling. ext3 is a very good and reliable filesystem. It has an additional hashed b-tree indexing option that enables high performance in almost all situations. In short, ext3 is an excellent filesystem.

ReiserFS is a B*-tree based filesystem that has very good overall performance and greatly outperforms both ext2 and ext3 when dealing with small files (files less than 4k), often by a factor of 10x-15x. ReiserFS also scales extremely well and has metadata journaling. As of kernel 2.4.18+, ReiserFS is solid and usable as both general-purpose filesystem and for extreme cases such as the creation of large filesystems, the use of many small files, very large files and directories containing tens of thousands of files.

XFS is a filesystem with metadata journaling which comes with a robust feature-set and is optimized for scalability. We only recommend using this filesystem on Linux systems with high-end SCSI and/or fibre channel storage and an uninterruptible power supply. Because XFS aggressively caches in-transit data in RAM, improperly designed programs (those that don't take proper precautions when writing files to disk and there are quite a few of them) can lose a good deal of data if the system goes down unexpectedly.

JFS is IBM's high-performance journaling filesystem. It has recently become production-ready and there hasn't been a sufficient track record to comment positively nor negatively on its general stability at this point.

Applying a Filesystem to a Partition

To create a filesystem on a partition or volume, there are tools available for each possible filesystem:

Filesystem Creation Command
ext2 mke2fs
ext3 mke2fs -j
reiserfs mkreiserfs
xfs mkfs.xfs
jfs mkfs.jfs

For instance, to have the root partition (/dev/sda2 in our example) in ext3, you would use:

Code Listing 4.1: Applying a filesystem on a partition

# mke2fs -j /dev/sda2

Now create the filesystems on your newly created partitions (or logical volumes).

Activating the Swap Partition

mkswap is the command that is used to initialize swap partitions:

Code Listing 4.2: Creating a Swap signature

# mkswap /dev/sda1

To activate the swap partition, use swapon:

Code Listing 4.3: Activating the swap partition

# swapon /dev/sda1

Create and activate the swap now.

4.e. Mounting

Now that your partitions are initialized and are housing a filesystem, it is time to mount those partitions. Use the mount command. Don't forget to create the necessary mount directories for every partition you created. As an example we mount the root and boot partition:

Code Listing 5.1: Mounting partitions

# mount /dev/sda2 /mnt/gentoo

Note: If you want your /tmp to reside on a separate partition, be sure to change its permissions after mounting: chmod 1777 /mnt/gentoo/tmp. This also holds for /var/tmp.

We will also have to mount the proc filesystem (a virtual interface with the kernel) on /proc. But first we will need to place our files on the partitions.

Continue with Installing the Gentoo Installation Files.

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Page updated November 09, 2004

Summary: To be able to install Gentoo, you must create the necessary partitions. This chapter describes how to partition a disk for future usage.

Sven Vermeulen

Daniel Robbins

Chris Houser

Jerry Alexandratos

Seemant Kulleen
Gentoo x86 Developer

Tavis Ormandy
Gentoo Alpha Developer

Jason Huebel
Gentoo AMD64 Developer

Guy Martin
Gentoo HPPA developer

Pieter Van den Abeele
Gentoo PPC developer

Joe Kallar
Gentoo SPARC developer

John P. Davis

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Jungmin Seo

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Jon Portnoy

Zack Gilburd

Jack Morgan

Benny Chuang


Joshua Kinard

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Gerald J. Normandin Jr.

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Lars Weiler

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