@evert @mcc Yeah, if the question is whether *any* such format exists, you could make a ZFS filesystem with dedup and compression enabled, write the data, then ‘zfs send’ it to a file. The file could then be ‘zfs receive’d or mounted. Since dedup only matters on write, you just don’t use dedup on the receiving end, and you don’t pay the RAM penalty.

That’s more or less how I make base images for zones.