Synology has solved it by running btrfs on top of mdraid, then patching it so that btrfs reports errors to mdraid and putting repair code into mdraid. This reli

Synology has solved it by running btrfs on top of mdraid, then patching it so that btrfs reports errors to mdraid and putting repair code into mdraid. This reliably works. I'm sure some would be interested in this, but Synology seems to not take GPLv2 too seriously and nobody seems to care about this. RAID 5/6 in btrfs is relatively neglected because the big users that pay developers to work on btrfs (and put the work up-stream) don't care that much about it. E.g. Facebook is probably using it for their HDFS storage, so HDFS gives them the RAID functionality.