Handbook:AMD64/Blocks/Disks

Partition tables
Although it is theoretically possible to use a raw, unpartitioned disk to house a Linux system (when creating a btrfs RAID for example), this is almost never done in practice. Instead, disk block devices are split up into smaller, more manageable block devices. On systems, these are called partitions. There are currently two standard partitioning technologies in use: MBR (sometimes also called DOS disklabel) and GPT; these are tied to the two boot process types: legacy BIOS boot and UEFI.

GUID Partition Table (GPT)
The GUID Partition Table (GPT) setup (also called GPT disklabel) uses 64-bit identifiers for the partitions. The location in which it stores the partition information is much bigger than the 512 bytes of the MBR partition table (DOS disklabel), which means there is practically no limit on the amount of partitions for a GPT disk. Also the size of a partition is bounded by a much greater limit (almost 8 ZiB - yes, zebibytes).

When a system's software interface between the operating system and firmware is UEFI (instead of BIOS), GPT is almost mandatory as compatibility issues will arise with DOS disklabel.

GPT also takes advantage of checksumming and redundancy. It carries CRC32 checksums to detect errors in the header and partition tables and has a backup GPT at the end of the disk. This backup table can be used to recover damage of the primary GPT near the beginning of the disk.

Master boot record (MBR) or DOS boot sector
The Master boot record boot sector (also called DOS boot sector or DOS disklabel) was first introduced in 1983 with PC DOS 2.x. MBR uses 32-bit identifiers for the start sector and length of the partitions, and supports three partition types: primary, extended, and logical. Primary partitions have their information stored in the master boot record itself - a very small (usually 512 bytes) location at the very beginning of a disk. Due to this small space, only four primary partitions are supported (for instance, to ).

In order to support more partitions, one of the primary partitions in the MBR can be marked as an extended partition. This partition can then contain additional logical partitions (partitions within a partition).

The Handbook authors suggest using GPT whenever possible for Gentoo installations.

Advanced storage
The Installation CDs provide support for Logical Volume Manager (LVM). LVM increases the flexibility offered by the partitioning setup. It allows to combine partitions and disks into volume groups and define RAID groups or caches on fast SSDs for slow HDs. The installation instructions below will focus on "regular" partitions, but it is good to know LVM is supported if that route is desired. Visit the LVM article for more details. Newcomers beware: although fully supported, LVM is outside the scope of this guide.

Default partitioning scheme
Throughout the remainder of the handbook, we will discuss and explain two cases: 1) GPT partition table and UEFI boot, and 2) MBR partition table and legacy BIOS boot. While it is possible to mix and match, that goes beyond the scope of this manual. As already stated above, installations on modern hardware should use GPT partition table and UEFI boot; as an exception from this rule, MBR and BIOS boot is still frequently used in virtualized (cloud) environments.

The following partitioning scheme will be used as a simple example layout:

If this suffices as information, the advanced reader can directly skip ahead to the actual partitioning.

Both and  are partitioning utilities. is well known, stable, and recommended for the MBR partition layout. was one of the first Linux block device management utilities to support GPT partitions, and provides an alternative. Here, is used since it has a better text-based user interface.

Before going to the creation instructions, the first set of sections will describe in more detail how partitioning schemes can be created and mention some common pitfalls.

What is the EFI System Partition (ESP)?
When installing Gentoo on a system that uses UEFI to boot the operating system (instead of BIOS), then it is important that an EFI System Partition (ESP) is created. The instructions below contain the necessary pointers to correctly handle this operation. The EFI system partition is not required when booting in BIOS/Legacy mode.

The ESP must be a FAT variant (sometimes shown as vfat on Linux systems). The official UEFI specification denotes FAT12, 16, or 32 filesystems will be recognized by the UEFI firmware, although FAT32 is recommended for the ESP. After partitioning, format the ESP accordingly:

What is the BIOS boot partition?
A BIOS boot partition is only needed when combining a GPT partition layout with GRUB2 in BIOS/Legacy mode. It is not required when booting in EFI/UEFI mode, and also not required when using a MBR table. It is a very small (1 to 2 MB) partition in which boot loaders like GRUB2 can put additional data that doesn't fit in the allocated storage. It will not be used in this guide.

Partitioning the disk with GPT for UEFI
The following parts explain how to create the example partition layout for a GPT / UEFI boot installation using. The example partition layout was mentioned earlier:

Change the partition layout according to personal preference.

Viewing the current partition layout
is a popular and powerful tool to split a disk into partitions. Fire up against the disk (in our example, we use ):

Use the key to display the disk's current partition configuration:

This particular disk was configured to house two Linux filesystems (each with a corresponding partition listed as "Linux") as well as a swap partition (listed as "Linux swap").

Creating a new disklabel / removing all partitions
Type to create a new GPT disklabel on the disk; this will remove all existing partitions.

For an existing GPT disklabel (see the output of above), alternatively consider removing the existing partitions one by one from the disk. Type to delete a partition. For instance, to delete an existing :

The partition has now been scheduled for deletion. It will no longer show up when printing the list of partitions (, but it will not be erased until the changes have been saved. This allows users to abort the operation if a mistake was made - in that case, type immediately and hit  and the partition will not be deleted.

Repeatedly type to print out a partition listing and then type  and the number of the partition to delete it. Eventually, the partition table will be empty:

Now that the in-memory partition table is empty, we're ready to create the partitions.

Creating the EFI system partition (ESP)
First create a small EFI system partition, which will also be mounted as /boot. Type to create a new partition, followed by  to select the first partition. When prompted for the first sector, make sure it starts from 2048 (which may be needed for the boot loader) and hit. When prompted for the last sector, type +256M to create a partition 256 Mbyte in size:

Mark the partition as EFI system partition:

Creating the swap partition
Next, to create the swap partition, type to create a new partition, then type  to create the second partition,. When prompted for the first sector, hit. When prompted for the last sector, type +4G (or any other size needed for the swap space) to create a partition 4GB in size.

After all this is done, type to set the partition type,  to select the partition just created and then type in 19 to set the partition type to "Linux Swap".

Creating the root partition
Finally, to create the root partition, type to create a new partition. Then type to create the third partition,. When prompted for the first sector, hit. When prompted for the last sector, hit to create a partition that takes up the rest of the remaining space on the disk. After completing these steps, typing should display a partition table that looks similar to this:

Saving the partition layout
To save the partition layout and exit, type.

With the partitions created, it is now time to put filesystems on them.

Partitioning the disk with MBR for BIOS / legacy boot
The following explains how to create the example partition layout for a MBR / BIOS legacy boot installation. The example partition layout mentioned earlier is now:

Change the partition layout according to personal preference.

Viewing the current partition layout
Fire up against the disk (in our example, we use ):

Use the key to display the disk's current partition configuration:

This particular disk was until now configured to house two Linux filesystems (each with a corresponding partition listed as "Linux") as well as a swap partition (listed as "Linux swap"), using a GPT table.

Creating a new disklabel / removing all partitions
Type to create a new MBR disklabel (here also named DOS disklabel) on the disk; this will remove all existing partitions.

For an existing DOS disklabel (see the output of above), alternatively consider removing the existing partitions one by one from the disk. Type to delete a partition. For instance, to delete an existing :

The partition has now been scheduled for deletion. It will no longer show up when printing the list of partitions (, but it will not be erased until the changes have been saved. This allows users to abort the operation if a mistake was made - in that case, type immediately and hit  and the partition will not be deleted.

Repeatedly type to print out a partition listing and then type  and the number of the partition to delete it. Eventually, the partition table will be empty:

Now we're ready to create the partitions.

Creating the boot partition
First, create a small partition which will be mounted as /boot. Type to create a new partition, followed by  for a primary partition and  to select the first primary partition. When prompted for the first sector, make sure it starts from 2048 (which may be needed for the boot loader) and hit. When prompted for the last sector, type +256M to create a partition 256 Mbyte in size:

Creating the swap partition
Next, to create the swap partition, type to create a new partition, then, then type  to create the second primary partition,. When prompted for the first sector, hit. When prompted for the last sector, type +4G (or any other size needed for the swap space) to create a partition 4GB in size.

After all this is done, type to set the partition type,  to select the partition just created and then type in 82 to set the partition type to "Linux Swap".

Creating the root partition
Finally, to create the root partition, type to create a new partition. Then type and  to create the third primary partition,. When prompted for the first sector, hit. When prompted for the last sector, hit to create a partition that takes up the rest of the remaining space on the disk. After completing these steps, typing should display a partition table that looks similar to this:

Saving the partition layout
To save the partition layout and exit, type.

Now it is time to put filesystems on the partitions.