[linsomniac]

>What I would really like to see is a two-tier cache system

Is there any sort of Linux HSM (Hieracrhical Storage Manager)? I haven't see any and have been a bit surprised nothing has really developed there. They can manage putting hot data in RAM, SSDs, colder or larger data on spinning rust, deep freezing onto a tape silo or a cloud storage...

Some of the NAS devices and RAID cards can support a two-tier caching or data migration using SSDs, where hot or highly-random data (usually identified by smaller write sizes) go to the SSDs, and then can migrate to the spinning discs.

I've done some "poor mans" version of this using LVM, where I can "pvmove" blocks of a logical volume between spinning discs and SSDs, which is pretty slick, but a very crude tool.

[folmar]

CASTOR comes to mind for a start.

Take a look a the CERN paper https://iopscience.iop.org/article/10.1088/1742-6596/331/5/0... as they have a large use case.

[tadfisher]

Not a general kernel facility that I know of. I use nfscache every day though; my Steam data directory lives on NFS, and I set up nfscache with a 100GB LRU storage. This way I can avoid the "backup/restore" dance and have all my games installed, at the cost of waiting up to a few minutes to warm the cache for a new game.

[pdimitar]

I don't know about a manager per se but `bcachefs` for Linux seems to do a good chunk of what you're after.