Re: synchronize btrfs snapshots over a unreliable, slow connection

[Roman Mamedov <rm@romanrm.net>]

On Tue, 5 Jan 2021 11:24:24 +0000
Graham Cobb <g.btrfs@cobb.uk.net> wrote:

> used that approach as the (old) NAS I was using had a very old linux
> version and didn't even run btrfs.

One anecdote --

I do use an old D-Link DNS-323 NAS with old kernel and distro (older Debian),
and only ~60 MB of RAM to serve a 8 TB disk or two. How does that even work?

Simple: it exports the disk(s) over the network as block devices via NBD, and
they are mounted remotely on a much more modern and powerful host.

A bit of a secret sauce surprisingly turned out to be the QEMU's NBD server
(qemu-nbd), it allows to set disk access modes inherited from QEMU itself, and
there with "--cache=none" the little thing doesn't choke (anymore) RAM-wise,
even with full jumbo-frames enabled on the network side.

(Other NBD servers were much less performant and/or reliable on the hardware).

Transfer speeds are around 17 MBytes/sec. That's on a Gbit LAN, and admittedly
running blockdevice-level access over a network does prefer to have a low ping
for good performance.

-- 
With respect,
Roman