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4.a. 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 IDE drive in a Linux system, namely /dev/hda. If your system uses SCSI or SATA 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.
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 x86 systems, these are called partitions.
Partitions are divided in three types: primary, extended and logical.
A primary partition is a partition which has its information stored in the MBR (master boot record). As an MBR is very small (512 bytes) only four primary partitions can be defined (for instance, /dev/hda1 to /dev/hda4).
An extended partition is a special primary partition (meaning the extended partition must be one of the four possible primary partitions) which contains more partitions. Such a partition didn't exist originally, but as four partitions were too few, it was brought to life to extend the formatting scheme without losing backward compatibility.
A logical partition is a partition inside the extended partition. Their definitions aren't placed inside the MBR, but are declared inside the extended partition.
The x86 Installation CDs provide support for EVMS and LVM2. EVMS and LVM2 increase the flexibility offered by your partitioning setup. During the installation instructions, we will focus on "regular" partitions, but it is still good to know EVMS and LVM2 are supported as well.
4.b. 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/hda3||ext3||Rest of the disk||Root partition|
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 partitioning your disk by reading 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:
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.
As an example partitioning, we show you one for a 20GB disk, used as a demonstration laptop (containing webserver, mailserver, gnome, ...):
Filesystem Type Size Used Avail Use% Mounted on /dev/hda5 ext3 509M 132M 351M 28% / /dev/hda2 ext3 5.0G 3.0G 1.8G 63% /home /dev/hda7 ext3 7.9G 6.2G 1.3G 83% /usr /dev/hda8 ext3 1011M 483M 477M 51% /opt /dev/hda9 ext3 2.0G 607M 1.3G 32% /var /dev/hda1 ext2 51M 17M 31M 36% /boot /dev/hda6 swap 516M 12M 504M 2% <not mounted> (Unpartitioned space for future usage: 2 GB)
/usr is rather full (83% used) here, but once all software is installed, /usr doesn't tend to grow that much. For /var, people might think the assigned space is too much. However, Gentoo compiles all programs inside /var/tmp/portage, so you should have /var with at least 1G free if you don't want to compile big programs and at least 3G free if compiling KDE or OpenOffice.org is no big deal for you.
4.c. Using fdisk to Partition your Disk
The following parts explain how to create the example partition layout described previously, namely:
Change your partition layout according to your own preference.
Viewing the Current Partition Layout
fdisk is a popular and powerful tool to split your disk into partitions. Fire up fdisk on your disk (in our example, we use /dev/hda):
# fdisk /dev/hda
Once in fdisk, you'll be greeted with a prompt that looks like this:
Command (m for help):
Type p to display your disk's current partition configuration:
Command (m for help): p Disk /dev/hda: 240 heads, 63 sectors, 2184 cylinders Units = cylinders of 15120 * 512 bytes Device Boot Start End Blocks Id System /dev/hda1 1 14 105808+ 83 Linux /dev/hda2 15 49 264600 82 Linux swap /dev/hda3 50 70 158760 83 Linux /dev/hda4 71 2184 15981840 5 Extended /dev/hda5 71 209 1050808+ 83 Linux /dev/hda6 210 348 1050808+ 83 Linux /dev/hda7 349 626 2101648+ 83 Linux /dev/hda8 627 904 2101648+ 83 Linux /dev/hda9 905 2184 9676768+ 83 Linux Command (m for help):
This particular disk is configured to house seven Linux filesystems (each with a corresponding partition listed as "Linux") as well as a swap partition (listed as "Linux swap").
Removing all Partitions
We will first remove all existing partitions from the disk. Type d to delete a partition. For instance, to delete an existing /dev/hda1:
Command (m for help): d Partition number (1-4): 1
The partition has been scheduled for deletion. It will no longer show up if you type p, but it will not be erased until your changes have been saved. If you made a mistake and want to abort without saving your changes, type q immediately and hit enter and your partition will not be deleted.
Now, assuming that you do indeed want to wipe out all the partitions on your system, repeatedly type p to print out a partition listing and then type d and the number of the partition to delete it. Eventually, you'll end up with a partition table with nothing in it:
Disk /dev/hda: 30.0 GB, 30005821440 bytes 240 heads, 63 sectors/track, 3876 cylinders Units = cylinders of 15120 * 512 = 7741440 bytes Device Boot Start End Blocks Id System Command (m for help):
Now that the in-memory partition table is empty, we're ready to create the partitions. We will use a default partitioning scheme as discussed previously. Of course, don't follow these instructions to the letter if you don't want the same partitioning scheme!
Creating the Boot Partition
We first create a small boot partition. Type n to create a new partition, then p to select a primary partition, followed by 1 to select the first primary partition. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, type +32M to create a partition 32 Mbyte in size:
Command (m for help): n Command action e extended p primary partition (1-4) p Partition number (1-4): 1 First cylinder (1-3876, default 1): (Hit Enter) Using default value 1 Last cylinder or +size or +sizeM or +sizeK (1-3876, default 3876): +32M
Now, when you type p, you should see the following partition printout:
Command (m for help): p Disk /dev/hda: 30.0 GB, 30005821440 bytes 240 heads, 63 sectors/track, 3876 cylinders Units = cylinders of 15120 * 512 = 7741440 bytes Device Boot Start End Blocks Id System /dev/hda1 1 14 105808+ 83 Linux
We need to make this partition bootable. Type a to toggle the bootable flag on a partition and select 1. If you press p again, you will notice that an * is placed in the "Boot" column.
Creating the Swap Partition
Let's now create the swap partition. To do this, type n to create a new partition, then p to tell fdisk that you want a primary partition. Then type 2 to create the second primary partition, /dev/hda2 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, 2 to select the partition you just created 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/hda: 30.0 GB, 30005821440 bytes 240 heads, 63 sectors/track, 3876 cylinders Units = cylinders of 15120 * 512 = 7741440 bytes Device Boot Start End Blocks Id System /dev/hda1 * 1 14 105808+ 83 Linux /dev/hda2 15 81 506520 82 Linux swap
Creating the Root Partition
Finally, let's create the root partition. To do this, type n to create a new partition, then p to tell fdisk that you want a primary partition. Then type 3 to create the third primary partition, /dev/hda3 in our case. When prompted for the first cylinder, hit enter. When prompted for the last cylinder, hit enter to create a partition that takes up the rest of the remaining space on your disk. After completing these steps, typing p should display a partition table that looks similar to this:
Command (m for help): p Disk /dev/hda: 30.0 GB, 30005821440 bytes 240 heads, 63 sectors/track, 3876 cylinders Units = cylinders of 15120 * 512 = 7741440 bytes Device Boot Start End Blocks Id System /dev/hda1 * 1 14 105808+ 83 Linux /dev/hda2 15 81 506520 82 Linux swap /dev/hda3 82 3876 28690200 83 Linux
Saving the Partition Layout
To save the partition layout and exit fdisk, type w.
Command (m for help): w
Now that your partitions are created, you can now continue with Creating Filesystems.
4.d. 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, ReiserFS, XFS and JFS as these are the most commonly used filesystems on Linux systems.
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/hda1 in our example) in ext2 and the root partition (/dev/hda3 in our example) in ext3 (as in our example), you would use:
# mke2fs /dev/hda1 # 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.
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:
Warning: Due to a bug in the e2fsprogs package, you need to explicitly use the mount -t ext3 option if you are using an ext3 filesystem.
# mount /dev/hda3 /mnt/gentoo (For an ext3 partition:) # mount -t ext3 /dev/hda3 /mnt/gentoo # mkdir /mnt/gentoo/boot # mount /dev/hda1 /mnt/gentoo/boot
Note: 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 partitions.
Continue with Installing the Gentoo Installation Files.
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