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.
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,
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. 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 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.
Using fdisk on HPPA to Partition your Disk
Use fdisk to create the partitions you want:
Code Listing 1.1: Partitioning the disk
# fdisk /dev/sda
HPPA machines use the PC standard DOS partition tables. To create a new
DOS partition table, simply use the o command.
Code Listing 1.1: Creating a DOS partition table
# fdisk /dev/sda
Command (m for help): o
Building a new DOS disklabel.
PALO (the HPPA bootloader) needs a special partition to work. You have
to create a partition of at least 16MB at the beginning of your disk.
The partition type must be of type f0 (Linux/PA-RISC boot).
If you ignore this and continue without a special PALO partition, your system
will stop loving you and fail to start. Also, if your disk is larger than 2GB,
make sure that the boot partition is in the first 2GB of your disk. PALO is
unable to read a kernel after the 2GB limit.
Code Listing 1.1: A simple default partition scheme
# cat /etc/fstab
/dev/sda2 /boot ext2 noauto,noatime 1 1
/dev/sda3 none swap sw 0 0
/dev/sda4 / ext4 noatime 0 0
# fdisk /dev/sda
Command (m for help): p
Disk /dev/sda: 4294 MB, 4294816768 bytes
133 heads, 62 sectors/track, 1017 cylinders
Units = cylinders of 8246 * 512 = 4221952 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 8 32953 f0 Linux/PA-RISC boot
/dev/sda2 9 20 49476 83 Linux
/dev/sda3 21 70 206150 82 Linux swap
/dev/sda4 71 1017 3904481 83 Linux
Now that your partitions are created, you can continue with 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...
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.
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/sda2 in our
example) in ext2 and the root partition (/dev/sda4 in our example)
in ext4 (as in our example), you would use:
Code Listing 1.1: Applying a filesystem on a partition
# mkfs.ext2 /dev/sda2
# mkfs.ext4 /dev/sda4
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 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 with the commands mentioned above.
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 1.1: Mounting partitions
# mount /dev/sda4 /mnt/gentoo
# mkdir /mnt/gentoo/boot
# mount /dev/sda2 /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
Continue with (Installing the Gentoo