On 2018-12-13 02:29 AM, Adam Borowski wrote:

> 
> For btrfs, a block device is a block device, it's not "racist".
> You can freely mix and/or replace.  If you want to, say, extend a SD
> card with NBD to remote spinning rust, it works well -- tested :p
> 

The possibility ff NBD certainly intrigues me, but I would be concerned
about the write barriers,, (needed to make sure cached writes are
committed to spinning rust in the correct order, so as to avoid
corruption in the case of interruption.

Assuming the underlaying block device on the remote device has a working
implementation, (as we know, not always a safe assumption...) does NBD
properly pass along the barriers commands/support?