ZFS also does better when the guests are stored on zvols. If volblocksize=4K is set on creation, it can avoid read-modify-write overhead. Regular files can be u

ZFS also does better when the guests are stored on zvols. If volblocksize=4K is set on creation, it can avoid read-modify-write overhead. Regular files can be used too, but those are somewhat higher overhead. In that case, recordsize=4K can be used. That said, I do not think the tests involved nesting CoW file systems.