Introduction to 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:
|Type of device||Default device handle||Editorial notes and considerations|
|SATA, SAS, SCSI, or USB flash||/dev/sda||Found on hardware from roughly 2007 until the present, this device handle is perhaps the most commonly used in Linux. These types of devices can be connected via the SATA bus, SCSI, USB bus as block storage. As example, the first partition on the first SATA device is called /dev/sda1.|
|NVM Express (NVMe)||/dev/nvme0n1||The latest in solid state technology, NVMe drives are connected to the PCI Express bus and have the fastest transfer block speeds on the market. Systems from around 2014 and newer may have support for NVMe hardware. The first partition on the first NVMe device is called /dev/nvme0n1p1.|
|MMC, eMMC, and SD||/dev/mmcblk0||embedded MMC devices, SD cards, and other types of memory cards can be useful for data storage. That said, many systems may not permit booting from these types of devices. It is suggested to not use these devices for active Linux installations; rather consider using them to transfer files, which is their design goal. Alternatively they could be useful for short-term backups.|
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.
Introduction to block devices
Placeholder for introduction to block devices specific to the relative architecture.
Designing a partition scheme
Placeholder for designing a partition scheme specific to the relative architecture.
Creating file systems
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...
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 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. The below is a non-exaustive list
- 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.
- 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.
- 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.
- IBM's high-performance journaling filesystem. JFS is a light, fast, and reliable B+tree-based filesystem with good performance in various conditions.
- 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.
- 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.
- Also known as FAT32, is supported by Linux but does not support standard UNIX permission settings. It is mostly used for interoperability/interchange with other operating systems (Microsoft Windows or Apple's OSX) but is also a necessity for some system bootloader firmware (like UEFI). Users of UEFI systems will need an EFI System Partition formatted with VFAT in order to boot.
- 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 filesystem for most cases. It should only be used for interoperability/interchange 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:
|Filesystem||Creation command||On minimal CD?||Package|
For instance, to have the root partition () as ext4 as used in the example partition structure, the following commands would be used:
When using 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:
mkfs.ext4 -T small /dev/<device>
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.
Mounting the root partition
Now that the partitions are initialized and are housing a filesystem, it is time to mount those partitions. Use the mount command, but don't forget to create the necessary mount directories for every partition created. As an example we mount the root partition:
If /tmp/ needs to reside on a separate partition, be sure to change its permissions after mounting:
chmod 1777 /mnt/gentoo/tmp
Later in the instructions the proc filesystem (a virtual interface with the kernel) as well as other kernel pseudo-filesystems will be mounted. But first we install the Gentoo installation files.