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[209.132.180.67]) by mx.google.com with ESMTP id i21-v6si18012541pgg.513.2018.08.13.11.44.00; Mon, 13 Aug 2018 11:44:15 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730285AbeHMV0a (ORCPT + 99 others); Mon, 13 Aug 2018 17:26:30 -0400 Received: from mx2.suse.de ([195.135.220.15]:33574 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729525AbeHMV0a (ORCPT ); Mon, 13 Aug 2018 17:26:30 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id EEC16AE0B; Mon, 13 Aug 2018 18:43:03 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 365C5DA7C5; Mon, 13 Aug 2018 20:42:54 +0200 (CEST) Date: Mon, 13 Aug 2018 20:42:52 +0200 From: David Sterba To: Naohiro Aota Cc: David Sterba , linux-btrfs@vger.kernel.org, Chris Mason , Josef Bacik , linux-kernel@vger.kernel.org, Hannes Reinecke , Damien Le Moal , Bart Van Assche , Matias Bjorling Subject: Re: [RFC PATCH 00/17] btrfs zoned block device support Message-ID: <20180813184251.GC24025@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Naohiro Aota , David Sterba , linux-btrfs@vger.kernel.org, Chris Mason , Josef Bacik , linux-kernel@vger.kernel.org, Hannes Reinecke , Damien Le Moal , Bart Van Assche , Matias Bjorling References: <20180809180450.5091-1-naota@elisp.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180809180450.5091-1-naota@elisp.net> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 10, 2018 at 03:04:33AM +0900, Naohiro Aota wrote: > This series adds zoned block device support to btrfs. Yay, thanks! As this a RFC, I'll give you some. The code looks ok for what it claims to do, I'll skip style and unimportant implementation details for now as there are bigger questions. The zoned devices bring some constraints so not all filesystem features cannot be expected to work, so this rules out any form of in-place updates like NODATACOW. Then there's list of 'how will zoned device work with feature X'? You disable fallocate and DIO. I haven't looked closer at the fallocate case, but DIO could work in the sense that open() will open the file but any write will fallback to buffered writes. This is implemented so it would need to be wired together. Mixed device types are not allowed, and I tend to agree with that, though this could work in principle. Just that the chunk allocator would have to be aware of the device types and tweaked to allocate from the same group. The btrfs code is not ready for that in terms of the allocator capabilities and configuration options. Device replace is disabled, but the changlog suggests there's a way to make it work, so it's a matter of implementation. And this should be implemented at the time of merge. RAID5/6 + zoned support is highly desired and lack of it could be considered a NAK for the whole series. The drive sizes are expected to be several terabytes, that sounds be too risky to lack the redundancy options (RAID1 is not sufficient here). The changelog does not explain why this does not or cannot work, so I cannot reason about that or possibly suggest workarounds or solutions. But I think it should work in principle. As this is first post and RFC I don't expect that everything is implemented, but at least the known missing points should be documented. You've implemented lots of the low-level zoned support and extent allocation, so even if the raid56 might be difficult, it should be the smaller part.