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.

They use it for everything except MySQL databases, which are stored on XFS.

There was a recent discussion on LKML where Chris Mason (I think) described all this.

Does XFS have better performance than ext4 for MySQL DBs?

I really don't trust Synology anymore, had so many destroyed Raid's in the past mainly with RS407.

I buy a old Workstation slap FreeBSD with ZFS (RaidZ1 or 2) on it, and i am much happier.

Exactly the same way ReadyNAS solves it...

I observed that on my ReadyNAS too and always thought it was a weird setup.

Now I know there are proper reasons for it. Good to know.