1. Introduction

In Unix-like systems, inodes are data structures that describe files and directories. The number of possible inodes is limited and set during partition creation. That means we can run out of them and be unable to create any new files, even if we have space on the device. In this tutorial, we’ll learn how to prevent this situation and how to deal with it if it happens.

2. Why inodes Are Limited

Each inode contains crucial information about its file, like its attributes and disk block locations. This data is necessary for the system to use the file. In file systems from the family of extended file systems, the default for Linux-based systems, inodes are stored in a fixed-sized table. The size of this table is decided upon the creation of the partition and can’t be changed later.

Other file systems (for example, APFS used on macOS) don’t use fixed-sized tables but, instead, use other data structures like B-trees. Thus, the number of possible inodes is much more flexible. It’s still limited by how big an index can be stored in a 64-bit integer (or 32-bit on older file systems), but that’s a limit that’s hard to hit.

3. Setting the inode Limit

The inode limit is set during partition creation. There are some sensible defaults in place, so we usually don’t need to worry about it. However, if we know that we’ll store a huge number of tiny files, we can override these defaults. For example, we could anticipate that the average file in our system will have a size of only 1 kB. Let’s create an ext4 partition with one inode for every thousand bytes:

$ sudo mkfs.ext4 -i 1000 /dev/sdev

Alternatively, we could choose another file system, like Btrfs, that doesn’t have the same problem with limited inodes.

4. Check and Free inodes

We can check the available inodes using the df command:

$ df -i
Filesystem     512-blocks      Used  Available Capacity iused      ifree %iused  Mounted on
/dev/disk1s5s1  489620264  46865488   34089872    58%  568975 2447532345    0%   /
/dev/disk2s1   1953456384 727555584 1225900800    38% 2842014    4788675   37%   /Volumes/T7

The “iused” column tells us the number of used inodes, “ifree” gives us the number of free inodes, and the “%iused” column tells us the percentage of used inodes.

Unfortunately, there is no way to free inodes other than deleting files we don’t need. The problem is that we sometimes don’t know where to look for files that drain the inode limit. One way to tackle this is to sort directories by the number of files in them. By doing so, we can quickly locate problematic directories.

We can achieve that by listing all the files, selecting only the first directory in the path, and then counting how many occurrences of each directory we have:

$ sudo find . -xdev -type f | cut -d "/" -f 2 | sort | uniq -c | sort -n
1585 Documents
7168 Pictures
113659 .nvm
669666 Library
980996 Projects

As we can see, we store the highest number of files in the “Projects” directory. At this moment, we can decide to investigate further or take some action like deleting files or moving that directory to some other drive.

5. Conclusion

In this tutorial, we examined what inodes are and why they are limited in number. We looked at preventing issues with inodes by configuring the file system or choosing a file system more suitable for our needs. Then, we learned how to analyze inode usage and locate problematic directories.

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