with hardware raid, if you have data corruption that doesn't bring down a whole drive, you have no idea which data copy is the correct one. (at least not in an

with hardware raid, if you have data corruption that doesn't bring down a whole drive, you have no idea which data copy is the correct one. (at least not in an automated way that doesn't require manual work) With zfs, it has checksumming so it knows which data copy is the correct one. As drives get bigger, the odds of bit decay, while being low on a per-bit level, become great enough for the drive in total, and is a concern. File systems have had checksumming, but if there is no exposure into the raid layer, the file system can't use this checksum to recover from bit decay errors since it can't control which drive processes a request. This is why zfs operates at both layers. zfs vs btrfs is harder, and i'd guess its the zfs layered read and write caching systems enticing people in.