Linux Filesystems

Filesystem management means creating, attaching, checking, and maintaining the structures that store files on disk. In Linux, good filesystem habits directly affect reliability and make troubleshooting faster when a system won't boot or a drive goes read-only. For CompTIA A+ Core 2 (220-1202), Domain 1, Objective 1.9, the focus stays practical: you should know how to check filesystems with fsck and how to attach storage with mount.

This topic feels abstract until something breaks. Then it becomes very concrete. A missing mount can make /home look empty, and a corrupted filesystem can block boot. The good news is that Linux gives clear tools to observe what's happening, as long as you use them in the right order.

Examples below use common distros (Ubuntu, Debian, and RHEL-based systems). Many commands require sudo, especially when mounting and repairing filesystems.

How Linux disks and filesystems fit together

Think of storage like a library building. The physical drive is the building, partitions are floors, and a filesystem is the catalog system on each floor. Without the catalog, the books might still exist, but you can't reliably find them.

Linux separates four ideas that beginners often mix up:

A device is the hardware (or virtual hardware) that stores data. On most systems, Linux names SATA and many USB drives as /dev/sda, /dev/sdb, and so on. NVMe devices often look like /dev/nvme0n1. These names identify the whole drive, not a usable slice.

A partition is a defined region on that device. Linux shows partitions with a suffix like /dev/sda1, /dev/sda2, or for NVMe, /dev/nvme0n1p1. Partitions let one disk hold separate purposes, such as a root filesystem and a data filesystem.

A filesystem is the on-disk structure that tracks files, folders, permissions, and free space. Common Linux filesystems include:

• ext4 (common default on many distros)
• xfs (common on RHEL-based systems)
• btrfs (often chosen for snapshots on some distros)
• vfat (common on USB sticks and the EFI System Partition)

A mount point is the folder where Linux attaches that filesystem into the system's single directory tree. Linux does not use drive letters. Instead, it presents one unified tree rooted at /.

Here's a practical mapping you'll see often:

• /dev/sda1 mounted at / (the root filesystem)
• /dev/sda2 mounted at /home (user data kept separate)
• A USB drive like /dev/sdb1 mounted at /mnt/usb for temporary access, or under /media/<username>/ on desktop systems

This model explains many "missing files" reports. If /home fails to mount, the system may boot anyway. However, you could land in a nearly empty /home folder that actually sits on the root filesystem. The data is usually still on disk, it's just not attached where you expect.

The key terms you must know for the exam

Use these definitions as a quick mental checklist before you run any disk command:

• Block device: A device file under /dev that supports block I/O, such as /dev/sda or /dev/nvme0n1.
• Partition: A numbered section of a disk, such as /dev/sda1, used to hold a filesystem or swap.
• Filesystem: The format that organizes data and metadata on a partition (for example, ext4 or xfs).
• Superblock: Key filesystem metadata that describes the filesystem's layout and state (high level, not a user file).
• Mount point: A directory (like /home or /mnt/usb) where a filesystem becomes accessible.
• Directory tree: Linux's single hierarchy starting at /, where all mounted filesystems appear.
• UUID: A unique identifier for a filesystem, often used in /etc/fstab because it stays stable.
• Swap: Disk space used as virtual memory, either as a partition or a swapfile.

Always confirm the target device before you act. A single wrong /dev/sdX can cause data loss, especially with repair tools.

Quick ways to identify what is mounted right now

Before mounting or repairing, first answer a simple question: what's attached, and where? These commands give that picture from different angles.

Command	Best for	What to look for
lsblk	A tree view of disks, partitions, and mount points	Device name, partition layout, mount point column
df -h	Space usage by mounted filesystem	Size, used/free space, and mount path
mount or findmnt	Mount details and options	Source device, mount point, options like ro or rw
blkid	Filesystem type and UUID	TYPE= and UUID= fields for /etc/fstab

When you read the output, focus on four fields: device name, filesystem type, mount point, and free space. For example, df -h might show /dev/sda2 mounted on /home at 92 percent used. That quickly points to a capacity issue, not a permission problem.

lsblk is often the safest first step because it ties the pieces together.