The nice feller over at phoronix brings us this handy to have bit of info:
It turns out the RAID5 and RAID6 code for the Btrfs file-system's built-in RAID support is faulty and users should not be making use of it if you care about your data.
There has been this mailing list thread since the end of July about Btrfs scrub recalculating the wrong parity in RAID5. The wrong parity and unrecoverable errors has been confirmed by multiple parties. The Btrfs RAID 5/6 code has been called as much as fatally flawed -- "more or less fatally flawed, and a full scrap and rewrite to an entirely different raid56 mode on-disk format may be necessary to fix it. And what's even clearer is that people /really/ shouldn't be using raid56 mode for anything but testing with throw-away data, at this point. Anything else is simply irresponsible."
Just as well I haven't gotten around to trying it then.
(Score: 2) by rleigh on Sunday August 07 2016, @06:38PM
Yep. I argued with my immediate boss about this at a previous job. He wouldn't have any of it, after all, the systems they had sold were all working fine and no customer had complained. A while later, a thunderstorm over a customer's shop caused a lightning strike, which went through the phone line, modem, serial port, mainboard, through the HDD cables and blew up the *same* chip on the HDD controller board on both HDDs--I saw the charred remains personally when we got them sent back. Total unrecoverable dataloss, even after sending the drives to a specialist data recovery firm. In the next revision of the hardware, we replaced one HDD with a USB port and had the customers do a nightly full backup and rotate them; they could then transfer that data elsewhere as they liked. But it took a huge disaster which showed up the incompetence, along with the financial/legal consequences, to effect any change; simple reason and logic was not sufficient, sadly. Before that, they were convinced that two disks in the same system were a backup, and told customers this. After, they finally realised how uniquely vulnerable the data is when it's all in that one place. They used to tell people to remove one (in a caddy) and put it in a safe; but when it's in place during regular operation there's simply no recovery from failure.
(Score: 2, Informative) by Anonymous Coward on Monday August 08 2016, @07:33AM
Morons.
In my experience, the primary use of backups is not hardware faults (including lightning-induced hardware faults), but human or software error overwriting or deleting stuff.
In this situation, RAID will ensure that the mistake is propagated to every drive within milliseconds.
Snapshots can remedy the human factor somewhat (as long as the person making the mistake doesn't have root access), but it doesn't protect against software problems (including controller and file system drivers).