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Fri, 1 Nov 2019 15:42:42 +0000 (UTC) Date: Fri, 1 Nov 2019 16:42:41 +0100 From: Kevin Wolf To: Max Reitz Subject: Re: [PATCH for-4.2 1/4] Revert "qcow2: skip writing zero buffers to empty COW areas" Message-ID: <20191101154241.GA23153@localhost.localdomain> References: <20191101100019.9512-1-mreitz@redhat.com> <20191101100019.9512-2-mreitz@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="cWoXeonUoKmBZSoM" Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.61 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: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, qemu-devel@nongnu.org, Anton Nefedov , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --cWoXeonUoKmBZSoM Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 01.11.2019 um 15:01 hat Max Reitz geschrieben: > On 01.11.19 13:40, Eric Blake wrote: > > On 11/1/19 11:00 AM, Max Reitz wrote: > >> This reverts commit c8bb23cbdbe32f5c326365e0a82e1b0e68cdcd8a. > >> > >> This commit causes fundamental performance problems on XFS (because > >> fallocate() stalls the AIO pipeline), and as such it is not clear that > >> we should unconditionally enable this behavior. > >> > >> We expect subclusters to alleviate the performance penalty of small > >> writes to newly allocated clusters, so when we get them, the originall= y > >> intended performance gain may actually no longer be significant. > >> > >> If we want to reintroduce something similar to c8bb23cbdbe, it will > >> require extensive benchmarking on various systems with subclusters > >> enabled. > >> > >> Cc: qemu-stable@nongnu.org > >> Signed-off-by: Max Reitz > >> --- > >=20 > >> +++ b/qapi/block-core.json > >> @@ -3304,8 +3304,6 @@ > >> =C2=A0 # > >> =C2=A0 # @cor_write: a write due to copy-on-read (since 2.11) > >> =C2=A0 # > >> -# @cluster_alloc_space: an allocation of file space for a cluster > >> (since 4.1) > >> -# > >> =C2=A0 # @none: triggers once at creation of the blkdebug node (since = 4.1) > >=20 > > Deleting released qapi is not backwards-compatible. >=20 > Right. :-/ >=20 > I=E2=80=99ll just not make changes to the QAPI schema. It doesn=E2=80=99= t hurt to leave > this in. Why would it be incompatible to drop an enum value that is only ever used in output and that QEMU doesn't generate? 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