Comment by Youden
6 years ago
If you use LVM/mdadm for RAID, it's not possible for btrfs to correct checksum mismatches (i.e. protect against bitrot).
6 years ago
If you use LVM/mdadm for RAID, it's not possible for btrfs to correct checksum mismatches (i.e. protect against bitrot).
That's a good point, though Synology (my brand of NAS) claims that they've developed analogous corruption checks operating at the LVM level, so you get the benefits of btrfs (including checksum checks and RAID scrubbing) without having to actually use its RAID implementation.
https://www.synology.com/en-global/knowledgebase/DSM/help/DS...
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_...