If you're using copy-on-write snapshots, then the total size of your snapshots should be small since most of the data in said immutable content store never changes. But the benefit is a bit-error between one mirroring operation and the next doesn't overwrite your unchanged, good data on the slaves.

The problem I think needs more attention here is ext4 silently corrupting data. ZFS has it exactly right with the built-in checksumming on write and read - it can't stop a disk going bad, but it can tell you exactly what's affected and _when_ - corruption would've been caught the moment the mirroring operation tried to read back bad data (and would've faulted the process, rather then happily return bad data).