Introduction to 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.
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
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.
There may be "Disk Driver" partitions on your disk such as
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.
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.
||Apple Partition Map
||Rest of Disk
||ext3, ext4, reiserfs, xfs
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_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.
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.
||Apple Partition Map
||Quik Boot Partition (quik only)
||Rest of Disk
||ext3, ext4, reiserfs, xfs
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.
||affs1 or ext2
||Rest of Disk
||ext3, ext4, reiserfs, xfs
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.
||PReP Boot Partition (Type 0x41)
||Linux Swap (Type 0x82)
||Rest of Disk
||ext3, ext4, reiserfs, xfs
||Linux Root (Type 0x83)
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.
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.
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
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
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.
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,
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
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
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
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
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:
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
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).
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.
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
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