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4. Preparing the Disks

• Introduction to Block Devices
• Designing a Partitioning Scheme
• Using fdisk on MIPS 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 SCSI HD in a Linux system, namely /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

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 partitions.

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 Monitor.
• 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 following is an example excerpt from an fdisk session. Read and tailor it to your needs...

# fdisk /dev/sda

Command (m for help): x

Expert command (m for help): m
Command action
   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 -----
Bootfile: /unix
----- Directory Entries -----

Command (m for help):

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.

Command (m for help): n
Partition number (1-16): 1
First cylinder (5-8682, default 5): 51
 Last cylinder (51-8682, default 8682): 101
(Notice how fdisk only allows Partition #1 to be re-created starting at a minimum of cylinder 5)
(Had you attempted to delete & re-create the SGI Volume Header this way, this is the same issue
 you would have encountered.)
(In our example, we want /boot to be 50MB, so we start it at cylinder 51 (the Volume Header needs to 
 start at cylinder 0, remember?), and set its ending cylinder to 101, which will roughly be 50MB (+/- 1-5MB))

Command (m for help): d
Partition number (1-16): 9
(Delete Partition #9 (SGI Volume Header))

Command (m for help): n
Partition number (1-16): 9
First cylinder (0-50, default 0): 0
 Last cylinder (0-50, default 50): 50
(Re-Create Partition #9, ending just before Partition #1)

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

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 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 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.

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:

# mke2fs /dev/sda1
# mke2fs -j /dev/sda3

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/sda2

To activate the swap partition, use swapon:

# swapon /dev/sda2

Create and activate the swap now.

4.e. 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/sda3 /mnt/gentoo
# mkdir /mnt/gentoo/boot
# mount /dev/sda1 /mnt/gentoo/boot

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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