1.20 2004-11-24
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/dev/hda1(Not applicable)(Partition Map)32kApple_partition_map/dev/hda2(Not needed)(Not applicable)(bootstrap)800kApple_Bootstrap/dev/hda3/dev/hda2/dev/hda1(swap)512MSwap partition/dev/hda4/dev/hda3/dev/hda2ext3Rest of the diskRoot partition
Partition NewWorld Partition OldWorld Partition Pegasos Filesystem Size Description
There are some partitions named like this: Apple_Driver43, Apple_Driver_ATA, Apple_FWDriver, Apple_Driver_IOKit, Apple_Patches. If you are not planning to use MacOS 9 you can delete them, because MacOS X and Linux don't need them. You might have to use parted in order to delete them, as mac-fdisk can't delete them yet.

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/IBM) to Partition your Disk or Alternative: Using parted (especially 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 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.

Default: Using mac-fdisk (Apple/IBM) 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. Usually the first partition on NewWorld machines (Apple_partition_map) could not be deleted.

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. PPC 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 -- 512MB is recommended though). When asked for a name, enter swap (mandatory).

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 (mandatory).

To finish up, write the partition to the disk using w and q to quit mac-fdisk.

To make sure everything is ok, you should run mac-fdisk once more 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.

Using parted (especially Pegasos) to Partition your Disk

parted, the Partition Editor, can now handle HFS+ partitions used by Mac OS and Mac OS X. With this tool you can shrink 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: 

# parted /dev/hda

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. Your changes aren't saved until you quit the application; 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 named "BI0" (BI zero) at the start of the drive. 50MB should be more than enough to store the MorphOS kernel. If you have a Pegasos I or intend to use reiserfs or xfs, you will also have to store your Linux kernel on this partition (the Pegasos II can boot from ext2/ext3 drives). To create the partition run mkpart primary affs1 START END where START and END should be replaced with the megabyte range (e.g. 5 55 creates a 50 MB partition starting at 5MB and ending at 55MB.

You need to create two partitions for Linux, one root filesystem for all your program files etc, and one swap partition. To create the root filesystem you must first decide which filesystem to use. Possible options are ext2, ext3, reiserfs and xfs. Unless you know what you are doing, use ext3. Run mkpart primary ext3 START END to create an ext3 partition. Again, replace START and END with the megabyte start and stop marks for the partition.

It is generally recommended that you create a swap partition the same size as the amount of RAM in your computer times two. You will probably get away with a smaller swap partition unless you intend to run a lot of applications at the same time (although at least 512MB is recommended). To create the swap partition, run mkpart primary linux-swap START END.

Write down the partition minor numbers as they are required during the installation process. To display the minor numbers run print. Your drives are accessed as /dev/hdaX where X is replaced with the minor number of the partition.

When you are done in parted simply run quit.

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, ext3, ReiserFS and XFS are found stable on the PPC architecture. 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 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.

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.xfs
Filesystem Creation Command

For instance, to have the root partition (/dev/hda4 in our example) in ext3 (as in our example), you would use: 

# mke2fs -j /dev/hda4

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

On OldWorld machines and the PegasosII your partition which holds the kernel must be ext2 or ext3. NewWorld machines can boot from any of ext2, ext3, XFS, ReiserFS or even HFS/HFS+ filesystems. Activating the Swap Partition

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

# mkswap /dev/hda3

To activate the swap partition, use swapon: 

# swapon /dev/hda3

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 create a mount-point and mount the root partition: 

# mkdir /mnt/gentoo
# mount /dev/hda4 /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 will 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 Installation Files.