Systemd Readahead split

Background

Systemd upstream decided to drop readahead since 217 version [1,2] even if no many alternatives for HDD users apart of migrating to SSD exist [3], because of that, this aims to keep supplying the readahead systemd implementation for now until a better alternative exists.
  1. readahead: wipe out readahead
  2. remove references of readahead
  3. [systemd-devel] [HEADS-UP] Intent to remove readahead from systemd
As usual, if you want to help, provide patches and so feel free to open a bug report to bugs.gentoo.org. The same if you want to report a bug of course ;)