Home-brew automatic tiered storage solutions with Linux? (Memory -> SSD -> HDD -> remote storage)

Solution 1:

Well, ZFS uses a storage layering called Hybrid Storage Pool (HSP):

1. Layer: memory
2. Layer: SSD-based read-caches (L2ARC) and write-caches (separate intent log aka slog).
3. Layer: (cheap) harddisks

With HSP its easy to automatically benefit from the advantages of SSDs compared to a harddisk-only solution. A system using HSP can be both faster and cheaper than the latter. See this link for some nice examples and more details.

I think there are plans regarding hierarchical storage management (HSM) for ZFS (see for example the Automatic Data Migration (ADM) OpenSolaris project) but I don't know its current status.

Solution 2:

Check out the LVM based "lvmts" (LVM Tiered Storage) solution this guy is cooking up:

https://bbs.archlinux.org/viewtopic.php?pid=1140640#p1140640

Pretty cool.