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Wed, 22 Jul 2020 17:15:41 +0000 (UTC) Message-ID: Subject: Re: [PATCH for-5.1 1/2] qcow2: Implement v2 zero writes with discard if possible From: Maxim Levitsky To: Kevin Wolf Date: Wed, 22 Jul 2020 20:15:40 +0300 In-Reply-To: <20200722171456.GC4838@linux.fritz.box> References: <20200720131810.177978-1-kwolf@redhat.com> <20200720131810.177978-2-kwolf@redhat.com> <498fc3712cd3a0cb0f6588331c47b5d12b7eac96.camel@redhat.com> <20200722171456.GC4838@linux.fritz.box> User-Agent: Evolution 3.36.3 (3.36.3-1.fc32) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=207.211.31.120; envelope-from=mlevitsk@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/22 00:40:35 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_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, 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, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 2020-07-22 at 19:14 +0200, Kevin Wolf wrote: > Am 22.07.2020 um 19:01 hat Maxim Levitsky geschrieben: > > On Mon, 2020-07-20 at 15:18 +0200, Kevin Wolf wrote: > > > qcow2 version 2 images don't support the zero flag for clusters, so for > > > write_zeroes requests, we return -ENOTSUP and get explicit zero buffer > > > writes. If the image doesn't have a backing file, we can do better: Just > > > discard the respective clusters. > > > > > > This is relevant for 'qemu-img convert -O qcow2 -n', where qemu-img has > > > to assume that the existing target image may contain any data, so it has > > > to write zeroes. Without this patch, this results in a fully allocated > > > target image, even if the source image was empty. > > > > > > Reported-by: Nir Soffer > > > Signed-off-by: Kevin Wolf > > > --- > > > block/qcow2-cluster.c | 9 ++++++++- > > > 1 file changed, 8 insertions(+), 1 deletion(-) > > > > > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > > > index 4b5fc8c4a7..a677ba9f5c 100644 > > > --- a/block/qcow2-cluster.c > > > +++ b/block/qcow2-cluster.c > > > @@ -1797,8 +1797,15 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset, > > > assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) || > > > end_offset >= bs->total_sectors << BDRV_SECTOR_BITS); > > > > > > - /* The zero flag is only supported by version 3 and newer */ > > > + /* > > > + * The zero flag is only supported by version 3 and newer. However, if we > > > + * have no backing file, we can resort to discard in version 2. > > > + */ > > > if (s->qcow_version < 3) { > > > + if (!bs->backing) { > > > + return qcow2_cluster_discard(bs, offset, bytes, > > > + QCOW2_DISCARD_REQUEST, false); > > > + } > > > return -ENOTSUP; > > > } > > > > > > > From my knowelege of nvme, I remember that discard doesn't have to zero the blocks. > > There is special namespace capability the indicates the contents of the discarded block. > > (Deallocate Logical Block Features) > > > > If and only if the discard behavier flag indicates that discarded areas are zero, > > then the write-zero command can have special 'deallocate' flag that hints the controller > > to discard the sectors. > > > > So woudn't discarding the clusters have theoretical risk of introducing garbage there? > > No, qcow2_cluster_discard() has a defined behaviour. For v2 images, it > unallocates the cluster in the L2 table (this is only safe without a > backing file), for v3 images it converts them to zero clusters. All right then! Best regards, Maxim Levitsky > > Kevin