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Thu, 7 May 2020 08:39:25 +0000 (UTC) Date: Thu, 7 May 2020 10:39:24 +0200 From: Kevin Wolf To: Max Reitz Subject: Re: [PATCH] qcow2: Fix preallocation on block devices Message-ID: <20200507083924.GA6019@linux.fritz.box> References: <20200505141801.1096763-1-mreitz@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200505141801.1096763-1-mreitz@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.81; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/07 00:55:34 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: qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 05.05.2020 um 16:18 hat Max Reitz geschrieben: > Calling bdrv_getlength() to get the pre-truncate file size will not > really work on block devices, because they have always the same length, > and trying to write beyond it will fail with a rather cryptic error > message. >=20 > Instead, we should use qcow2_get_last_cluster() and bdrv_getlength() > only as a fallback. >=20 > Before this patch: > $ truncate -s 1G test.img > $ sudo losetup -f --show test.img > /dev/loop0 > $ sudo qemu-img create -f qcow2 -o preallocation=3Dfull /dev/loop0 64M > Formatting '/dev/loop0', fmt=3Dqcow2 size=3D67108864 cluster_size=3D65536 > preallocation=3Dfull lazy_refcounts=3Doff refcount_bits=3D16 > qemu-img: /dev/loop0: Could not resize image: Failed to resize refcount > structures: No space left on device >=20 > With this patch: > $ sudo qemu-img create -f qcow2 -o preallocation=3Dfull /dev/loop0 64M > Formatting '/dev/loop0', fmt=3Dqcow2 size=3D67108864 cluster_size=3D65536 > preallocation=3Dfull lazy_refcounts=3Doff refcount_bits=3D16 > qemu-img: /dev/loop0: Could not resize image: Failed to resize > underlying file: Preallocation mode 'full' unsupported for this > non-regular file >=20 > So as you can see, it still fails, but now the problem is missing > support on the block device level, so we at least get a better error > message. >=20 > Note that we cannot preallocate block devices on truncate by design, > because we do not know what area to preallocate. Their length is always > the same, the truncate operation does not change it. >=20 > Signed-off-by: Max Reitz Thanks, applied to the block branch. Kevin