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4. Preparing the Disks
4.a. 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 SCSI HD in a Linux system, namely
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. These are called
4.b. Designing a Partitioning Scheme
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. There is also a 15-partition
limit for SCSI and SATA.
4.c. Using fdisk on MIPS to Partition your Disk
Creating an SGI Disk Label
All disks in an SGI System require an SGI Disk Label, which serves a
similar function as Sun & MS-DOS disklabels -- It stores information about
the disk partitions. Creating a new SGI Disk Label will create two special
partitions on the disk:
SGI Volume Header (9th partition): This partition is important. It
is where the kernel images will go. To store kernel images, you will utilize
the tool known as dvhtool to copy kernel images to this partition.
You will then be able to boot kernels from this partition via the SGI PROM
SGI Volume (11th partition): This partition is similar in purpose to
the Sun Disklabel's third partition of "Whole Disk". This partition spans
the entire disk, and should be left untouched. It serves no special purpose
other than to assist the PROM in some undocumented fashion (or it is used by
IRIX in some way).
The SGI Volume Header must begin at cylinder 0. Failure to do so means
you won't be able to boot from the disk.
The following is an example excerpt from an fdisk session. Read and
tailor it to your needs...
Code Listing 3.1: Creating an SGI Disklabel
# fdisk /dev/sda
Command (m for help): x
Expert command (m for help): m
b move beginning of data in a partition
c change number of cylinders
d print the raw data in the partition table
e list extended partitions
f fix partition order
g create an IRIX (SGI) partition table
h change number of heads
m print this menu
p print the partition table
q quit without saving changes
r return to main menu
s change number of sectors/track
v verify the partition table
w write table to disk and exit
Expert command (m for help): g
Building a new SGI disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content will be unrecoverably lost.
Expert command (m for help): r
Command (m for help): p
Disk /dev/sda (SGI disk label): 64 heads, 32 sectors, 17482 cylinders
Units = cylinders of 2048 * 512 bytes
----- partitions -----
Pt# Device Info Start End Sectors Id System
9: /dev/sda1 0 4 10240 0 SGI volhdr
11: /dev/sda2 0 17481 35803136 6 SGI volume
----- Bootinfo -----
----- Directory Entries -----
Command (m for help):
If your disk already has an existing SGI Disklabel, then fdisk will not allow
the creation of a new label. There are two ways around this. One is to create a
Sun or MS-DOS disklabel, write the changes to disk, and restart fdisk. The
second is to overwrite the partition table with null data via the following
command: dd if=/dev/zero of=/dev/sda bs=512 count=1.
Getting the SGI Volume Header to just the right size
Now that an SGI Disklabel is created, partitions may now be defined. In the
above example, there are already two partitions defined for you. These are the
special partitions mentioned above and should not normally be altered. However,
for installing Gentoo, we'll need to load multiple kernel images directly into
the volume header, as there is no supported SGI Bootloader available in Portage
yet. The volume header itself can hold up to eight images of any size,
with each image allowed eight-character names.
The process of making the volume header larger isn't exactly straight-forward --
there's a bit of a trick to it. One cannot simply delete and re-add the volume
header due to odd fdisk behavior. In the example provided below, we'll create a
50MB Volume header in conjunction with a 50MB /boot partition. The actual layout
of your disk may vary, but this is for illustrative purposes only.
Code Listing 3.2: Resizing the SGI Volume Header correctly
Command (m for help): n
Partition number (1-16): 1
First cylinder (5-8682, default 5): 51
Last cylinder (51-8682, default 8682): 101
Command (m for help): d
Partition number (1-16): 9
Command (m for help): n
Partition number (1-16): 9
First cylinder (0-50, default 0): 0
Last cylinder (0-50, default 50): 50
Final partition layout
Once this is done, you are safe to create the rest of your partitions as you see
fit. After all your partitions are laid out, make sure you set the partition ID
of your swap partition to 82, which is Linux Swap. By default, it will be
83, Linux Native.
Now that your partitions 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. Ext2 and ext3 are found stable on the
MIPS architectures, others are experimental.
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
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:
For instance, to have the boot partition (/dev/sda1 in our
example) in ext2 and the root partition (/dev/sda3 in our example)
in ext3, you would use:
Code Listing 4.1: Applying a filesystem on a partition
# mke2fs /dev/sda1
# mke2fs -j /dev/sda3
Now create the filesystems on your newly created partitions (or logical
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/sda2
To activate the swap partition, use swapon:
Code Listing 4.3: Activating the swap partition
# swapon /dev/sda2
Create and activate the swap now.
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/sda3 /mnt/gentoo
# mkdir /mnt/gentoo/boot
# mount /dev/sda1 /mnt/gentoo/boot
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
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