Understanding Disk Usage Difference in Ubuntu/Linux (df -h)

Overview

When checking disk usage in Ubuntu/Linux using the df -h command, administrators may notice that the total disk space does not exactly match the sum of used and available space.

Example:

df -h

Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        29G   15G   15G  50% /

At first glance:

15G Used + 15G Available = 30G

However, the total disk size displayed is only 29G.

Cause

This behavior is normal in Linux filesystems and occurs due to:

• Filesystem reserved blocks
• Filesystem metadata and journaling
• Human-readable rounding by df -h

Linux filesystems such as EXT4 reserve a percentage of disk space (typically 5%) for:

• Root/system processes
• Filesystem recovery
• Preventing complete disk exhaustion

Verification Steps

Check Exact Disk Usage in MB

Run:

df -BM

Example:

Filesystem     1M-blocks  Used Available Use%
/dev/root         30720M 14890M   14830M  50%

This shows the exact allocation without human-readable rounding.

Check Exact Usage in Bytes

Run:

df -B1

This displays precise byte-level usage information.

Check Reserved Block Information

Run:

sudo tune2fs -l /dev/root | grep "Reserved block"

Example output:

Reserved block count:     380000
Reserved blocks uid:      0
Reserved blocks gid:      0

Check Reserved Block Percentage

Run:

sudo tune2fs -l /dev/root | grep "Reserved block percentage"

Typically, EXT4 reserves approximately 5% of total disk space.