It was my understanding that both dedup and caching work on block level. So if you have identical on-disk blocks (same original data past same compression and encryption), they turn into one(*) on-disk block with several references from DDT. And that one block is only cached once, saving ARC space.
* (Technically, for very-often referenced blocks there is a number of copies, controlled by ditto attribute). HTH, //Jim Klimov _______________________________________________ zfs-discuss mailing list firstname.lastname@example.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss