e4rat (“Ext4 - Reducing Access Times”) is a toolset to accelerate the boot process as well as application startups. Through physical file realloction e4rat eliminates both seek times and rotational delays. This leads to a high disk transfer rate.
In other words, E4rat works out which files your machine uses while booting up (and immediately afterwards), then puts them in order on your disk so they can be read more quickly. As the name suggests, it only works with the ext4 filesystem.
Via the Arch Forums, where some users have cut their boot time in half with E4rat