Handbook:AMD64/Blocks/Disks

Partitions
Although it is theoretically possible to use a full disk to house a Linux system (when creating a btrfs RAID), this is almost never done in practice. Instead, full disk block devices are split up in smaller, more manageable block devices. On systems, these are called partitions. There are currently two standard partitioning technologies in use: MBR and GPT.

MBR
The MBR (Master Boot Record) setup 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 ).

To support more partitions, one of the primary partitions can be marked as an extended partition. This partition can then contain logical partitions (partitions within a partition).

Each partition is limited to 2 TB in size (due to the 32-bit identifiers). Also, the MBR setup does not provide any backup-MBR, so if an application or user overwrites the MBR, all partition information is lost.

GPT
The GPT (GUID Partition table) setup uses 64-bit identifiers for the partitions. The location in which it stores the partition information is also much bigger than the 512 bytes of an MBR, and there is no limit on the amount of partitions. Also the size of a partition is bounded by a much greater limit (almost 8 ZB - yes, zettabytes).

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 MBR.

GPT also has the advantage that it has a backup GPT at the end of the disk, which can be used to recover damage of the primary GPT at the beginning. GPT also carries CRC32 checksums to detect errors in the header and partition tables.

GPT or MBR
From the description above, one might think that using GPT should always be the recommended approach. But there are a few caveats with this.

Using GPT on a BIOS-based computer works, but then one cannot dual-boot with a Microsoft Windows operating system. The reason is that Microsoft Windows will boot in EFI mode if it detects a GPT partition label.

Some buggy BIOSes or EFIs configured to boot in BIOS/CSM/legacy mode might also have problems with booting from GPT labeled disks. If that is the case, it might be possible to work around the problem by adding the boot/active flag on the protective MBR partition which has to be done through with the   option to force it to read the partition table using the MBR format.

In this case, launch and toggle the flag using the  key. Press to select the first partition, then press the  key to write the changes to the disk and exit the  application:

Using UEFI
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 is created. The instructions for below contain the necessary pointers for this.

The EFI system partition also needs to be a FAT32 partition (or vfat as it is shown on Linux systems). The instructions at the end of this chapter use ext2 as the example file system for the partition. Make sure to use vfat, like so:

Advanced storage
The Installation CDs provide support for LVM2. LVM2 increases the flexibility offered by the partitioning setup. During the installation instructions, we will focus on "regular" partitions, but it is still good to know LVM2 is supported as well.

Default partitioning scheme
Throughout the remainder of the handbook, the following partitioning scheme is used. If this suffices, then the reader can immediately jump to Default: Using parted to partition the disk or Alternative: Using fdisk to partition the disk. Both are partitioning tools, is well known and stable and recommended for the MBR partition layout, while  is more recent and recommended for GPT layouts.

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

What is the BIOS boot partition?
A BIOS boot partition 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 (a few hundred bytes in case of MBR) and cannot be placed elsewhere.

Such partitions are not always necessary, but considering the low space consumption and the difficulties we have with documenting the plethora of partitioning differences otherwise, it is recommended to create it in either case.

For completeness, the BIOS boot partition is needed when GPT partition layout is used with GRUB2, or when the MBR partition layout is used with GRUB2 when the first partition starts earlier than the 1 MB location on the disk.

Default: Using parted to partition the disk
In this chapter, the example partition layout mentioned earlier in the instructions will be used:

Change the partition layout according to personal preference.

Viewing the current partition layout with parted
The application offers a simple interface for partitioning the disks and supports very large partitions (more than 2 TB). Fire up against the disk (in our example, we use ). It is recommended to ask to use optimal partition alignment:

Alignment means that partitions are started on well-known boundaries within the disk, ensuring that operations on the disk from the operating system level (retrieve pages from the disk) use the least amount of internal disk operations. Misaligned partitions might require the disk to fetch two pages instead of one even if the operating system asked for a single page.

To find out about all options supported by parted, type and press return.

Setting the GPT label
Most disks on the or  architectures are prepared using an msdos label. Using, the command to put a GPT label on the disk is :

To have the disk with MBR layout, use.

Removing all partitions with parted
If this isn't done yet (for instance through the operation earlier, or because the disk is a freshly formatted one), first remove all existing partitions from the disk. Type to view the current partitions, and  where NUMBER is the partition to remove.

Do the same for all other partitions that aren't needed. However, make sure to not make any mistakes here - parted executes the changes immediately (unlike which stages them, allowing a user to "undo" his changes before saving or exiting fdisk).

Creating the partitions
Now create the partitions. Creating partitions with isn't very difficult - all we need to do is inform parted about the following settings:


 * The partition type to use. This usually is primary. If the msdos partition label is used, keep in mind that there can be no more than 4 primary partitions. If more than 4 partitions are needed, make one of the first four partitions extended and create logical partitions inside it.
 * The start location of a partition (which can be expressed in MB, GB, ...)
 * The end location of the partition (which can be expressed in MB, GB, ...)

First, tell parted that the size unit we work with is megabytes (actually mebibytes, abbreviated as MiB which is the "standard" notation, but we will use MB in the text throughout as it is much more common):

Now create a 2 MB partition that will be used by the GRUB2 boot loader later. Use the command for this, and inform  to start from 1 MB and end at 3 MB (creating a partition of 2 MB in size).

Do the same for the boot partition (128 MB), swap partition (in the example, 512 MB) and the root partition that spans the remaining disk (for which the end location is marked as -1, meaning the end of the disk minus one MB, which is the farthest a partition can go).

When using the UEFI interface to boot the system (instead of BIOS), mark the boot partition as the EFI System Partition. Parted does this automatically when the boot option is set on the partition:

The end result looks like so:

Use the command to exit parted.

Alternative: Using fdisk to partition the disk
The following parts explain how to create the example partition layout 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 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 with fdisk
First remove all existing partitions 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 enter 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 BIOS boot partition
First create a very small BIOS boot partition. Type to create a new partition, then  to select a primary partition, followed by  to select the first primary partition. When prompted for the first sector, make sure it starts from 2048 (which is needed for the boot loader) and hit enter. When prompted for the last sector, type +2M to create a partition 2 Mbyte in size:

Mark the partition for EFI purposes:

Creating the boot partition
Now create a small boot partition. Type to create a new partition, then  to select a primary partition, followed by  to select the second primary partition. When prompted for the first sector, accept the default by hitting. When prompted for the last sector, type +128M to create a partition 128 Mbyte in size:

Now, when pressing, the following partition printout is displayed:

Type to toggle the bootable flag on a partition and select. After pressing again, notice that an * is placed in the "Boot" column.

Creating the swap partition
To create the swap partition, type to create a new partition, then  to tell fdisk to create a primary partition. Then type to create the third primary partition,. When prompted for the first sector, hit. When prompted for the last sector, type +512M (or any other size needed for the swap space) to create a partition 512MB 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  to tell  to create a primary partition. Then type to create the fourth 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.

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