Metadata writes can be considered atomic, so an integrated checksum is written at the time the metadata block is written, or overwritten. Whereas with data, you

Metadata writes can be considered atomic, so an integrated checksum is written at the time the metadata block is written, or overwritten. Whereas with data, you can't do overwrites of either data or checksum, it's not atomic and any kind of crash or powerfailure will result in mismatching data to checksum. So unless you have something really clever to work around this, you need a copy on write file system to do data checksums.