What is an Inode?

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What is an Inode?
Inode or index node is part of the UNIX/LINUX filesystem. It contains information that is needed in accessing a particular file. This information includes the inode number, file type, file owner identifier (UID), group ID of the owner (GID), file access permissions, time when file was accessed, modified, or changed, number of links to the file, size of the file, and the contents for the disk address in a file.

Inode is a flat array that stores index numbers for each file, but it does not contain any information about the filename. Every inode is unique for a particular file in the UNIX Operating System. One inode is retained in memory for a particular file, so when there are multiple processes in UNIX that would like to access the same file, only one copy of the inode is needed. This particular inode is called “incore inode”. Common in many file system types today, available inodes are fixed when a filesytem is created. This means there is a limit or maximum number of files that a particular system can hold. In normal instances, only 1% is devoted to inodes when creating a file system.

When somebody opens a particular file, this file’s inode will then be read by the kernel. In application, if you open multiple files or folders, multiple inodes will also be used, leading to more system resources to be consumed. This explains the reason that inodes may be limited by some web hosting services. To control usage of particular server, some web hosts will offer a fix limit on inodes. When a particular subscriber reaches the limit, his/her account may be suspended. According to experts, 250,000 inodes could be enough for normal usage. But one must check before any web hosting subscription is done to make sure that accounts will not be suspended by a limit on inode usage.

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