Does anyone know if there has been any talk about implementing HSM (Hierarchical Storage Management) in Linux? I'm aware there are one or two (very expensive) proprietary solutions out there, but it strikes me that now is a good time to at least consider how you would implement it and what you need from the various layers to handle it. Since we have two potential new generic file systems in the works, whose on-disk layout hasn't been fixed yet I can't think of a better time.
Obviously HSM is one of those things that crosses the traditional layering, but BTRFS at least is already handling multi layer issues.
Implementing a linux native HSM strikes me as one of those game changers, we'd have a huge feature none of the other OS's can currently match without large expenditure. I've lost count of the number of situations where organizations have bought hugely expensive SCSI or FC storage systems with loads of capacity, where what they actually needed was just a few high performance disks (or even SSDs nowadays) backed by a slower but high capacity set of SATA disks. Even small servers or desktops probably have a use for this, that new disk you just bought to expand capacity is probably faster that the old one.
Using tape libraries at the second or third level of the HSM has a few more complications, but could be tackled later.