Torvalds disagrees with you, at least wrt to the fundamental limitation here (there may be other issues layered on top of it of course).
> No, you need not just the blocks, you need the actual cache chain data structures themselves. For doing things like good read-ahead, you need to be able to (efficiently) look up trivial things like "is that block
already in the cache".
> So you need not only the data, you need the _tags_ too.
> In other words, your filesystem needs to have access to the whole
disk cache layer, not just the contents. Or it will not perform well.
https://yarchive.net/comp/microkernels.html
Edit: and the context of this discussion was fairly ancient systems with tagged TLBs and simple in order cores with syscalls nearly as cheap as regular user space call instructions. They were still ungodly slow with microkernels, and his explanation is the meat of his view as to why. It's all about having the data in the right place with as little synchronization required.
> No, you need not just the blocks, you need the actual cache chain data structures themselves. For doing things like good read-ahead, you need to be able to (efficiently) look up trivial things like "is that block already in the cache".
> So you need not only the data, you need the _tags_ too.
> In other words, your filesystem needs to have access to the whole disk cache layer, not just the contents. Or it will not perform well.
https://yarchive.net/comp/microkernels.html
Edit: and the context of this discussion was fairly ancient systems with tagged TLBs and simple in order cores with syscalls nearly as cheap as regular user space call instructions. They were still ungodly slow with microkernels, and his explanation is the meat of his view as to why. It's all about having the data in the right place with as little synchronization required.