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1.  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 drive in a Linux system, namely /dev/sda. SCSI and Serial ATA drives are both labeled /dev/sd*; even IDE drives are labeled /dev/sd* with the new libata framework in the kernel. If you're using the old device framework, then your first IDE drive is /dev/hda.

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 most systems, these are called partitions.

1.  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. Choose the filesystem layout that best matches the type of PowerPC system you are installing on.

Apple New World

Apple New World machines are fairly straightforward to configure. The first partition is always an Apple Partition Map. This partition keeps track of the layout of the disk. You cannot remove this partition. The next partition should always be a bootstrap partition. This partition contains a small (800k) HFS filesystem that holds a copy of the bootloader Yaboot and its configuration file. This partition is not the same as a /boot partition as found on other architectures. After the boot partition, the usual Linux filesystems are placed, according to the scheme below. The swap partition is a temporary storage place for when your system runs out of physical memory. The root partition will contain the filesystem that Gentoo is installed on. If you wish to dual boot, the OSX partition can go anywhere after the bootstrap partition to insure that yaboot starts first.

Note: There may be "Disk Driver" partitions on your disk such as Apple_Driver63, Apple_Driver_ATA, Apple_FWDriver, Apple_Driver_IOKit, and Apple_Patches. These are used to boot MacOS, so if you have no need for this, you can remove them by initializing the disk with mac-fdisk's i option. This will completely erase the disk! If you are in doubt, just let them be.

Note: If you partitioned this disk with Apple's Disk Utility, there may be 128Mb spaces between partitions which Apple reserves for "future use". You can safely remove these.

Partition Size Filesystem Description
/dev/sda1 32k None Apple Partition Map
/dev/sda2 800k HFS Apple Bootstrap
/dev/sda3 512Mb Swap Linux Swap
/dev/sda4 Rest of Disk ext3, ext4, reiserfs, xfs Linux Root

Apple Old World

Apple Old World machines are a bit more complicated to configure. The first partition is always an Apple Partition Map. This partition keeps track of the layout of the disk. You cannot remove this partition. If you are using BootX, the configuration below assumes that MacOS is installed on a seperate disk. If this is not the case, there will be additional partitions for "Apple Disk Drivers" such as Apple_Driver63, Apple_Driver_ATA, Apple_FWDriver, Apple_Driver_IOKit, Apple_Patches and the MacOS install. If you are using Quik, you will need to create a boot partition to hold the kernel, unlike other Apple boot methods. After the boot partition, the usual Linux filesystems are placed, according to the scheme below. The swap partition is a temporary storage place for when your system runs out of physical memory. The root partition will contain the filesystem that Gentoo is installed on.

Note: If you are using an OldWorld machine, you will need to keep MacOS available. The layout here assumes MacOS is installed on a separate drive.

Partition Size Filesystem Description
/dev/sda1 32k None Apple Partition Map
/dev/sda2 32Mb ext2 Quik Boot Partition (quik only)
/dev/sda3 512Mb Swap Linux Swap
/dev/sda4 Rest of Disk ext3, ext4, reiserfs, xfs Linux Root


The Pegasos partition layout is quite simple compared to the Apple layouts. The first partition is a Boot Partition, which contains kernels to be booted, along with an Open Firmware script that presents a menu on boot. After the boot partition, the usual Linux filesystems are placed, according to the scheme below. The swap partition is a temporary storage place for when your system runs out of physical memory. The root partition will contain the filesystem that Gentoo is installed on.

Partition Size Filesystem Description
/dev/sda1 32Mb affs1 or ext2 Boot Partition
/dev/sda2 512Mb Swap Linux Swap
/dev/sda3 Rest of Disk ext3, ext4, reiserfs, xfs Linux Root

IBM PReP (RS/6000)

The IBM PowerPC Reference Platform (PReP) requires a small PReP boot partition on the disk's first partition, followed by the swap and root partitions.

Partition Size Filesystem Description
/dev/sda1 800k None PReP Boot Partition (Type 0x41)
/dev/sda2 512Mb Swap Linux Swap (Type 0x82)
/dev/sda3 Rest of Disk ext3, ext4, reiserfs, xfs Linux Root (Type 0x83)

Warning: parted is able to resize partitions including HFS+. Unfortunately there may be issues with resizing HFS+ journaled filesystems, so, for the best results, switch off journaling in Mac OS X before resizing. Remember that any resizing operation is dangerous, so attempt at your own risk! Be sure to always have a backup of your data before resizing!

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 Default: Using mac-fdisk (Apple) to Partition your Disk or Alternative: Using parted (IBM/Pegasos) 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 received mail is stored in /var. A good choice of filesystem will then maximise your performance. Game servers should have a separate /opt as most game servers are installed there. The reason is similar for /home: security and backups. Whatever layout you chose, you will definitely want to keep /usr large: not only will it contain the majority of applications, the Portage tree alone takes more than 500Mb 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 disadvantages as well. If not configured properly, you will have a system with lots of free space on one partition and none on another. Another nuisance is that separate partitions - especially for important mountpoints like /usr or /var - often require the administrator to boot with an initramfs to mount the partition before other boot scripts start. This isn't always the case though, so your results may vary.

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

1.  Default: Using mac-fdisk (Apple) Partition your Disk

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

Code Listing 1.1: Starting mac-fdisk

# mac-fdisk /dev/sda

If you used Apple's Disk Utility to leave space for Linux, first delete the partitions you have created previously to make room for your new install. Use d in mac-fdisk to delete those partition(s). It will ask for the partition number to delete. Usually the first partition on NewWorld machines (Apple_partition_map) cannot be deleted. If you would like to start with a clean disk, you can simply initialize the disk by pressing i. This will completely erase the disk, so use this with caution.

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. Apple 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 -- a minimum of 512MB is recommended, but 2 times your physical memory is the generally accepted size). When asked for a name, enter swap.

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.

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 -l 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 continue with Creating Filesystems.

1.  Using parted to Partition your Disk (Pegasos and RS/6000)

parted, the Partition Editor, can now handle HFS+ partitions used by Mac OS and Mac OS X. With this tool you can resize your Mac partitions and create space for your Linux partitions. Nevertheless, the example below describes partitioning for Pegasos machines only.

To begin let's fire up parted:

Code Listing 1.1: Starting parted

# parted /dev/sda

If the drive is unpartitioned, run mklabel amiga to create a new disklabel for the drive.

You can type print at any time in parted to display the current partition table. If at any time you change your mind or made a mistake you can press Ctrl-c to abort parted.

If you intend to also install MorphOS on your Pegasos create an affs1 filesystem at the start of the drive. 32MB should be more than enough to store the MorphOS kernel. If you have a Pegasos I or intend to use any filesystem besides ext2 or ext3, you will also have to store your Linux kernel on this partition (the Pegasos II can only boot from ext2/ext3 or affs1 partitions). To create the partition run mkpart primary affs1 START END where START and END should be replaced with the megabyte range (e.g. 0 32) which creates a 32 MB partition starting at 0MB and ending at 32MB. If you chose to create an ext2 or ext3 partition instead, substitute ext2 or ext3 for affs1 in the mkpart command.

You will need to create two partitions for Linux, one root filesystem and one swap partition. Run mkpart primary START END to create each partition, replacing START and END with the desired megabyte boundries.

It is generally recommended that you create a swap partition that is two times bigger than the amount of RAM in your computer, but at least 512Mb is recommended. To create the swap partition, run mkpart primary linux-swap START END with START and END again denoting the partition boundries.

When you are done in parted simply type quit.

1.  Creating Filesystems


Now that your partitions are created, it is time to place a filesystem on them. If you're not sure which filesystems to choose and are happy with our defaults, continue with Applying a Filesystem to a Partition. Otherwise, read on to learn about the available filesystems.


The Linux kernel supports various filesystems. We'll explain ext2, ext3, ext4, ReiserFS, XFS and JFS as these are the most commonly used filesystems on Linux systems.

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.

ext4 is a filesystem created as a fork of ext3 bringing new features, performance improvements and removal of size limits with moderate changes to the on-disk format. It can span volumes up to 1 EB and with maximum file size of 16 TB. Instead of the classic ext2/3 bitmap block allocation ext4 uses extents, which improve large file performance and reduce fragmentation. Ext4 also provides more sophisticated block allocation algorithms (delayed allocation and multiblock allocation) giving the filesystem driver more ways to optimise the layout of data on the disk. The ext4 filesystem is a compromise between production-grade code stability and the desire to introduce extensions to an almost decade old filesystem. Ext4 is the recommended all-purpose all-platform filesystem.

If you intend to install Gentoo on a small partition (less than 8GB), then you'll need to tell ext2, ext3 or ext4 (if available) to reserve enough inodes when you create the filesystem. The mke2fs application uses the "bytes-per-inode" setting to calculate how many inodes a file system should have. By running mke2fs -T small /dev/<device> (ext2) or mke2fs -j -T small /dev/<device> (ext3/ext4) the number of inodes will generally quadruple for a given file system as its "bytes-per-inode" reduces from one every 16kB to one every 4kB. You can tune this even further by using mke2fs -i <ratio> /dev/<device> (ext2) or mke2fs -j -i <ratio> /dev/<device> (ext3/ext4).

JFS is IBM's high-performance journaling filesystem. JFS is a light, fast and reliable B+tree-based filesystem with good performance in various conditions.

ReiserFS is a B+tree-based journaled filesystem that has good overall performance, especially when dealing with many tiny files at the cost of more CPU cycles. ReiserFS appears to be less maintained than other filesystems.

XFS is a filesystem with metadata journaling which comes with a robust feature-set and is optimized for scalability. XFS seems to be less forgiving to various hardware problems.

Activating the Swap Partition

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

Code Listing 1.1: Creating a swap signature

# mkswap /dev/sda3

To activate the swap partition, use swapon:

Code Listing 1.1: Activating the swap partition

# swapon /dev/sda3

Create and activate the swap now before creating other filesystems.

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
ext4 mkfs.ext4
reiserfs mkreiserfs
xfs mkfs.xfs

For instance, to make an ext4 filesystem on the root partition (/dev/sda4 in our example), you would use:

Code Listing 1.1: Applying a filesystem on a partition

# mkfs.ext4 /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).

Note: On the PegasosII your partition which holds the kernel must be ext2, ext3 or affs1. NewWorld machines can boot from any of ext2, ext3, XFS, ReiserFS or even HFS/HFS+ filesystems. On OldWorld machines booting with BootX, the kernel must be placed on an HFS partition, but this will be completed when you configure your bootloader.

1.  Mounting

Now that your partitions are initialized and are housing a filesystem, it is time to mount those partitions. Use the mount command. As an example we mount the root partition:

Code Listing 1.1: Mounting partitions

# 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 and unpacking with chmod 1777 /mnt/gentoo/tmp. This is also true for /var/tmp.

Continue with (Installing the Gentoo Installation Files).

Page updated February 23, 2013

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

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