Bit Rot functionality.

[2ndNAS]

Before I received my RN202, I read about Bit Rot (http://arstechnica.com/information-
technology/2014/01/bitrot-and-atomic-cows-inside-next-gen-filesystems/2/) and got the impression that Bit Rot was active during file read and would detect media degradation and (with data dedundancy) repair the read errors. 

Reading more about Bit Rot I understod that (at least with Readynas) Bit Rot is active during rewrite, and rewrites modified blocks to another disk area.

Then I found ftp://ftp.netgear-info.de/download/PRESSE/Retail-Business-Unit_B2C/Storage/ReadyNAS%20New%20Family/R...

Under the photos it describes that before the introduction of Bit Rot Protection from NETGEAR it was possible to fix read errors with the correct bit off the mirrored device. This equals the description in the arstechnica article.
Then it says "ReadyNAS does not need multiple devices in a mirrored or RAID setup to protect against bit  rot." Does it mean that Readynas fixes Bit Rot read errors in a single disk setup? I don't know what to believe.

Can you explain the advantage of Checksum (with both Bit Rot and snapshot disabled) in a single disk  environment? In a Raid 1 environment? 
Does Checksum enable BTRFS to correct read errors in a Raid 1 environment?
I have read that for every block written a CRC32 checksum is saved in metadata, and expect that CRC32 alone only can be used for error detection. 

- What happens when BIT Rot protection is enabled (snapshots disabled) in a single disk environment? In a  Raid 1 environment?

 Does Bit Rot enable BTRFS to correct read errors in a Raid 1 environment? 

I have not found any mention of why Bit Rot uses CoW, would be good to understand.

[StephenB]

The bit rot algorithm in OS6 was developed by Netgear, and is not the method built into btrfs that was described in your ars technica link.

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@2ndNAS wrote:

Then it says "ReadyNAS does not need multiple devices in a mirrored or RAID setup to protect against bit  rot." Does it mean that Readynas fixes Bit Rot read errors in a single disk setup? 

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No. That's a very carefully crafted sentence - "multiple devices" means multiple NAS not multiple disks.

The OS 6 algorithm does require RAID - later on in the same writeup it says "the ReadyNAS performs bit rot protection on 2 layers of storage – the BTRFS layer and the lower MDADM RAID layer."  So it is using btrfs checksums together with RAID parity.

Going any further would just be speculation, since Netgear isn't giving the details.