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=-11.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, NICE_REPLY_A,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 74D30C433E1 for ; Mon, 17 Aug 2020 15:59:45 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1CC4520729 for ; Mon, 17 Aug 2020 15:59:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Do6vVEQR" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1CC4520729 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:55504 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1k7hY0-0004MD-87 for qemu-devel@archiver.kernel.org; Mon, 17 Aug 2020 11:59:44 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:55766) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1k7hAc-0002mX-V0 for qemu-devel@nongnu.org; Mon, 17 Aug 2020 11:35:34 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:57477 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1k7hAa-0006Qr-MT for qemu-devel@nongnu.org; Mon, 17 Aug 2020 11:35:34 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1597678531; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=DnEM542Mr5GicBRNuKWc7x2V8iKSHq35OT4nth8iEP8=; b=Do6vVEQRMTkNFotnIIU7YUKBfc0xZDGYATsIjshz3iH6d0vPKsbcfzt9dyvnIYWr4sgaXO ZyX8D1z5CTdfCO/arevvH9KgHL5HLE7TGbuCg+iylAJYC2kRnbVnzqE7MjnjlEeLRV6uQD EQH7Z9xdaaNZKKms48v5uCQS3FmS45U= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-274-Ww0ptaVsOXqPyiG2ofvSLg-1; Mon, 17 Aug 2020 11:35:27 -0400 X-MC-Unique: Ww0ptaVsOXqPyiG2ofvSLg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B1FC5801AAE; Mon, 17 Aug 2020 15:35:26 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-113-146.ams2.redhat.com [10.36.113.146]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9A1BE7A1C1; Mon, 17 Aug 2020 15:35:25 +0000 (UTC) Subject: Re: [RFC PATCH 09/22] nbd: Add writethrough to block-export-add To: Kevin Wolf References: <20200813162935.210070-1-kwolf@redhat.com> <20200813162935.210070-10-kwolf@redhat.com> <20200817131359.GL11402@linux.fritz.box> <6c07d929-d7f0-50d8-88b1-b9fa89209c7d@redhat.com> <20200817143219.GN11402@linux.fritz.box> From: Max Reitz Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: Date: Mon, 17 Aug 2020 17:35:23 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200817143219.GN11402@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0.002 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="ipPWo6Uju0uQMt9u0WqjPcwdXLeVAkpSG" Received-SPF: pass client-ip=205.139.110.61; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/17 05:03:47 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --ipPWo6Uju0uQMt9u0WqjPcwdXLeVAkpSG Content-Type: multipart/mixed; boundary="tW7DOyUwUj8GrrdYNijHUqXLGbZksnrfv" --tW7DOyUwUj8GrrdYNijHUqXLGbZksnrfv Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 17.08.20 16:32, Kevin Wolf wrote: > Am 17.08.2020 um 15:51 hat Max Reitz geschrieben: >> On 17.08.20 15:13, Kevin Wolf wrote: >>> Am 17.08.2020 um 14:56 hat Max Reitz geschrieben: >>>> On 13.08.20 18:29, Kevin Wolf wrote: >>>>> qemu-nbd allows use of writethrough cache modes, which mean that writ= e >>>>> requests made through NBD will cause a flush before they complete. >>>>> Expose the same functionality in block-export-add. >>>>> >>>>> Signed-off-by: Kevin Wolf >>>>> --- >>>>> qapi/block-export.json | 7 ++++++- >>>>> blockdev-nbd.c | 2 +- >>>>> 2 files changed, 7 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/qapi/block-export.json b/qapi/block-export.json >>>>> index 1fdc55c53a..4ce163411f 100644 >>>>> --- a/qapi/block-export.json >>>>> +++ b/qapi/block-export.json >>>>> @@ -167,10 +167,15 @@ >>>>> # Describes a block export, i.e. how single node should be exported = on an >>>>> # external interface. >>>>> # >>>>> +# @writethrough: If true, caches are flushed after every write reque= st to the >>>>> +# export before completion is signalled. (since: 5.2; >>>>> +# default: false) >>>>> +# >>>>> # Since: 4.2 >>>>> ## >>>>> { 'union': 'BlockExportOptions', >>>>> - 'base': { 'type': 'BlockExportType' }, >>>>> + 'base': { 'type': 'BlockExportType', >>>>> + '*writethrough': 'bool' }, >>>>> 'discriminator': 'type', >>>>> 'data': { >>>>> 'nbd': 'BlockExportOptionsNbd' >>>> >>>> Hm. I find it weird to have @writethrough in the base but @device in >>>> the specialized class. >>>> >>>> I think everything that will be common to all block exports should be = in >>>> the base, and that probably includes a node-name. I=E2=80=99m aware t= hat will >>>> make things more tedious in the code, but perhaps it would be a nicer >>>> interface in the end. Or is the real problem that that would create >>>> problems in the storage daemon=E2=80=99s command line interface, becau= se then >>>> the specialized (legacy) NBD interface would no longer be compatible >>>> with the new generalized block export interface? >>> >>> Indeed. I think patch 15 has what you're looking for. >> >> Great. :) >> >> Discussions where both participants have the same opinion from the >> start are the best ones. >=20 > Makes things a lot easier. >=20 > Maybe I should try to move patch 15 earlier. The series is mostly just > in the order that I wrote things, but there were also a few nasty > dependencies in the part the generalises things from NBD to BlockExport. > So I'm not sure if this is a patch that can be moved. >=20 >>>> Anyway, @writable might be a similar story. A @read-only may make sen= se >>>> in general, I think. >>> >>> Pulling @writable up is easier than a @read-only, but that's a naming >>> detail. >> >> Sure. >> >>> In general I agree, but this part isn't addressed in this series yet. >>> Part of the reason why this is an RFC was to find out if I should >>> include things like this or if we'll do it when we add another export >>> type or common functionality that needs the same option. >> >> Sure, sure. >=20 > So should I or not? :-) Can we delay it until after this series? I.e., as long as it retains the name =E2=80=9Cwritable=E2=80=9D, would pulling it up into BlockExportOp= tions a compatible change? If so, then I suppose we could do it afterwards. But I think it does make the most sense to =E2=80=9Cjust=E2=80=9D do it as part of this series. >> Meta: I personally don=E2=80=99t like RFC patches very much. RFC to me = means >> everything is fair game, and reviewers should be free to let their >> thoughts wander and come up with perhaps wild ideas, just trying to >> gauge what everyone thinks. >> >> When I=E2=80=99m the submitter, I tend to get defensive then, because I= =E2=80=99ve >> invested time in writing the code already, so I tend to be biased >> against fundamental changes. (Horrible personal trait. I=E2=80=99m wor= king >> on it.) >=20 > This makes sense. Nobody likes having to rewrite their RFC series. >=20 > But there is one thing I dread even more: Polishing the RFC series for > another week until I can send it out as non-RFC and _then_ having to > rewrite it. Yes. Especially bad with tests. >> As a reviewer, the code and thus some fleshed out design is there >> already, so it=E2=80=99s difficult to break free from that and find comp= letely >> different solutions to the original problem. >> (I kind of ventured in that direction for this patch, and it seems like >> you immediately noticed that my response was different from usual and >> pointed out the RFC status, perhaps to make me feel more comfortable in >> questioning the fundamental design more. Which I noticed, hence this >> wall of text.) >=20 > Basically just telling you that I was already interested in your input > for this point specifically when I sent the series. OK :) >> Perhaps I=E2=80=99m wrong. Perhaps it=E2=80=99s just myself (the points= I=E2=80=99ve just >> listed are definitely my own personal weaknesses), but I can=E2=80=99t h= elp but >> project and assume that others may feel similar, at least in part. >> So I feel like RFCs that consist of patches tend to at least lock me in >> to the solution that=E2=80=99s present. I find them difficult to handle= , both >> as a submitter and as a reviewer. >> >> All in all, that means on either side I tend to handle patch RFCs as >> =E2=80=9CStandard series, just tests missing=E2=80=9D. Not sure if that= =E2=80=99s ideal. Or >> maybe that=E2=80=99s exactly what patch RFCs are? >> >> (Of course, it can and should be argued that even for standard series, I >> shouldn=E2=80=99t be afraid of questioning the fundamental design still.= But >> that=E2=80=99s hard...) >=20 > I usually send RFC patches when I know that I wouldn't consider them > mergable yet, but I don't want to invest the time to polish them before > I know that other people agree with the approach and the time won't be > wasted. >=20 >> But, well. The alternative is writing pure design RFCs, and then you >> tend to get weeks of slow discussion, drawing everything out. Which >> isn=E2=80=99t ideal either. Or is that just a baseless prejudice I have= ? >=20 > In many cases (and I think this is one of them in large parts), I only > really learn what the series will look like when I write it. That=E2=80=99s true. With a pure design RFC, it=E2=80=99s often difficult = to know even the scope of the design until you=E2=80=99ve begun to write code. So there= =E2=80=99s a danger of just writing a bunch of uncontroversial basic design stuff because one has no idea of what may actually become problematic and questionable. :/ > I could have sent a design RFC for the QAPI part, but I didn't expect > this to be contentious because it's just the normal add/del/query thing > that exists for pretty much everything else, too. Yeah, the functions themselves are clear. Hm. Perhaps software engineering just is actually difficult, and there=E2=80=99s no way around it. >>>> Basically, I think that the export code should be separate from the co= de >>>> setting up the BlockBackend that should be exported, so all options >>>> regarding that BB should be common; and those options are @node-name, >>>> @writethrough, and @read-only. (And perhaps other things like >>>> @resizable, too, even though that isn=E2=80=99t something to consider = for NBD.) >>> >>> Do you mean that the BlockBackend should already be created by the >>> generic block export layer? >> >> It would certainly be nice, if it were feasible, don=E2=80=99t you think= ? >> >> We don=E2=80=99t have to bend backwards for it, but maybe it would force= us to >> bring the natural separation of block device and export parameters to >> the interface. >=20 > I can try. I seem to remember that you had a reason not to do this the > last time we discussed generalised exports, but I'm not sure what it > was. >=20 > The obvious one could be that the block export layer doesn't know which > permissions are needed. But it can always start with minimal permissions > and let the driver do a blk_set_perm() if it needs more. Trying sounds good. Since there shouldn=E2=80=99t be consequences for the = QMP interface, we=E2=84=A2 can always try again later (i.e., when adding more e= xport types). Max --tW7DOyUwUj8GrrdYNijHUqXLGbZksnrfv-- --ipPWo6Uju0uQMt9u0WqjPcwdXLeVAkpSG Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl86o7wACgkQ9AfbAGHV z0CDHAgAhbAaz38b//zIvL4kqFTkCzKxPaZog71mj+J6EdZC8tsj8K3FYwMdT1gb qJjlzSy9JK1ivxJhOxiPHV3V80nTY5gj0ppzeqxYhyiiY+nf/jDq6BLSEwIFpxmM IWxJxQZN3sSucJk+f3YvHR8oFpUcHk97BAq2hbfxsWU2r8/4p9pztfjOEjDynxWX 8Did2Kzj5KaVEs3YF/+vGoc3RA7NadwbTZ8s5b3/MVH4iJ7NNEhri9efN+fuVZ4/ +vIzpSRZKRrUVJUNef8cONfAQ2GalxEc0rq7PHgdUogwyVt2/1sSBMgy3GmTCcyt 5OX0Jh86QuP9Xu8o3LLn4m28jaUBmQ== =2Cch -----END PGP SIGNATURE----- --ipPWo6Uju0uQMt9u0WqjPcwdXLeVAkpSG--