Primary logical partition linux

Table of Contents

This chapter continues on the hard disk devices from the previous one. Here we will put partitions on those devices.

This chapter prepares you for the next chapter, where we put file systems on our partitions.

about partitions

primary, extended and logical

Linux requires you to create one or more partitions . The next paragraphs will explain how to create and use partitions.

A partition’s geometry and size is usually defined by a starting and ending cylinder (sometimes by sector). Partitions can be of type primary (maximum four), extended (maximum one) or logical (contained within the extended partition). Each partition has a type field that contains a code. This determines the computers operating system or the partitions file system.

TableВ 5.1.В primary, extended and logical partitions

Partition Type	naming
Primary (max 4)	1-4
Extended (max 1)	1-4
Logical	5-

partition naming

We saw before that hard disk devices are named /dev/hdx or /dev/sdx with x depending on the hardware configuration. Next is the partition number, starting the count at 1. Hence the four (possible) primary partitions are numbered 1 to 4. Logical partition counting always starts at 5. Thus /dev/hda2 is the second partition on the first ATA hard disk device, and /dev/hdb5 is the first logical partition on the second ATA hard disk device. Same for SCSI, /dev/sdb3 is the third partition on the second SCSI disk.

TableВ 5.2.В Partition naming

partition	device
/dev/hda1	first primary partition on /dev/hda
/dev/hda2	second primary or extended partition on /dev/hda
/dev/sda5	first logical drive on /dev/sda
/dev/sdb6	second logical on /dev/sdb

The picture below shows two (spindle) disks with partitions. Note that an extended partition is a container holding logical drives.

discovering partitions

fdisk -l

In the fdisk -l example below you can see that two partitions exist on /dev/sdb . The first partition spans 31 cylinders and contains a Linux swap partition. The second partition is much bigger.

/proc/partitions

The /proc/partitions file contains a table with major and minor number of partitioned devices, their number of blocks and the device name in /dev . Verify with /proc/devices to link the major number to the proper device.

The major number corresponds to the device type (or driver) and can be found in /proc/devices . In this case 3 corresponds to ide and 8 to sd . The major number determines the device driver to be used with this device.

The minor number is a unique identification of an instance of this device type. The devices.txt file in the kernel tree contains a full list of major and minor numbers.

parted and others

You may be interested in alternatives to fdisk like parted , cfdisk , sfdisk and gparted . This course mainly uses fdisk to partition hard disks.

parted is recommended by some Linux distributions for handling storage with gpt instead of mbr .

Below a screenshot of parted on CentOS.

partitioning new disks

In the example below, we bought a new disk for our system. After the new hardware is properly attached, you can use fdisk and parted to create the necessary partition(s). This example uses fdisk , but there is nothing wrong with using parted .

recognising the disk

First, we check with fdisk -l whether Linux can see the new disk. Yes it does, the new disk is seen as /dev/sdb, but it does not have any partitions yet.

opening the disk with fdisk

Then we create a partition with fdisk on /dev/sdb. First we start the fdisk tool with /dev/sdb as argument. Be very very careful not to partition the wrong disk!!

empty partition table

Inside the fdisk tool, we can issue the p command to see the current disks partition table.

create a new partition

No partitions exist yet, so we issue n to create a new partition. We choose p for primary, 1 for the partition number, 1 for the start cylinder and 14 for the end cylinder.

We can now issue p again to verify our changes, but they are not yet written to disk. This means we can still cancel this operation! But it looks good, so we use w to write the changes to disk, and then quit the fdisk tool.

display the new partition

Let’s verify again with fdisk -l to make sure reality fits our dreams. Indeed, the screenshot below now shows a partition on /dev/sdb.

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Ubuntu Documentation

Partitioning basics

When you buy a new hard drive, it usually comes in an unformatted state. There are no partitions or file systems on it and it is ready for partitioning and formatting. It usually comes with a disk with a formatting tool on it which will create one or more partitions on this hard drive based on Microsoft’s FAT file system, but partition creation and formatting can be done with any partitioning tool, though there are certain issues when dealing with Windows on multi-boot systems, and with using Linux partition tools like GParted to modify Windows partitions). Upon launching GPartEd from the Ubuntu Live CD (selecting «System/Administration/Partition Editor» from the Launch Bar at top) and (if necessary) selecting the hard drive, the drive will look like this:

Notice that the entire drive is marked «Unallocated» and is called «Free Space.» The drive is ready to create partitions on. I will start with the types of partitions that can be created and their purposes. Drives which have already been partitioned and formatted will follow these conventions, so in order to successfully and efficiently repartition them requires knowledge of these conventions.

There are three types of partitions with which you will be dealing; Primary, Extended, and Logical partitions.

Partitioning Conventions

In «Partitioning Basics», I mentioned the «Unallocated Free Space» on a new drive. All partitions created in this Free Space are created under «Primary partition» rules and conventions.

Primary Partition Rules and Conventions

You can only create four Primary partitions on any single physical hard drive. This partition limit extends to the Linux Swap partition as well as for any Operating System installation or extra special purpose partitions, such as separate /root, /home, /boot, etc., that you might want to create.

If you attempt to create more than four Primary Partitions, you will see the following warning:

Notice that when I tried to create a fifth partition, GPartEd gave me a message warning that this could not be accomplished. How do we circumvent this limitation and create more partitions? With an Extended Partition.

Extended Partitions

While in most circumstances, such as a «Guided Install» from the Live CD on a new Windows computer, four partitions are enough, there are circumstances in which you might need to create more than four partitions. This is the reason for an Extended partition.

An Extended partition is a special type of partition that contains «Free Space» in which more than the four Primary partitions can be created. Partitions created within the Extended partition are called Logical partitions, and any number of Logical partitions can be created within an Extended partition. The following conventions apply to Extended partitions:

When you create an Extended partition, it occupies one of the four Primary partition spots. When an Extended partition exists on a hard drive, only three Primary partitions may co-exist with it. (See Primary Partitions Rules and Conventions) If there are four Primary partitions already on a hard drive and you wish to create an Extended partition in which to create more, one of the Primary partitions must be deleted in order to create it.

Only one extended partition may be created on a hard drive (though you can have one extended partition on each of the hard drives connected to your system). The partition editor will not allow it, and it would serve no purpose at any rate. If you need the extra Extended partition space, you only need expand the one you have.

An Extended partition cannot be formatted with a filesystem, such as ext4, FAT, or NTFS, nor can it directly hold data. That is the function of the Logical Drives which are created within it.

Logical Partitions

A partition created within an Extended partition is called a Logical partition. Any number of Logical partitions may be created within an Extended partition, and they may be formatted with any filesystem.

All Operating Systems that I am aware of are able to access any Logical partition that is formatted to a compatible filesystem. For instance, while Windows will not recognize a Linux ext2 partition without a third-party driver, it will be able to access any partition formatted with FAT or NTFS, depending on the version of Windows. Linux, of course, will access all of these.

HowtoPartition/PartitioningBasics (последним исправлял пользователь ben64 2018-01-15 10:28:37)

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Partitioning

Disk partitioning or disk slicing is the creation of one or more regions on secondary storage, so that each region can be managed separately.

An entire disk may be allocated to a single partition, or multiple ones for cases such as dual-booting, maintaining a swap partition, or to logically separate data such as audio and video files. The partitioning scheme is stored in a partition table such as Master Boot Record (MBR) or GUID Partition Table (GPT).

Partition tables are created and modified using one of many partitioning tools. The tools available for Arch Linux are listed in the #Partitioning tools section.

Partitions usually contain a file system directly which is accomplished by creating a file system on (a.k.a. formatting) the partition. Alternatively, partitions can contain LVM, block device encryption or RAID, which ultimately provide device files on which a file system can be placed (or the devices can be stacked further).

Any block device (e.g. disk, partition, LUKS device, LVM logical volume or RAID array) that directly contains a mountable file system is called a volume.

Partition table

There are two main types of partition table available. These are described below in the #Master Boot Record (MBR) and #GUID Partition Table (GPT) sections along with a discussion on how to choose between the two. A third, less common alternative is using a partitionless disk, which is also discussed.

Use a partitioning tool to view the partition table of a block device.

Master Boot Record

The Master Boot Record (MBR) is the first 512 bytes of a storage device. It contains an operating system bootloader and the storage device’s partition table. It plays an important role in the boot process under BIOS systems. See Wikipedia:Master boot record#Disk partitioning for the MBR structure.

Master Boot Record (bootstrap code)

The first 440 bytes of MBR are the bootstrap code area. On BIOS systems it usually contains the first stage of the boot loader. The bootstrap code can be backed up, restored from backup or erased using dd.

Master Boot Record (partition table)

In the MBR partition table (also known as DOS or MS-DOS partition table) there are 3 types of partitions:

Primary partitions can be bootable and are limited to four partitions per disk or RAID volume. If the MBR partition table requires more than four partitions, then one of the primary partitions needs to be replaced by an extended partition containing logical partitions within it.

Extended partitions can be thought of as containers for logical partitions. A hard disk can contain no more than one extended partition. The extended partition is also counted as a primary partition so if the disk has an extended partition, only three additional primary partitions are possible (i.e. three primary partitions and one extended partition). The number of logical partitions residing in an extended partition is unlimited. A system that dual boots with Windows will require for Windows to reside in a primary partition.

The customary numbering scheme is to create primary partitions sda1 through sda3 followed by an extended partition sda4. The logical partitions on sda4 are numbered sda5, sda6, etc.

GUID Partition Table

GUID Partition Table (GPT) is a partitioning scheme that is part of the Unified Extensible Firmware Interface specification; it uses globally unique identifiers (GUIDs), or UUIDs in the Linux world, to define partitions and partition types. It is designed to succeed the Master Boot Record partitioning scheme method.

At the start of a GUID Partition Table disk there is a protective Master Boot Record (PMBR) to protect against GPT-unaware software. This protective MBR just like an ordinary MBR has a bootstrap code area which can be used for BIOS/GPT booting with boot loaders that support it.

Choosing between GPT and MBR

GUID Partition Table (GPT) is an alternative, contemporary, partitioning style; it is intended to replace the old Master Boot Record (MBR) system. GPT has several advantages over MBR which has quirks dating back to MS-DOS times. With the recent developments to the formatting tools, it is equally easy to get good dependability and performance for GPT or MBR.

Some points to consider when choosing:

To dual-boot with Windows (both 32-bit and 64-bit) using Legacy BIOS, the MBR scheme is required.
To dual-boot Windows 64-bit using UEFI mode instead of BIOS, the GPT scheme is required.
If you are installing on older hardware, especially on old laptops, consider choosing MBR because its BIOS might not support GPT (but see below how to fix it).
If you are partitioning a disk that is larger than 2 TiB, you need to use GPT.
It is recommended to always use GPT for UEFI boot, as some UEFI implementations do not support booting to the MBR while in UEFI mode.
If none of the above apply, choose freely between GPT and MBR. Since GPT is more modern, it is recommended in this case.

Some advantages of GPT over MBR are:

Provides a unique disk GUID and unique partition GUID (PARTUUID) for each partition — A good filesystem-independent way of referencing partitions and disks.
Provides a filesystem-independent partition name (PARTLABEL).
Arbitrary number of partitions — depends on space allocated for the partition table — No need for extended and logical partitions. By default the GPT table contains space for defining 128 partitions. However if you want to define more partitions, you can allocate more space to the partition table (currently only gdisk is known to support this feature).
Uses 64-bit LBA for storing Sector numbers — maximum addressable disk size is 2 ZiB. MBR is limited to addressing 2 TiB of space per drive.[1]
Stores a backup header and partition table at the end of the disk that aids in recovery in case the primary ones are damaged.
CRC32 checksums to detect errors and corruption of the header and partition table.

The section on #Partitioning tools contains a table indicating which tools are available for creating and modifying GPT and MBR tables.

Partitionless disk

This article or section needs expansion.

Partitionless disk a.k.a. superfloppy refers to a storage device without a partition table, having one file system occupying the whole storage device. The boot sector present on a partitionless device is called a volume boot record (VBR).

Btrfs partitioning

Btrfs can occupy an entire data storage device and replace the MBR or GPT partitioning schemes. See the Btrfs#Partitionless Btrfs disk instructions for details.

Partition scheme

This article or section needs expansion.

There are no strict rules for partitioning a hard drive, although one may follow the general guidance given below. A disk partitioning scheme is determined by various issues such as desired flexibility, speed, security, as well as the limitations imposed by available disk space. It is essentially personal preference. If you would like to dual boot Arch Linux and a Windows operating system please see Dual boot with Windows.

Single root partition

This scheme is the simplest and should be enough for most use cases. A swapfile can be created and easily resized as needed. It usually makes sense to start by considering a single / partition and then separate out others based on specific use cases like RAID, encryption, a shared media partition, etc.

Discrete partitions

This article or section needs expansion.

Separating out a path as a partition allows for the choice of a different filesystem and mount options. In some cases like a media partition, they can also be shared between operating systems.

Below are some example layouts that can be used when partitioning, and the following subsections detail a few of the directories which can be placed on their own separate partition and then mounted at mount points under / . See file-hierarchy(7) for a full description of the contents of these directories.

The root directory is the top of the hierarchy, the point where the primary filesystem is mounted and from which all other filesystems stem. All files and directories appear under the root directory / , even if they are stored on different physical devices. The contents of the root filesystem must be adequate to boot, restore, recover, and/or repair the system. Therefore, certain directories under / are not candidates for separate partitions.

The / partition or root partition is necessary and it is the most important. The other partitions can be replaced by it.

/ traditionally contains the /usr directory, which can grow significantly depending upon how much software is installed. 15–20 GiB should be sufficient for most users with modern hard disks. If you plan to store a swap file here, you might need a larger partition size.

The /boot directory contains the kernel and ramdisk images as well as the boot loader configuration file and boot loader stages. It also stores data that is used before the kernel begins executing user-space programs. /boot is not required for normal system operation, but only during boot and kernel upgrades (when regenerating the initial ramdisk).

A suggested size for /boot is 200 MiB unless you are using EFI system partition as /boot , in which case at least 260 MiB is recommended.

The /home directory contains user-specific configuration files, caches, application data and media files.

Separating out /home allows / to be re-partitioned separately, but note that you can still reinstall Arch with /home untouched even if it is not separate—the other top-level directories just need to be removed, and then pacstrap can be run.

You should not share home directories between users on different distributions, because they use incompatible software versions and patches. Instead, consider sharing a media partition or at least using different home directories on the same /home partition. The size of this partition varies.

The /var directory stores variable data such as spool directories and files, administrative and logging data, pacman’s cache, etc. It is used, for example, for caching and logging, and hence frequently read or written. Keeping it in a separate partition avoids running out of disk space due to flunky logs, etc.

It exists to make it possible to mount /usr as read-only. Everything that historically went into /usr that is written to during system operation (as opposed to installation and software maintenance) must reside under /var .

/var will contain, among other data, the pacman cache. Retaining these packages is helpful in case a package upgrade causes instability, requiring a downgrade to an older, archived package. The pacman cache will grow as the system is expanded and updated, but it can be safely cleared if space becomes an issue. 8–12 GiB on a desktop system should be sufficient for /var , depending on how much software will be installed.

One can consider mounting a «data» partition to cover various files to be shared by all users. Using the /home partition for this purpose is fine as well. The size of this partition varies.

A swap is a file or partition that provides disk space used as virtual memory. Swap files and swap partitions are equally performant, but swap files are much easier to resize as needed. A swap partition can potentially be shared between operating systems, but not if hibernation is used.

Historically, the general rule for swap partition size was to allocate twice the amount of physical RAM. As computers have gained ever larger memory capacities, this rule is outdated. For example, on average desktop machines with up to 512 MiB RAM, the 2× rule is usually adequate; if a sufficient amount of RAM (more than 1024 MiB) is available, it may be possible to have a smaller swap partition.

To use hibernation (a.k.a suspend to disk) it is advised to create the swap partition at the size of RAM. Although the kernel will try to compress the suspend-to-disk image to fit the swap space there is no guarantee it will succeed if the used swap space is significantly smaller than RAM. See Power management/Suspend and hibernate#Hibernation for more information.

Example layouts

This article or section needs expansion.

The following examples use /dev/sda as the example disk with /dev/sda1 as the first partition. The block device naming scheme will differ if you are partitioning a NVMe disk (e.g. /dev/nvme0n1 with partitions starting from /dev/nvme0n1p1 ) or an SD card or eMMC disk (e.g. /dev/mmcblk0 with partitions starting from /dev/mmcblk0p1 ). See Device file#Block device names for more information.

UEFI/GPT layout example

Mount point on the installed system	Partition	Partition type GUID	Partition attributes
/boot or /efi 1	/dev/sda1	C12A7328-F81F-11D2-BA4B-00A0C93EC93B : EFI system partition	At least 260 MiB
[SWAP]	/dev/sda2	0657FD6D-A4AB-43C4-84E5-0933C84B4F4F : Linux swap	More than 512 MiB
/	/dev/sda3	4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709 : Linux x86-64 root (/)	Remainder of the device

BIOS/MBR layout example

Mount point on the installed system	Partition	Partition type ID	Boot flag	Suggested size
[SWAP]	/dev/sda1	82 : Linux swap	No	More than 512 MiB
/	/dev/sda2	83 : Linux	Yes	Remainder of the device
N/A	Unallocated space 2	N/A	N/A	At least 16.5 KiB at the end of the disk

BIOS/GPT layout example

Mount point on the installed system	Partition	Partition type GUID	Partition attributes
None	/dev/sda1	21686148-6449-6E6F-744E-656564454649 : BIOS boot partition 3	1 MiB
[SWAP]	/dev/sda2	0657FD6D-A4AB-43C4-84E5-0933C84B4F4F : Linux swap	More than 512 MiB
/	/dev/sda3	4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709 : Linux x86-64 root (/)	Remainder of the device

The ESP can be mounted to /efi if the used boot loader is capable of accessing the file system (and everything above it) on which the kernel and initramfs images are located. See EFI system partition#Typical mount points and the warning in Arch boot process#Boot loader for details.
An unpartitioned space of at least 33 512-byte sectors (16.5 KiB) at the end of the disk to allow converting to GPT in the future. The space will be required for the backup GPT header. The recommendation to preserve an unpartitioned space applies to all MBR partitioned disks.
A BIOS boot partition is only required when using GRUB for BIOS booting from a GPT disk. The partition has nothing to do with /boot , and it must not be formatted with a file system or mounted.

Tools

Partitioning tools

The following programs are used to create and/or manipulate device partition tables and partitions. See the linked articles for the exact commands to be used.

This table will help you to choose utility for your needs:

MBR	GPT
Dialog	fdisk parted	fdisk gdisk parted
Pseudo-graphics	cfdisk	cfdisk cgdisk
Non-interactive	sfdisk parted	sfdisk sgdisk parted
Graphical	GParted gnome-disk-utility partitionmanager	GParted gnome-disk-utility partitionmanager

fdisk

fdisk and its related utilities are described in the fdisk article.

fdisk ( util-linux )
- fdisk(8) – Dialog-driven program for creation and manipulation of partition tables.
- cfdisk(8) – Curses-based variant of fdisk.
- sfdisk(8) – Scriptable variant of fdisk.

GPT fdisk

gdisk and its related utilities are described in the gdisk article.

GNU Parted

These group of tools are described in the GNU Parted article.

Backup

fdisk can create a backup of the partitions table. See fdisk#Backup and restore partition table.
GPT fdisk can create a binary backup consisting of the protective MBR, the main GPT header, the backup GPT header, and one copy of the partition table. See GPT fdisk#Backup and restore partition table.

Recovery

gpart — A utility that guesses the contents of a destroyed MBR partition table. Its usage is explained in the gpart(8) man page.

https://github.com/baruch/gpart || gpart

GPT fdisk — A partitioning tool that can restore the primary GPT header (located at the start of the disk) from the secondary GPT header (located at the end of the disk) or vice versa.

https://www.rodsbooks.com/gdisk/ || gptfdisk

TestDisk — A utility that supports recovering lost partitions on both MBR and GPT.

https://www.cgsecurity.org/index.html?testdisk.html || testdisk

Partition alignment

fdisk, gdisk and parted handle alignment automatically. See GNU Parted#Check alignment if you want to verify your alignment after partitioning.

For certain drives Advanced Format might be able to provide a better-performing alignment.

GPT kernel support

The CONFIG_EFI_PARTITION option in the kernel config enables GPT support in the kernel (despite the name, EFI PARTITION). This option must be built in the kernel and not compiled as a loadable module. This option is required even if GPT disks are used only for data storage and not for booting. This option is enabled by default in all Arch’s officially supported kernels. In case of a custom kernel, enable this option by doing CONFIG_EFI_PARTITION=y .

Troubleshooting

This article or section needs expansion.

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