Handbook:SPARC/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.

Partitions

Although it is theoretically possible to use the entire 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. These are known as partitions or slices.

The first partition on the first SCSI disk is /dev/sda1, the second /dev/sda2 and so on.

The third partition on Sun systems is set aside as a special "whole disk" slice. This partition must not contain a file system.

Users who are used to the DOS partitioning scheme should note that Sun disklabels do not have "primary" and "extended" partitions. Instead, up to eight partitions are available per drive, with the third of these being reserved.

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.

Default partition scheme

The table below suggests a suitable starting point for most systems. Note that this is only an example, so feel free to use different partitioning schemes.

Firing up fdisk

Start fdisk with the disk as argument:

Command (m for help):

To view the available partitions, type in p:

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes
  
   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda2           488       976    499712   82  Linux swap
/dev/sda3             0      8635   8842240    5  Whole disk
/dev/sda4           976      1953   1000448   83  Linux native
/dev/sda5          1953      2144    195584   83  Linux native
/dev/sda6          2144      8635   6646784   83  Linux native

Note the Sun disk label in the output. If this is missing, the disk is using the DOS-partitioning, not the Sun partitioning. In this case, use s to ensure that the disk has a Sun partition table:

Building a new sun disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.
  
Drive type
   ?   auto configure
   0   custom (with hardware detected defaults)
   a   Quantum ProDrive 80S
   b   Quantum ProDrive 105S
   c   CDC Wren IV 94171-344
   d   IBM DPES-31080
   e   IBM DORS-32160
   f   IBM DNES-318350
   g   SEAGATE ST34371
   h   SUN0104
   i   SUN0207
   j   SUN0327
   k   SUN0340
   l   SUN0424
   m   SUN0535
   n   SUN0669
   o   SUN1.0G
   p   SUN1.05
   q   SUN1.3G
   r   SUN2.1G
   s   IOMEGA Jaz
Select type (? for auto, 0 for custom): 0
Heads (1-1024, default 64): 
Using default value 64
Sectors/track (1-1024, default 32): 
Using default value 32
Cylinders (1-65535, default 8635): 
Using default value 8635
Alternate cylinders (0-65535, default 2): 
Using default value 2
Physical cylinders (0-65535, default 8637): 
Using default value 8637
Rotation speed (rpm) (1-100000, default 5400): 10000
Interleave factor (1-32, default 1): 
Using default value 1
Extra sectors per cylinder (0-32, default 0): 
Using default value 0

The right values can be found in the documentation of the hard disk itself. The 'auto configure' option does not usually work.

Deleting existing partitions

It's time to delete any existing partitions. To do this, type d and hit Enter. Give the partition number to delete. To delete a pre-existing /dev/sda1, type:

Partition number (1-4): 1

Do not delete partition 3 (whole disk). This is required. If this partition does not exist, follow the "Creating a Sun Disklabel" instructions above.

After deleting all partitions except the Whole disk slice,a partition layout similar to the following should show up:

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes
  
   Device Flag    Start       End    Blocks   Id  System
/dev/sda3             0      8635   8842240    5  Whole disk

Creating the root partition

Next create the root partition. To do this, type n to create a new partition, then type 1 to create the partition. When prompted for the first cylinder, hit Enter. When prompted for the last cylinder, type +512M to create a partition 512 MB in size. Make sure that the entire root partition fits within the first 2 GB of the disk. The output of these steps is as follows:

Partition number (1-8): 1
First cylinder (0-8635): (press Enter)
Last cylinder or +size or +sizeM or +sizeK (0-8635, default 8635): +512M

When listing the partitions (through p), the following partition printout is shown:

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes
  
   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda3             0      8635   8842240    5  Whole disk

Creating a swap partition

Next, let's create the swap partition. To do this, type n to create a new partition, then 2 to create the second partition, /dev/sda2 in our case. When prompted for the first cylinder, hit Enter. When prompted for the last cylinder, type +512M to create a partition 512 MB in size. After this, type t to set the partition type, 2 to select the partition 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:

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes
  
   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda2           488       976    499712   82  Linux swap
/dev/sda3             0      8635   8842240    5  Whole disk

Creating the usr, var and home partitions

Finally, let's create the /usr, /var and /home partitions. As before, type n to create a new partition, then type 4 to create the third partition (we do not count the whole disk as being a partition), /dev/sda4 in our case. When prompted for the first cylinder, hit Enter. When prompted for the last cylinder, enter +2048M to create a partition 2 GB in size. Repeat this process for /dev/sda5 and sda6, using the desired sizes. When finished, the partition table will look similar to the following:

Disk /dev/sda (Sun disk label): 64 heads, 32 sectors, 8635 cylinders
Units = cylinders of 2048 * 512 bytes
  
   Device Flag    Start       End    Blocks   Id  System
/dev/sda1             0       488    499712   83  Linux native
/dev/sda2           488       976    499712   82  Linux swap
/dev/sda3             0      8635   8842240    5  Whole disk
/dev/sda4           976      1953   1000448   83  Linux native
/dev/sda5          1953      2144    195584   83  Linux native
/dev/sda6          2144      8635   6646784   83  Linux native

Save and exit

Save the partition layout and exit fdisk by typing w:

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 sparc 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 root partition (/dev/sda1) 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.