Take a walk through EXT4’s history, features, and optimal use, and learn how it differs from previous iterations of the EXT filesystem. You’ll also learn why we don’t have to defrag this filesystem. Although there are alreday newer file systems starting to go mainstream on Linux, EXT4 is still probably the most widely used filesystem.
See https://opensource.com/article/17/5/introduction-ext4-filesystem 
An introduction to Linux’s EXT4 filesystem | Opensource.com
Take a walk through EXT4’s history, features, and optimal use, and learn how it differs from previous iterations of the EXT filesystem.