Handbook:Alpha/Installation/Disks

Block devices

Let's take a good look at disk-oriented aspects of Gentoo Linux and Linux in general, including block devices, partitions, and Linux filesystems. Once the ins and outs of disks are understood, partitions and filesystems can be established for installation.

To begin, let's look at block devices. SCSI and Serial ATA drives are both labeled under device handles such as: /dev/sda, /dev/sdb, /dev/sdc, etc. On more modern machines, PCI Express based NVMe solid state disks have device handles such as /dev/nvme0n1, /dev/nvme0n2, etc.

The following table will help readers determine where to find a certain type of block device on the system:

The block devices above represent an abstract interface to the disk. User programs can use these block devices to interact with the disk without worrying about whether the drives are SATA, SCSI, or something else. The program can simply address the storage on the disk as a bunch of contiguous, randomly-accessible 4096-byte (4K) blocks.

Slices

Although it is theoretically possible to use a full disk to house a Linux system, this is almost never done in practice. Instead, full disk block devices are split up in smaller, more manageable block devices. On Alpha systems, these are called slices.

How many partitions and how big?

The design of disk partition layout is highly dependent on the demands of the system and the file system(s) applied to the device. If there are lots of users, then it is advised to have /home on a separate partition which will increase security and make backups and other types of maintenance easier. If Gentoo is being installed to perform as a mail server, then /var should be a separate partition as all mails are stored inside the /var directory. Game servers may have a separate /opt partition since most gaming server software is installed therein. The reason for these recommendations is similar to the /home directory: security, backups, and maintenance.

In most situations on Gentoo, /usr and /var should be kept relatively large in size. /usr hosts the majority of applications available on the system and the Linux kernel sources (under /usr/src). By default, /var hosts the Gentoo ebuild repository (located at /var/db/repos/gentoo) which, depending on the file system, generally consumes around 650 MiB of disk space. This space estimate excludes the /var/cache/distfiles and /var/cache/binpkgs directories, which will gradually fill with source files and (optionally) binary packages respectively as they are added to the system.

How many partitions and how big very much depends on considering the trade-offs and choosing the best option for the circumstance. Separate partitions or volumes have the following advantages:

• Choose the best performing filesystem for each partition or volume.
• The 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 realized 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 certain disadvantages as well:

• If not configured properly, the system might have lots of free space on one partition and little free space on another.
• A separate partition for /usr/ may require the administrator to boot with an initramfs to mount the partition before other boot scripts start. Since the generation and maintenance of an initramfs is beyond the scope of this handbook, we recommend that newcomers do not use a separate partition for /usr/.
• There is also a 15-partition limit for SCSI and SATA unless the disk uses GPT labels.

What about swap space?

There is no perfect value for the swap partition. The purpose of swap space is to provide disk storage to the kernel when internal memory (RAM) is under pressure. A swap space allows for the kernel to move memory pages that are not likely to be accessed soon to disk (swap or page-out), freeing memory. Of course, if that memory is suddenly needed, these pages need to be put back in memory (page-in) which will take a while (as disks are very slow compared to internal memory).

When the system is not going to run memory intensive applications or has lots of memory available, then it probably does not need much swap space. However, swap space is used to store the entire contents of memory in case of hibernation. If the system requires support for hibernation, then swap space larger than or equal to the amount of memory is necessary.

As a general rule of thumb, the swap space size is recommended to be twice the internal memory (RAM). For systems with multiple hard disks, it is wise to create one swap partition on each disk so that they can be utilized for parallel read/write operations. The faster a disk can swap, the faster the system will run when data in swap space must be accessed. When choosing between rotational and solid state disks, it is better for performance to put swap on the SSD. Also, swap files can be used as an alternative to swap partitions; this is mostly interesting for systems with very limited disk space.

Identifying available disks

To figure out what disks are running in the system, use the following commands:

For IDE disks:

For SCSI disks:

The output will show what disks were detected and their respective /dev/ entry. In the following parts we assume that the disk is a SCSI disk on /dev/sda.

Now fire up fdisk:

Deleting all slices

If the hard drive is completely blank, then first create a BSD disklabel.

/dev/sda contains no disklabel.
Do you want to create a disklabel? (y/n) y
A bunch of drive-specific info will show here
3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  c:        1      5290*     5289*    unused        0     0

We start with deleting all slices except the 'c'-slice (a requirement for using BSD disklabels). The following shows how to delete a slice (in the example we use 'a'). Repeat the process to delete all other slices (again, except the 'c'-slice).

Use p to view all existing slices. d is used to delete a slice.

8 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  a:        1       235*      234*    4.2BSD     1024  8192    16
  b:      235*      469*      234*      swap
  c:        1      5290*     5289*    unused        0     0
  d:      469*     2076*     1607*    unused        0     0
  e:     2076*     3683*     1607*    unused        0     0
  f:     3683*     5290*     1607*    unused        0     0
  g:      469*     1749*     1280     4.2BSD     1024  8192    16
  h:     1749*     5290*     3541*    unused        0     0

Partition (a-h): a

After repeating this process for all slices, a listing should show something similar to this:

3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  c:        1      5290*     5289*    unused        0     0

Creating the swap slice

On Alpha based systems there is no need for a separate boot slice. However, the first cylinder cannot be used as the aboot image will be placed there.

We will create a swap slice starting at the third cylinder, with a total size of 1 GB. Use n to create a new slice. After creating the slice, we will change its type to 1 (one), meaning swap.

Partition (a-p): a
First cylinder (1-5290, default 1): 3
Last cylinder or +size or +sizeM or +sizeK (3-5290, default 5290): +1024M

Partition (a-c): a
Hex code (type L to list codes): 1

After these steps a layout similar to the following should be shown:

3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  a:        3      1003      1001       swap
  c:        1      5290*     5289*    unused        0     0

Creating the root slice

We will now create the root slice, starting from the first cylinder after the swap slice. Use the p command to view where the swap slice ends. In our example, this is at 1003, making the root slice start at 1004.

Another problem is that there is currently a bug in fdisk making it think the number of available cylinders is one above the real number of cylinders. In other words, when asked for the last cylinder, decrease the cylinder number (in this example: 5290) with one.

When the slice is created, we change the type to 8, for ext2.

Partition (a-p): b
First cylinder (1-5290, default 1): 1004
Last cylinder or +size or +sizeM or +sizeK (1004-5290, default 5290): 5289

Partition (a-c): b
Hex code (type L to list codes): 8

The resulting slice layout should now be similar to this:

3 partitions:
#       start       end      size     fstype   [fsize bsize   cpg]                                    
  a:        3      1003      1001       swap
  b:     1004      5289      4286       ext2
  c:        1      5290*     5289*    unused        0     0

Save the slice layout and exit

Exit the fdisk application by typing w. This will also save the slice layout.

Identifying the available disks

To figure out what disks are running, use the following commands:

For IDE disks:

For SCSI disks:

From this output it should be easy to see what disks were detected and their respective /dev/ entry. In the following parts we assume that the disk is a SCSI disk on /dev/sda.

Now fire up fdisk:

Deleting all partitions

If the hard drive is completely blank, then first create a DOS disklabel.

Building a new DOS disklabel.

We start with deleting all partitions. The following shows how to delete a partition (in the example we use '1'). Repeat the process to delete all other partitions.

Use p to view all existing partitions. d is used to delete a partition.

Disk /dev/sda: 9150 MB, 9150996480 bytes
64 heads, 32 sectors/track, 8727 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
  
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1         478      489456   83  Linux
/dev/sda2             479        8727     8446976    5  Extended
/dev/sda5             479        1433      977904   83  Linux Swap
/dev/sda6            1434        8727     7469040   83  Linux

Partition number (1-6): 1

Creating the boot partition

On Alpha systems which use MILO to boot, we have to create a small vfat boot partition.

Command action
  e   extended
  p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-8727, default 1): 1
Last cylinder or +size or +sizeM or +sizeK (1-8727, default 8727): +16M

Selected partition 1
Hex code (type L to list codes): 6
Changed system type of partition 1 to 6 (FAT16)

Creating the swap partition

We will create a swap partition with a total size of 1 GB. Use n to create a new partition.

Command action
  e   extended
  p   primary partition (1-4)
p
Partition number (1-4): 2
First cylinder (17-8727, default 17): 17
Last cylinder or +size or +sizeM or +sizeK (17-8727, default 8727): +1000M

Partition number (1-4): 2
Hex code (type L to list codes): 82
Changed system type of partition 2 to 82 (Linux swap)

After these steps a layout similar to the following is shown:

Disk /dev/sda: 9150 MB, 9150996480 bytes
64 heads, 32 sectors/track, 8727 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
  
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1          16       16368    6  FAT16
/dev/sda2              17         971      977920   82  Linux swap

Creating the root partition

We will now create the root partition. Again, just use the n command.

Command action
  e   extended
  p   primary partition (1-4)
p
Partition number (1-4): 3
First cylinder (972-8727, default 972): 972
Last cylinder or +size or +sizeM or +sizeK (972-8727, default 8727): 8727

After these steps a layout similar to the following should be shown:

Disk /dev/sda: 9150 MB, 9150996480 bytes
64 heads, 32 sectors/track, 8727 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
  
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1          16       16368    6  FAT16
/dev/sda2              17         971      977920   82  Linux swap
/dev/sda3             972        8727     7942144   83  Linux

Save the partition layout and exit

Save the changes made in fdisk by typing w.

Now that the partitions are created, continue with Creating filesystems.

Introduction

Now that the partitions have been created, it is time to place a filesystem on them. In the next section the various file systems that Linux supports are described. Readers that already know which filesystem to use can continue with Applying a filesystem to a partition. The others should read on to learn about the available filesystems...

Filesystems

Linux supports several dozen filesystems, although many of them are only wise to deploy for specific purposes. Only certain filesystems may be found found stable on the alpha architecture - it is advised to read up on the filesystems and their support state before selecting a more experimental one for important partitions. Ext4 is the recommended all-purpose all-platform filesystem.

btrfs

A next generation filesystem that provides many advanced features such as snapshotting, self-healing through checksums, transparent compression, subvolumes, and integrated RAID. Kernels prior to 5.4.y are not guaranteed to be safe to use with btrfs in production because fixes for serious issues are only present in the more recent releases of the LTS kernel branches. Filesystem corruption issues are common on older kernel branches, with anything older than 4.4.y being especially unsafe and prone to corruption. Corruption is more likely on older kernels (than 5.4.y) when compression is enabled. RAID 5/6 and quota groups unsafe on all versions of btrfs. Furthermore, btrfs can counter-intuitively fail filesystem operations with ENOSPC when df reports free space due to internal fragmentation (free space pinned by DATA + SYSTEM chunks, but needed in METADATA chunks). Additionally, a single 4K reference to a 128M extent inside btrfs can cause free space to be present, but unavailable for allocations. This can also cause btrfs to return ENOSPC when free space is reported by df. Installing sys-fs/btrfsmaintenance and configuring the scripts to run periodically can help to reduce the possibility of ENOSPC issues by rebalancing btrfs, but it will not eliminate the risk of ENOSPC when free space is present. Some workloads will never hit ENOSPC while others will. If the risk of ENOSPC in production is unacceptable, you should use something else. If using btrfs, be certain to avoid configurations known to have issues. With the exception of ENOSPC, information on the issues present in btrfs in the latest kernel branches is available at the btrfs wiki status page.

ext2

This 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 the system is booted and the filesystem happens to be in an inconsistent state.

ext3

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. It uses an HTree index that enables high performance in almost all situations. In short, ext3 is a very good and reliable filesystem.

ext4

Initially created as a fork of ext3, ext4 brings new features, performance improvements, and removal of size limits with moderate changes to the on-disk format. It can span volumes up to 1 EB and with maximum file size of 16TB. Instead of the classic ext2/3 bitmap block allocation ext4 uses extents, which improve large file performance and reduce fragmentation. Ext4 also provides more sophisticated block allocation algorithms (delayed allocation and multiblock allocation) giving the filesystem driver more ways to optimize the layout of data on the disk. Ext4 is the recommended all-purpose all-platform filesystem.

f2fs

The Flash-Friendly File System was originally created by Samsung for the use with NAND flash memory. As of Q2, 2016, this filesystem is still considered immature, but it is a decent choice when installing Gentoo to microSD cards, USB drives, or other flash-based storage devices.

JFS

IBM's high-performance journaling filesystem. JFS is a light, fast, and reliable B+tree-based filesystem with good performance in various conditions.

ReiserFS

A B+tree-based journaled filesystem that has good overall performance, especially when dealing with many tiny files at the cost of more CPU cycles. ReiserFS version 3 is included in the mainline Linux kernel, but is not recommended to be used when initially installing a Gentoo system. Newer versions of the ReiserFS filesystem exist, however they require additional patching of the mainline kernel to be utilized.

XFS

A filesystem with metadata journaling which comes with a robust feature-set and is optimized for scalability. XFS seems to be less forgiving to various hardware problems, but has been continuously upgraded to include modern features.

VFAT

Also known as FAT32, is supported by Linux but does not support standard UNIX permission settings. It is mostly used for interoperability with other operating systems (Microsoft Windows or Apple's OSX) but is also a necessity for some system bootloader firmware (like UEFI).

NTFS

This "New Technology" filesystem is the flagship filesystem of Microsoft Windows since Windows NT 3.1. Similar to vfat above it does not store UNIX permission settings or extended attributes necessary for BSD or Linux to function properly, therefore it should not be used as a root filesystem. It should only be used for interoperability with Microsoft Windows systems (note the emphasis on only).

Applying a filesystem to a partition

To create a filesystem on a partition or volume, there are user space utilities available for each possible filesystem. Click the filesystem's name in the table below for additional information on each filesystem:

For instance, to have the EFI system partition partition (/dev/sda1) as FAT32 and the root partition (/dev/sda3) as ext4 as used in the example partition structure, the following commands would be used:

When using ext2, ext3, or ext4 on a small partition (less than 8 GiB), then the file system must be created with the proper options to reserve enough inodes. This can be done using one of the following commands, respectively:

This will generally quadruple the number of inodes for a given file system as its "bytes-per-inode" reduces from one every 16kB to one every 4kB.

Now create the filesystems on the newly created partitions (or logical volumes).

Activating the swap partition

mkswap is the command that is used to initialize swap partitions:

To activate the swap partition, use swapon:

Create and activate the swap with the commands mentioned above.