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

[Duncan <1i5t5.duncan@cox.net>]

Austin S. Hemmelgarn posted on Tue, 17 Oct 2017 15:19:09 -0400 as
excerpted:

>> It's a single-device filesystem, thus disconnects are obviously fatal. 
>> But,
>> they never caused even a single bit of damage (as scrub goes), thus
>> proving btrfs handles this kind of disconnects well.  Unlike times
>> past, the kernel doesn't get confused thus no reboot is needed, merely
>> an unmount, "service nbd-client restart", mount, restart the rebuild
>> jobs.
> That's expected behavior though.  _Single_ device BTRFS has nothing to
> get out of sync most of the time, the only time there's any possibility
> of an issue is when you die after writing the first copy of a block
> that's in a dup profile chunk, but even that is not very likely to cause
> problems (you'll just lose at most the last <commit-time> worth of
> data).  The moment you add another device though, that simplicity goes
> out the window.

This is why I said if you /must/ work with USB-connected devices, I'd 
suggest single-device btrfs, tho I'd go full dup (data too) to take 
advantage of the btrfs ability to detect checksum errors and rewrite a 
bad copy from the second, hopefully good, copy.

Single device simply doesn't have the sync issues of multi-device, even 
in dup mode, so end up being much more reliable in cases like USB 
connection (and NBD as in the example above), where connection 
reliability is a problem.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman