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.

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.

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:

/dev/hda1(Not needed)(bootstrap)800kApple_Bootstrap/dev/hda2/dev/hda1(swap)512MSwap partition/dev/hda3/dev/hda2ext3Rest of the diskRoot partition
Partition NewWorld Partition OldWorld Filesystem Size Description

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 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.

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 most performant 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.

Using mac-fdisk on PPC to Partition your Disk

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

# mac-fdisk /dev/hda

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. If you previously selected 3 as partition number, enter 3p.

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 a boot partition.

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 3 before to create the Apple_Bootstrap partition, you now have to enter 4p. When you're asked for the size, enter 512M (or whatever size you want -- 512 is recommended though). When asked for a name, enter swap (mandatory).

To create the root partition, enter c, followed by 5p to select from what block the root partition should start. When asked for the size, enter 5p again. mac-fdisk will interprete 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.

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

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?

Several filesystems are available. Ext2 and ext3 are found stable on the PPC architecture, reiserfs and xfs are in experimental stage. jfs is unsupported.

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 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. 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 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 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:

ext2mke2fsext3mke2fs -jreiserfsmkreiserfsxfsmkfs.xfsjfsmkfs.jfs
Filesystem Creation Command

For instance, to have the root partition (/dev/hda3 in our example) in ext3 (as in our example), you would use: 

# mke2fs -j /dev/hda3

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

Activating the Swap Partition

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

# mkswap /dev/hda2

To activate the swap partition, use swapon: 

# swapon /dev/hda2

Create and activate the swap now.

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 mount the root and boot partition: 

# mount /dev/hda3 /mnt/gentoo
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 also need to mount the proc filesystem (a virtual interface with the kernel) on /proc. We first create the /mnt/gentoo/proc mountpoint and then mount the filesystem: 

# mkdir /mnt/gentoo/proc
# mount -t proc none /mnt/gentoo/proc

Now continue with Installing the Gentoo Installation Files.