Re: Is it safe to use btrfs on top of different types of devices?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-10-19 10:42, Zoltan wrote:
> On Thu, Oct 19, 2017 at 4:27 PM, Austin S. Hemmelgarn
> <ahferroin7@gmail.com> wrote:
> 
>> and thus when the same device reappears (as it will when the disconnect was
>> due to a transient bus error, which happens a lot), it shows up as a
>> different device node, which gets scanned for filesystems by udev, and BTRFS
>> then gets really confused because it now sees 3 (or more) devices for a 2
>> device filesystem.
> 
> And what would happen with a regular, single-device BTRFS volume after
> a reconnect? Isn't this issue just as bad for that case?
No, because the multi-device code only gets used if the filesystem 
claims to have more than one device, and it's a bug in the multi-device 
code that causes this problem.  From a data safety perspective, the 
disconnect will look like a power loss event if it was a single device 
filesystem, and BTRFS handles that situation fine (though you would 
probably need to remount the filesystem).

FWIW, the same bug causes similar data loss problems with block-level 
copies of BTRFS filesystems (if you then mount either the original or 
the copy while both are visible to the system), and allows you to screw 
up multi-device filesystems by connecting a storage device with a 
carefully crafted bogus BTRFS filesystem on it.  Overall though, it's 
not been seen as a high priority bug because:

1. Nobody has come up with a reliable method of handling it that doesn't 
break anything or require revising the on-disk layout.
2. It's easy to work around (don't do block level copies and ensure 
proper physical security of the system like you should be doing anyway).