I have looked forward a way to cache a big slow drive on another quick drive (in a similar way as Windows7 ReadyBoost already do with flash usb drives).
I have found out this interesting project, flashcache (GitHub) which is a facebook released code;
Flashcache works at the block device level and take advantege of quick SSD drives.
Microsoft ReadBoost caches little files by taking advantage of slower flash device (access time of slower flash device is still lower than hard disk one).
So, ReadyBosst and flashcache are similar but different.
I'll surely try flashcache in the future but it's a shame it doesn't come bundled with vanilla kernels.
More infos here: Facebook FlashCache Post
