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Mon, 17 Aug 2020 13:14:00 +0000 (UTC) Date: Mon, 17 Aug 2020 15:13:59 +0200 From: Kevin Wolf To: Max Reitz Subject: Re: [RFC PATCH 09/22] nbd: Add writethrough to block-export-add Message-ID: <20200817131359.GL11402@linux.fritz.box> References: <20200813162935.210070-1-kwolf@redhat.com> <20200813162935.210070-10-kwolf@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0.001 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Xm/fll+QQv+hsKip" Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=kwolf@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/17 00:24:04 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 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_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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" --Xm/fll+QQv+hsKip Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 write > > requests made through NBD will cause a flush before they complete. > > Expose the same functionality in block-export-add. > >=20 > > Signed-off-by: Kevin Wolf > > --- > > qapi/block-export.json | 7 ++++++- > > blockdev-nbd.c | 2 +- > > 2 files changed, 7 insertions(+), 2 deletions(-) > >=20 > > 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 request= 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' >=20 > 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 that= 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, because = 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. > Anyway, @writable might be a similar story. A @read-only may make sense > in general, I think. Pulling @writable up is easier than a @read-only, but that's a naming detail. 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. > Basically, I think that the export code should be separate from the code > 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? 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