I have used e4rat before to speed up Ubuntu. My understanding of it is that it moves files around on the disk to speed up boot times. However, I am also under the impression that ureadahead fulfills the same purpose. Am I mistaken in their purposes, or are they redundant? And if so, which should I use? Finally, is ureadahead a standard GNU component, or is it Ubuntu-specific?
e4rat is intended for older hard disks with mechanical parts. It works by moving everything to the beginning of the platter. Ureadahead works by packing up files together used during boot -- thus it works no matter what the physical disk type is like. They cannot both be used at the same time, and if your using an ssd you can only use ureadahead.