Inodes
Concept
An inode is a filesystem data structure that stores metadata about a file or directory (permissions, owner, size, timestamps, pointers to data blocks, etc.).
The actual filename lives in the directory entry; the inode holds everything else.
Every filesystem has a fixed number of inodes created at format time (or dynamically managed in some modern filesystems).
Why it matters
A filesystem can report “No space left on device” even when df -h shows free space, if it has run out of inodes.
This is especially common with millions of small files (mail queues, session files, container layers, monitoring metrics, etc.).
Mental Model
Filesystem capacity has two limits:
1. Data blocks → shown by df -h
2. Inodes → shown by df -i
You can have free blocks but zero free inodes (or the reverse).
Key Commands
# Inode usage overview
df -i
# Detailed view for one filesystem
df -i /var
# Find directories containing huge numbers of files
find /var -xdev -type d -exec sh -c 'echo $(find "$1" -maxdepth 1 -type f | wc -l) files in $1' _ {} \; 2>/dev/null | sort -nr | head -20
# Count inodes used under a path
find /path -xdev -printf "%i\n" | sort -u | wc -lCommon Failure Modes & Symptoms
| Symptom | Likely cause | First checks |
|---|---|---|
| “No space left on device” but df -h OK | Inode exhaustion | df -i |
| Cannot create new files/directories | No free inodes | df -i, find large directories |
| Sudden inode exhaustion | Application creating millions of small files | Check mail, sessions, tmp, containers |
Investigation Tips
- Always run both
df -handdf -iwhen investigating space issues. - Common culprits: PHP sessions, mail spools, Nginx/Apache cache, Docker overlay, monitoring agents, CI artifacts.
- On XFS and some other filesystems inode allocation is more flexible, but you can still run out.