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4. Preparing the Disks

Content:

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 IDE drive in a Linux system, namely /dev/hda. If your system uses SCSI drives, then your first hard drive would be /dev/sda. Serial ATA drives are also /dev/sda even if they are IDE drives.

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.

Partitions and Slices

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 most systems, these are called partitions. Other architectures use a similar technique, called slices.

4.b. Designing a Partitioning Scheme

Default Partitioning Scheme

If you are not interested in drawing up a partitioning scheme for your system, you can use the partitioning scheme we use throughout this book:

Partition Filesystem Size Description
/dev/sda1 Partition map 31.5k Partition map
/dev/sda2 (bootstrap) 800k Apple_Bootstrap
/dev/sda3 (swap) 512M Swap partition
/dev/sda4 ext3 Rest of the disk Root partition

Note: There are some partitions named like this: Apple_Driver43, Apple_Driver_ATA, Apple_FWDriver, Apple_Driver_IOKit, and Apple_Patches. If you are not planning to use MacOS 9 you can delete them, because MacOS X and Linux don't need them. You might have to use parted in order to delete them, as mac-fdisk can't delete them yet.

If you are interested in knowing how big a partition should be, or even how many partitions you need, read on. Otherwise continue now with Apple G5: Using mac-fdisk to Partition your Disk or IBM pSeries: 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. You will definitely want to keep /usr big: not only will it contain the majority of applications, the Portage tree alone takes around 500 Mbyte excluding the various sources that are stored in it.

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. There is also a 15-partition limit for SCSI and SATA.

4.c. Default: Using mac-fdisk (Apple G5) to Partition your Disk

At this point, create your partitions using mac-fdisk:

Code Listing 3.1: Starting mac-fdisk

# mac-fdisk /dev/sda

First delete the partitions you have cleared previously to make room for your Linux partitions. Use d in mac-fdisk to delete those partition(s). It will ask for the partition number to delete.

Second, create an Apple_Bootstrap partition by using b. It will ask for what block you want to start. Enter the number of your first free partition, followed by a p. For instance this is 2p.

Note: This partition is not a "boot" partition. It is not used by Linux at all; you don't have to place any filesystem on it and you should never mount it. PPC users don't need an extra partition for /boot.

Now create a swap partition by pressing c. Again mac-fdisk will ask for what block you want to start this partition from. As we used 2 before to create the Apple_Bootstrap partition, you now have to enter 3p. When you're asked for the size, enter 512M (or whatever size you want). When asked for a name, enter swap (mandatory).

To create the root partition, enter c, followed by 4p to select from what block the root partition should start. When asked for the size, enter 4p again. mac-fdisk will interpret this as "Use all available space". When asked for the name, enter root (mandatory).

To finish up, write the partition to the disk using w and q to quit mac-fdisk.

Note: To make sure everything is ok, you should run mac-fdisk once more and check whether all the partitions are there. If you don't see any of the partitions you created, or the changes you made, you should reinitialize your partitions by pressing i in mac-fdisk. Note that this will recreate the partition map and thus remove all your partitions.

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

4.d. IBM pSeries, iSeries and OpenPower: using fdisk to Partition your Disk

Note: If you are planning to use a RAID disk array for your Gentoo installation and you are using POWER5-based hardware, you should now run iprconfig to format the disks to Advanced Function format and create the disk array. You should emerge iprutils after your install is complete.

If you have an ipr-based SCSI adapter, you should start the ipr utilities now.

Code Listing 4.1: Starting ipr utilities

# /etc/init.d/iprinit start

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

Partition Description
/dev/sda1 PPC PReP Boot partition
/dev/sda2 Swap partition
/dev/sda3 Root partition

Change your partition layout according to your own preference.

Viewing the Current Partition Layout

fdisk is a popular and powerful tool to split your disk into partitions. Fire up fdisk on your disk (in our example, we use /dev/sda):

Code Listing 4.2: Starting fdisk

# fdisk /dev/sda

Once in fdisk, you'll be greeted with a prompt that looks like this:

Code Listing 4.3: fdisk prompt

Command (m for help):

Type p to display your disk's current partition configuration:

Code Listing 4.4: An example partition configuration

Command (m for help): p

Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1          12       53266+  83  Linux
/dev/sda2              13         233      981571+  82  Linux swap
/dev/sda3             234         674     1958701+  83  Linux
/dev/sda4             675        6761    27035410+   5  Extended
/dev/sda5             675        2874     9771268+  83  Linux
/dev/sda6            2875        2919      199836   83  Linux
/dev/sda7            2920        3008      395262   83  Linux
/dev/sda8            3009        6761    16668918   83  Linux

Command (m for help):

This particular disk is configured to house six Linux filesystems (each with a corresponding partition listed as "Linux") as well as a swap partition (listed as "Linux swap").

Removing all Partitions

We will first remove all existing partitions from the disk. Type d to delete a partition. For instance, to delete an existing /dev/sda1:

Note: If you don't want to delete all partitions just delete those you want to delete. At this point the author recommends a backup of your data to avoid the lose of it.

Code Listing 4.5: Deleting a partition

Command (m for help): d
Partition number (1-4): 1

The partition has been scheduled for deletion. It will no longer show up if you type p, but it will not be erased until your changes have been saved. If you made a mistake and want to abort without saving your changes, type q immediately and hit enter and your partition will not be deleted.

Now, assuming that you do indeed want to wipe out all the partitions on your system, repeatedly type p to print out a partition listing and then type d and the number of the partition to delete it. Eventually, you'll end up with a partition table with nothing in it:

Code Listing 4.6: An empty partition table

Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes

Device Boot    Start       End    Blocks   Id  System

Command (m for help):

Now that the in-memory partition table is empty, we're ready to create the partitions. We will use a default partitioning scheme as discussed previously. Of course, don't follow these instructions to the letter if you don't want the same partitioning scheme!

Creating the PPC PReP boot partition

We first create a small PReP boot partition. Type n to create a new partition, then p to select a primary partition, followed by 1 to select the first primary partition. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, type +7M to create a partition 7 Mbyte in size. After you've done this, type t to set the partition type, 1 to select the partition you just created and then type in 41 to set the partition type to "PPC PReP Boot". Finally, you'll need to mark the PReP partition as bootable.

Note: The PReP partition has to be smaller than 8 MByte!

Code Listing 4.7: Creating the PReP boot partition

Command (m for help): p

Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes

   Device Boot      Start         End      Blocks   Id  System

Command (m for help): n
Command action
      e   extended
      p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-6761, default 1): 
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-6761, default
6761): +8M

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): 41
Changed system type of partition 1 to 41 (PPC PReP Boot)

Command (m for help): a
Partition number (1-4): 1
Command (m for help):

Now, when you type p, you should see the following partition information:

Code Listing 4.8: Created boot partition

Command (m for help): p

Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1  *            1           3       13293   41  PPC PReP Boot

Command (m for help):

Creating the Swap Partition

Let's now create the swap partition. To do this, type n to create a new partition, then p to tell fdisk that you want a primary partition. Then type 2 to create the second primary partition, /dev/sda2 in our case. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, type +512M to create a partition 512MB in size. After you've done this, type t to set the partition type, 2 to select the partition you just created and then type in 82 to set the partition type to "Linux Swap". After completing these steps, typing p should display a partition table that looks similar to this:

Code Listing 4.9: Partition listing after creating a swap partition

Command (m for help): p

Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1           3       13293   41  PPC PReP Boot
/dev/sda2               4         117      506331   82  Linux swap

Command (m for help):

Creating the Root Partition

Finally, let's create the root partition. To do this, type n to create a new partition, then p to tell fdisk that you want a primary partition. Then type 3 to create the third primary partition, /dev/sda3 in our case. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, hit enter to create a partition that takes up the rest of the remaining space on your disk. After completing these steps, typing p should display a partition table that looks similar to this:

Code Listing 4.10: Partition listing after creating the root partition

Command (m for help): p

Disk /dev/sda: 30.7 GB, 30750031872 bytes
141 heads, 63 sectors/track, 6761 cylinders
Units = cylinders of 8883 * 512 = 4548096 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1           3       13293   41  PPC PReP Boot
/dev/sda2               4         117      506331   82  Linux swap
/dev/sda3             118        6761    29509326   83  Linux

Command (m for help):

Saving the Partition Layout

To save the partition layout and exit fdisk, type w.

Code Listing 4.11: Save and exit fdisk

Command (m for help): w

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

4.e. Creating Filesystems

Introduction

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...

Filesystems?

Note: Several filesystems are available. ext2, ext3 and ReiserFS support is built in the Installation CD kernels. JFS and XFS support is available through kernel modules.

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. It uses an HTree index that enables high performance in almost all situations. In short, ext3 is a very good and reliable 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. ReiserFS is solid and usable as both general-purpose filesystem and for extreme cases such as the creation of large filesystems, very large files and directories containing tens of thousands of small files.

XFS is a filesystem with metadata journaling that is fully supported under Gentoo Linux's xfs-sources kernel. It 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 a 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.

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/sda4 in our example) in ext3 (as in our example), you would use:

Code Listing 5.1: Applying a filesystem on a partition

# mke2fs -j /dev/sda4

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

Important: If you choose to use ReiserFS for /, do not change its default block size if you will also be using yaboot as your bootloader, as explained in Configuring the Bootloader.

Activating the Swap Partition

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

Code Listing 5.2: Creating a Swap signature

# mkswap /dev/sda3

To activate the swap partition, use swapon:

Code Listing 5.3: Activating the swap partition

# swapon /dev/sda3

Create and activate the swap with the commands mentioned above.

4.f. 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 create a mount point and mount the root partition:

Code Listing 6.1: Mounting partitions

# mkdir /mnt/gentoo
# mount /dev/sda4 /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.

Continue with Installing the Gentoo Installation Files.


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Page updated June 26, 2007

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
Author

Grant Goodyear
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Roy Marples
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Daniel Robbins
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Chris Houser
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Jerry Alexandratos
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Gentoo x86 Developer

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