Comment by Youden

I wasn't actually able to find any real documentation on how Synology's SHR works.

Their recovery documentation [0] indicates that SHR is just plain mdadm + LVM and a couple of NAS recovery sites [1,2] indicate the same.

In the end I got a Reddit post [3] with a response from a Synology representative who says that the btrfs filesystem will request a read from a redundant copy from mdadm in order to correct checksum errors.

I wonder whether this is unique to Synology or whether the change has been upstreamed into the main Linux kernel.

[0]: https://www.synology.com/en-global/knowledgebase/DSM/tutoria...

[1]: https://support.reclaime.com/kb/article/8-synology-shr-raid/

[2]: http://www.nas-recovery.com/kb_hybrydraid.php

[3]: https://www.reddit.com/r/DataHoarder/comments/5yb13m/anyone_...