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4. Preparing the Disks

• Introduction to Block Devices
• Designing a Partitioning Scheme
• Using fdisk to Partition your Disk
• Creating Filesystems
• Mounting

4.a. 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 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

Although it is theoretically possible to use the entire 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 known as partitions.

The first partition on the first SCSI disk is /dev/sda1, the second /dev/sda2 and so on.

The third partition on Sun systems is set aside as a special "whole disk" slice. This partition must not contain a file system.

Users who are used to the DOS partitioning scheme should note that Sun disklabels do not have "primary" and "extended" partitions. Instead, up to eight partitions are available per drive, with the third of these being reserved.

4.b. Designing a Partitioning Scheme

Default Partitioning Scheme

If you are not interested in drawing up a partitioning scheme, the table below suggests a suitable starting point for most systems.

Note that a separate /boot partition is generally not recommended on SPARC, as it complicates the bootloader configuration.

4.c. Using fdisk to Partition your Disk

The following parts explain how to create the example partition layout described previously, namely:

Change the partition layout as required. Remember to keep the root partition entirely within the first 2 GBytes of the disk for older systems. There is also a 15-partition limit for SCSI and SATA.

Firing up fdisk

Start fdisk with your disk as argument:

# fdisk /dev/sda

You should be greeted with the fdisk prompt:

Command (m for help):

To view the available partitions, type in p:

Command (m for help): p

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes

   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda2           488       976    499712   82  Linux swap
/dev/sda3             0      8635   8842240    5  Whole disk
/dev/sda4           976      1953   1000448   83  Linux native
/dev/sda5          1953      2144    195584   83  Linux native
/dev/sda6          2144      8635   6646784   83  Linux native

Note the Sun disk label in the output. If this is missing, the disk is using the DOS-partitioning, not the Sun partitioning. In this case, use s to ensure that the disk has a sun partition table:

Command (m for help): s
Building a new sun disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.

Drive type
   ?   auto configure
   0   custom (with hardware detected defaults)
   a   Quantum ProDrive 80S
   b   Quantum ProDrive 105S
   c   CDC Wren IV 94171-344
   d   IBM DPES-31080
   e   IBM DORS-32160
   f   IBM DNES-318350
   g   SEAGATE ST34371
   h   SUN0104
   i   SUN0207
   j   SUN0327
   k   SUN0340
   l   SUN0424
   m   SUN0535
   n   SUN0669
   o   SUN1.0G
   p   SUN1.05
   q   SUN1.3G
   r   SUN2.1G
   s   IOMEGA Jaz
Select type (? for auto, 0 for custom): 0
Heads (1-1024, default 64): 
Using default value 64
Sectors/track (1-1024, default 32): 
Using default value 32
Cylinders (1-65535, default 8635): 
Using default value 8635
Alternate cylinders (0-65535, default 2): 
Using default value 2
Physical cylinders (0-65535, default 8637): 
Using default value 8637
Rotation speed (rpm) (1-100000, default 5400): 10000
Interleave factor (1-32, default 1): 
Using default value 1
Extra sectors per cylinder (0-32, default 0): 
Using default value 0

You can find the correct values in your disk's documentation. The 'auto configure' option does not usually work.

Deleting Existing Partitions

It's time to delete any existing partitions. To do this, type d and hit Enter. You will then be prompted for the partition number you would like to delete. To delete a pre-existing /dev/sda1, you would type:

Command (m for help): d
Partition number (1-4): 1

You should not delete partition 3 (whole disk). This is required. If this partition does not exist, follow the "Creating a Sun Disklabel" instructions above.

After deleting all partitions except the Whole disk slice, you should have a partition layout similar to the following:

Command (m for help): p

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes

   Device Flag    Start       End    Blocks   Id  System
/dev/sda3             0      8635   8842240    5  Whole disk

Creating the Root Partition

We're ready to create the root partition. To do this, type n to create a new partition, then type 1 to create the partition. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, type +512M to create a partition 512MBytes in size. Make sure that the entire root partition fits within the first 2GBytes of the disk. You can see output from these steps below:

Command (m for help): n
Partition number (1-8): 1
First cylinder (0-8635): (press Enter)
Last cylinder or +size or +sizeM or +sizeK (0-8635, default 8635): +512M

Now, when you type p, you should see the following partition printout:

Command (m for help): p

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes

   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda3             0      8635   8842240    5  Whole disk

Creating a swap partition

Next, let's create the swap partition. To do this, type n to create a new partition, then 2 to create the second partition, /dev/sda2 in our case. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, type +512M to create a partition 512MB in size. After you've done this, type t to set the partition type, and then type in 82 to set the partition type to "Linux Swap". After completing these steps, typing p should display a partition table that looks similar to this:

Command (m for help): p

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes

   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda2           488       976    499712   82  Linux swap
/dev/sda3             0      8635   8842240    5  Whole disk

Creating the /usr, /var and /home partitions

Finally, let's create the /usr, /var and /home partitions. As before, type n to create a new partition, then type 4 to create the third partition, /dev/sda4 in our case. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, enter +2048M to create a partition 2 GBytes in size. Repeat this process for sda5 and sda6, using the desired sizes. Once you're done, you should see something like this:

Command (m for help): p

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes

   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda2           488       976    499712   82  Linux swap
/dev/sda3             0      8635   8842240    5  Whole disk
/dev/sda4           976      1953   1000448   83  Linux native
/dev/sda5          1953      2144    195584   83  Linux native
/dev/sda6          2144      8635   6646784   83  Linux native

Save and Exit

To save your partition layout and exit fdisk, type w:

Command (m for help): w

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

4.d. 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 is used 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, some are known to be stable on the SPARC architecture. Ext2, ext3 and ext4, for example, are known to work well. Alternate filesystems may not function correctly.

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. If you intend to install Gentoo on a very small disk (less than 4GB), then you'll need to tell ext2 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> 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>.

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 reliable filesystem. If you intend to install Gentoo on a very small disk (less than 4GB), then you'll need to tell ext3 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 -j -T small /dev/<device> 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 -j -i <ratio> /dev/<device>.

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 extents, 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.

Applying a Filesystem to a Partition

To create a filesystem on a partition or volume, tools specific to the chosen filesystem are available:

For instance, to create the root partition (/dev/sda1 in our example) as ext2, and the /usr, /var, and /home partitions (/dev/sda4, 5 and 6 in our example, respectively) as ext3, you would use:

# mke2fs /dev/sda1
# mke2fs -j /dev/sda4
# mke2fs -j /dev/sda5
# mke2fs -j /dev/sda6

Activating the Swap Partition

mkswap is the command used to initialize swap partitions:

# mkswap /dev/sda2

To activate the swap partition, use swapon:

# swapon /dev/sda2

Create and activate the swap with the commands mentioned above.

4.e. Mounting

Now that your partitions are initialized and are housing a filesystem, it is time to mount them using the mount command. Don't forget to first create the necessary mount directories for every partition you created. For example:

# mount /dev/sda1 /mnt/gentoo
# mkdir /mnt/gentoo/usr
# mount /dev/sda4 /mnt/gentoo/usr
# mkdir /mnt/gentoo/var
# mount /dev/sda5 /mnt/gentoo/var
# mkdir /mnt/gentoo/home
# mount /dev/sda6 /mnt/gentoo/home

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 Installation Files.

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