All the parts (tiered caching, compression, checksums, redundancy, deduplication, journaling, network availability, FS migration...) likely exist separately; having them in a single filesystem (especially that doesn't require kernel patches, just a module) is quite pleasant.

And some of those things certainly benefit from being integrated into the FS.

I wonder if any knowledge at the filesystem level (as opposed to the block level, where bcache and dm-cache operate) could help L2ARC make better caching choices.

That maybe true, but not everybody runs ZFS on Linux and some of these features makes better sense to group in the filesystem driver anyway.

Plus it's slightly hypocritical to comment about how ZFS is growing features that already exist in Linux when Btrfs is the embodiment of reinventing features.

L2ARC has been around quite a while so it's much more likely that it's the other way around.

I was speaking about the preserved-across-reset portion (which this submission is related to) only.