Add some form of Automatic Storage Tiering (AHSM)

This can potentially bring enormous performance gains at relatively small cost and could give RHEL a unique selling point. What I mean by Automatic Storage Tiering is the movement of blocks between fast and slow storage, based on their usage (SSD <-> SCSI <-> SATA). There are allready some things floating around: There is flashcache from facebook, dm-cache, and this is also a nice initative: https://bbs.archlinux.org/viewtopic.php?id=113529 . I like the approach to make it a device-mapper module. This means it can work with LVM, raw disk, etc. another idea could be to bring it into the new gluster product, allthough I don't know wether that would fit.