@daftaupe @fink @martin
Just checked this link, thanks.
Metadata miroring is a FS feature. For exemple ext4 make many copy of the superbloc. Guess that one day ext(5?) will offer more. In fact, I don't feel the need to have multiple copy of metadata because :

hardware problems (badblock) can't be an issue anymore with raidX

Logical problem (silent corruption) is also an issue already solved by mdadm with raid6. (callded "self healing" on synology link)

For last feature, snapshots >LVM.