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Tue, 2 Jun 2020 11:06:49 +0000 (UTC) Date: Tue, 2 Jun 2020 13:06:48 +0200 From: Kevin Wolf To: Lin Ma Subject: Re: [question] Partial sector issue while discard in qcow2 image Message-ID: <20200602110648.GE5940@linux.fritz.box> References: MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii 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 = 2020/06/01 23:49:17 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, 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=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN 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: "pbonzini@redhat.com" , "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 02.06.2020 um 09:45 hat Lin Ma geschrieben: > Hi all, > > During woring to add GET LBA STATUS support in qemu scsi emulation > layer, I encountered an unmap issue with qcow2 image, It's likely > about how to unmap partial clusters. e.g.: > > With these default values: > * the default value of s->qdev.blocksize: 512 > * the default value of s->cluster_size of qcow2 image: 65536 > > Running 'sg_unmap -l 1024 -n 32 /dev/sda' hits the condition > 'if (!QEMU_IS_ALIGNED(offset | bytes, s->cluster_size))' in the > function qcow2_co_pdiscard, Then it won't perform > qcow2_cluster_discard(), So the corresponding clusters won't be > discard in this situation, Correct? Yes, this is correct. Discard is just a hint, so doing nothing is a perfectly valid implementation. In the case of qcow2, only full clusters are discarded. > Of cause, with the default blocksize and cluster_size, The below > examples won't hit the condition 'if (!QEMU_IS_ALIGNED...'. > sg_unmap -l 1024 -n 128 /dev/sda > or > sg_unmap -l 256 -n 128 /dev/sda Yes, and when discarding whole block devices (e.g. while creating a new filesystem on them) or large files, you'll probably get this case for most parts. Kevin