On 2015-10-14 05:13, Hugo Mills wrote:
> On Wed, Oct 14, 2015 at 05:08:17AM +0000, Duncan wrote:
>> Carmine Paolino posted on Tue, 13 Oct 2015 23:21:49 +0200 as excerpted:
>>
>>> I have an home server with 3 hard drives that I added to the same btrfs
>>> filesystem. Several hours ago I run `btrfs balance start -dconvert=raid0
>>> /` and as soon as I run `btrfs fi show /` I lost my ssh connection to
>>> the machine. The machine is still on, but it doesn’t even respond to
>>> ping[. ...]
>>>
>>> (I have a 250gb internal hard drive, a 120gb usb 2.0 one and a 2TB usb
>>> 2.0 one so the transfer speeds are pretty low)
>>
>> I won't attempt to answer the primary question[1] directly, but can point
>> out that in many cases, USB-connected devices simply don't have a stable
>> enough connection to work reliably in a multi-device btrfs.  There's
>> several possibilities for failure, including flaky connections (sometimes
>> assisted by cats or kids), unstable USB host port drivers, and unstable
>> USB/ATA translators.  A number of folks have reported problems with such
>> filesystems with devices connected over USB, that simply disappear if
>> they direct-connect the exact same devices to a proper SATA port.  The
>> problem seems to be /dramatically/ worse with USB connected devices, than
>> it is with, for instance, PCIE-based SATA expansion cards.
>>
>> Single-device btrfs with USB-attached devices seem to work rather better,
>> because at least in that case, if the connection is flaky, the entire
>> filesystem appears and disappears at once, and btrfs' COW, atomic-commit
>> and data-integrity features, kick in to help deal with the connection's
>> instability.
>>
>> Arguably, a two-device raid1 (both data/metadata, with metadata including
>> system) should work reasonably well too, as long as scrubs are done after
>> reconnection when there's trouble with one of the pair, because in that
>> case, all data appears on both devices, but single and raid0 modes are
>> likely to have severe issues in that sort of environment, because even
>> temporary disconnection of a single device means loss of access to some
>> data/metadata on the filesystem.  Raid10, 3+-device-raid1, and raid5/6,
>> are more complex situations.  They should survive loss of at least one
>> device, but keeping the filesystem healthy in the presence of unstable
>> connections is... complex enough I'd hate to be the one having to deal
>> with it, which means I can't recommend it to others, either.
>
>     Note also that RAID-0 is a poor choice for this configuration,
> because you'll only get 640 GB usable space out of it. With single,
> you'll get the full sum of 2370 GB usable. With RAID-1, you'll have
> 320 GB usable. The low figures for the RAID-0 and -1 come from the
> fact that you've got two small devices, and that both RAID-0 and
> RAID-1 have a minimum of two devices per block group. You can play
> around with the configurations at http://carfax.org.uk/btrfs-usage
>
>     But I second Duncan's advice about not using USB. It's really not a
> reliable configuration with btrfs.
I'd also second that statement, but go even further and say to not use 
USB for anything except backups and transferring data between computers 
unless you have absolutely no other option, and be wary of using any 
externally connected storage device for other use cases (I've seen 
similar issues with eSATA drives and BTRFS, and have heard of such 
issues with some Thunderbolt connected storage devices).