Disk management and filesystems

Every piece of data on your Linux system lives on a block device: a hard drive, SSD, or virtual disk. Managing these devices means partitioning them, creating filesystems on the partitions, and mounting those filesystems into the directory tree. This article covers the full workflow.

Prerequisites

You should understand the Linux filesystem layout and be comfortable with command line tools.

Block devices

A block device is a storage device that reads and writes in fixed-size blocks. Check your block devices:

lsblk

Output:

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda      8:0    0   500G  0 disk 
├─sda1   8:1    0   512M  0 part /boot/efi
├─sda2   8:2    0   480G  0 part /
└─sda3   8:3    0  19.5G  0 part [SWAP]
sdb      8:16   0   100G  0 disk 

graph TD
  A[Physical Disk /dev/sdb 100GB] --> B[Partition Table GPT/MBR]
  B --> C[Partition 1 /dev/sdb1 50GB]
  B --> D[Partition 2 /dev/sdb2 50GB]
  C --> E[Filesystem ext4]
  D --> F[Filesystem xfs]
  E --> G[Mount point /data]
  F --> H[Mount point /backup]
  style A fill:#64b5f6,stroke:#1976d2,color:#000
  style E fill:#81c784,stroke:#388e3c,color:#000
  style F fill:#81c784,stroke:#388e3c,color:#000

sda is your first disk with three partitions. sdb is a second disk with no partitions yet.

# More detail about disk sizes
sudo fdisk -l /dev/sdb

Output:

Disk /dev/sdb: 100 GiB, 107374182400 bytes, 209715200 sectors
Disk model: Virtual disk
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes

Partition tables: MBR vs GPT

Use GPT for any modern system. MBR is only needed for legacy BIOS systems.

Checking disk space

# Disk usage summary by filesystem
df -h

Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda2       480G  123G  333G  27% /
/dev/sda1       511M   12M  500M   3% /boot/efi
tmpfs           7.8G   45M  7.8G   1% /tmp

# Directory sizes
du -sh /var/log/*  2>/dev/null | sort -rh | head -10

Output:

234M	/var/log/journal
45M	/var/log/syslog
12M	/var/log/kern.log
8.5M	/var/log/auth.log
3.2M	/var/log/dpkg.log

# Find what is using the most space
du -sh /* 2>/dev/null | sort -rh | head -10

Output:

123G	/home
45G	/var
12G	/usr
1.2G	/opt
456M	/boot

Example 1: Partition a disk, create ext4, and mount it

Let’s set up the unused /dev/sdb disk. We will create two partitions: one for data (80GB) and one for backups (20GB).

Step 1: Create partitions with fdisk

sudo fdisk /dev/sdb

Inside fdisk:

Command (m for help): g        # Create new GPT partition table
Created a new GPT disklabel.

Command (m for help): n        # New partition
Partition number (1-128, default 1): 1
First sector (2048-209715166, default 2048): 
Last sector: +80G

Created a new partition 1 of type 'Linux filesystem' and of size 80 GiB.

Command (m for help): n        # Second partition
Partition number (2-128, default 2): 2
First sector (167774208-209715166, default 167774208): 
Last sector (167774208-209715166, default 209715166):     # Use remaining space

Created a new partition 2 of type 'Linux filesystem' and of size 20 GiB.

Command (m for help): p        # Print partition table
Device       Start       End   Sectors  Size Type
/dev/sdb1     2048 167774207 167772160   80G Linux filesystem
/dev/sdb2 167774208 209715166  41940959   20G Linux filesystem

Command (m for help): w        # Write and exit
The partition table has been altered.

Verify:

lsblk /dev/sdb

Output:

NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
sdb      8:16   0  100G  0 disk 
├─sdb1   8:17   0   80G  0 part 
└─sdb2   8:18   0   20G  0 part 

Step 2: Create filesystems

# ext4 for the data partition
sudo mkfs.ext4 -L data /dev/sdb1

Output:

mke2fs 1.47.0 (5-Feb-2023)
Creating filesystem with 20971520 4k blocks and 5242880 inodes
Filesystem UUID: a1b2c3d4-e5f6-7890-abcd-ef1234567890
Superblock backups stored on blocks: 
	32768, 98304, 163840, 229376, 294912, 819200, 884736
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

# ext4 for the backup partition
sudo mkfs.ext4 -L backup /dev/sdb2

Step 3: Create mount points and mount

# Create directories
sudo mkdir -p /data /backup

# Mount the partitions
sudo mount /dev/sdb1 /data
sudo mount /dev/sdb2 /backup

# Verify
df -h /data /backup

Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb1        79G   24K   75G   1% /data
/dev/sdb2        20G   24K   19G   1% /backup

Step 4: Make mounts permanent with fstab

Without fstab entries, the mounts will be lost on reboot.

# Get the UUIDs (more reliable than device names)
sudo blkid /dev/sdb1 /dev/sdb2

Output:

/dev/sdb1: LABEL="data" UUID="a1b2c3d4-e5f6-7890-abcd-ef1234567890" TYPE="ext4"
/dev/sdb2: LABEL="backup" UUID="f1e2d3c4-b5a6-7890-cdef-123456789abc" TYPE="ext4"

# Back up fstab first!
sudo cp /etc/fstab /etc/fstab.bak

# Add entries
echo 'UUID=a1b2c3d4-e5f6-7890-abcd-ef1234567890 /data   ext4 defaults 0 2' | sudo tee -a /etc/fstab
echo 'UUID=f1e2d3c4-b5a6-7890-cdef-123456789abc /backup ext4 defaults 0 2' | sudo tee -a /etc/fstab

# Test fstab syntax (unmount and remount all)
sudo umount /data /backup
sudo mount -a

# Verify
df -h /data /backup

The fstab fields:

UUID=...     /data    ext4    defaults    0    2
|            |        |       |           |    |
device       mount    fs      options     dump fsck
             point    type                     order

• defaults = rw, suid, dev, exec, auto, nouser, async
• 0 = do not dump (backup)
• 2 = check filesystem second (after root /)

Example 2: Check disk health and usage

Monitor your disks to catch problems before they cause data loss:

# Check filesystem for errors (must be unmounted)
sudo umount /data
sudo fsck -f /dev/sdb1

Output:

e2fsck 1.47.0 (5-Feb-2023)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
data: 11/5242880 files (0.0% non-contiguous), 420325/20971520 blocks

# Remount after check
sudo mount /data

# Check SMART health (for physical disks)
sudo apt install smartmontools -y
sudo smartctl -a /dev/sda | head -30

Output:

=== START OF INFORMATION SECTION ===
Model Family:     Samsung SSD 870
Device Model:     Samsung SSD 870 EVO 500GB
Serial Number:    S1234567890
Firmware Version: SVT04B6Q
User Capacity:    500,107,862,016 bytes [500 GB]
SMART overall-health self-assessment test result: PASSED

# Monitor I/O in real time
iostat -x 1 3

Output:

Device     r/s     w/s   rkB/s   wkB/s  %util
sda       12.00   45.00  480.00  1800.00  5.20
sdb        0.00    2.00    0.00    32.00   0.10

LVM basics

LVM (Logical Volume Manager) adds a layer of abstraction between physical disks and filesystems. It lets you resize volumes, span multiple disks, and take snapshots.

# Install LVM tools
sudo apt install lvm2 -y

# Create a physical volume
sudo pvcreate /dev/sdc

# Create a volume group
sudo vgcreate mygroup /dev/sdc

# Create a logical volume (50GB)
sudo lvcreate -L 50G -n mydata mygroup

# Create a filesystem on the LV
sudo mkfs.ext4 /dev/mygroup/mydata

# Mount it
sudo mkdir /mydata
sudo mount /dev/mygroup/mydata /mydata

# Extend the volume later (add 20GB)
sudo lvextend -L +20G /dev/mygroup/mydata
sudo resize2fs /dev/mygroup/mydata

The main advantage: you can resize volumes without unmounting them (for ext4 with resize2fs). This is critical for production servers where downtime is expensive.

What comes next

The next article covers Logs and system monitoring, where you will learn to read system logs, use journalctl, and track down problems.

For the security perspective on disk management, understanding where data lives is essential for incident response and forensic analysis.