Disk spin-down problem, RN104, fw 6.2.0

dsm1212 wrote:
Ah, ok, I think what it does then is read from both the mirrors and if it gets an IO error from one side of the mirror it will rewrite the data successfully read from the other side. md supports this automatically, you just need to trigger a read from both sides and it will happen. So bitrot protection will wake up the disks unless the program driving it checks to see if the disks are in standby.

Well, normal RAID handles read IO errors so there's no need to add more protection for that. Bitrot protection deals with the case where the data was "silently" corrupted w/o an I/O error to trigger the recovery. And the bitrot might only affect the checksums themselves, so that possibility needs to be covered.

So bitrot recovery has to kick in after the btrfs checksum fails but when no read errors have occured. Once you know the checksum is bad, there are several strategies you could attempt to use. The simplest I can think of is to simulate read errors on each of the data blocks covered by the checksum, and see if substituting these recovered blocks results in a good checksum. We are just speculating of course, since Netgear isn't saying.