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It's still not great. Linux native caching keeps fighting with the ZFS cache and it's not quite as freeable as native cache. If you try to launch some applications that allocates big chunks of memory quickly, it may fail due to ZFS not freeing its memory fast enough. I had this issue before limiting the max ZFS cache size. If ZFS could integrate the caching better with Linux it would be perfect to me.

How does FreeBSD handles it?

But is it double buffering pages?

No, ZoL bypasses the usual page cache.

Is it doing it for mmap though? 2 years ago, it didn't double-buffer apart from mmaped files. Here's a note about priorities: https://news.ycombinator.com/item?id=11697901

Also seems to be solved in this port: https://www.crossmeta.io/another-zfs-port-on-linux/

Given that said port predates feature flags and has no source posted, making any assertions about what it actually does versus claims to do seems premature.

My problem is exactly that; something is periodically dropping a 16G cache...