From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 192D5C433DB for ; Tue, 5 Jan 2021 11:54:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D3E07229CA for ; Tue, 5 Jan 2021 11:54:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729527AbhAELyO (ORCPT ); Tue, 5 Jan 2021 06:54:14 -0500 Received: from rin.romanrm.net ([51.158.148.128]:60024 "EHLO rin.romanrm.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726074AbhAELyN (ORCPT ); Tue, 5 Jan 2021 06:54:13 -0500 Received: from natsu (unknown [IPv6:fd39::e99e:8f1b:cfc9:ccb8]) by rin.romanrm.net (Postfix) with SMTP id 9B2D37A9; Tue, 5 Jan 2021 11:53:31 +0000 (UTC) Date: Tue, 5 Jan 2021 16:53:31 +0500 From: Roman Mamedov To: Graham Cobb Cc: Forza , Cedric.dewijs@eclipso.eu, linux-btrfs@vger.kernel.org Subject: Re: synchronize btrfs snapshots over a unreliable, slow connection Message-ID: <20210105165331.31aa8aee@natsu> In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Tue, 5 Jan 2021 11:24:24 +0000 Graham Cobb 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