COW and volume management?

> Maybe you could find and list at least one flaw?

ARC is not well-integrated with the rest of the memory management system, so the system can OOM before ARC detects memory pressure and shrinks itself (so it's more than just an "ARC presents itself as used memory which confuses people" UI issue).

The cache for mapped files is unnecessarily duplicated in both ARC and the page cache.

There's still no support for defragmentation, which is an issue on high-speed IOPS-limited NVMe drives as CoW filesystems tend to experience heavy fragmentation.