edafc70c0c
Since commit 5a37b60a61c, qemu-img create will pre-zero the target image if it isn't already zero-initialised (most importantly, for host block devices, but also iscsi etc.), so that writing explicit zeros wouldn't be necessary later. This could speed up the operation significantly, in particular when the source image file was only sparsely populated. However, it also means that some block are written twice: Once when pre-zeroing them, and then when they are overwritten with actual data. On a full image, the pre-zeroing is wasted work because everything will be overwritten. In practice, write_zeroes typically turns out faster than writing explicit zero buffers, but slow enough that first zeroing everything and then overwriting parts can be a significant net loss. Meanwhile, qemu-img convert was rewritten in 690c7301600 and zero blocks are now written to the target using bdrv_co_pwrite_zeroes() if the target could be pre-zeroed. This way we already make use of the faster write_zeroes operation, but avoid writing any blocks twice. Remove the pre-zeroing because these days this former optimisation has actually turned into a pessimisation in the common case. Reported-by: Nir Soffer <nsoffer@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com> Message-Id: <20200622151203.35624-1-kwolf@redhat.com> Tested-by: Nir Soffer <nsoffer@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
=========== QEMU README =========== QEMU is a generic and open source machine & userspace emulator and virtualizer. QEMU is capable of emulating a complete machine in software without any need for hardware virtualization support. By using dynamic translation, it achieves very good performance. QEMU can also integrate with the Xen and KVM hypervisors to provide emulated hardware while allowing the hypervisor to manage the CPU. With hypervisor support, QEMU can achieve near native performance for CPUs. When QEMU emulates CPUs directly it is capable of running operating systems made for one machine (e.g. an ARMv7 board) on a different machine (e.g. an x86_64 PC board). QEMU is also capable of providing userspace API virtualization for Linux and BSD kernel interfaces. This allows binaries compiled against one architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a different architecture ABI (e.g. the Linux x86_64 ABI). This does not involve any hardware emulation, simply CPU and syscall emulation. QEMU aims to fit into a variety of use cases. It can be invoked directly by users wishing to have full control over its behaviour and settings. It also aims to facilitate integration into higher level management layers, by providing a stable command line interface and monitor API. It is commonly invoked indirectly via the libvirt library when using open source applications such as oVirt, OpenStack and virt-manager. QEMU as a whole is released under the GNU General Public License, version 2. For full licensing details, consult the LICENSE file. Building ======== QEMU is multi-platform software intended to be buildable on all modern Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety of other UNIX targets. The simple steps to build QEMU are: .. code-block:: shell mkdir build cd build ../configure make Additional information can also be found online via the QEMU website: * `<https://qemu.org/Hosts/Linux>`_ * `<https://qemu.org/Hosts/Mac>`_ * `<https://qemu.org/Hosts/W32>`_ Submitting patches ================== The QEMU source code is maintained under the GIT version control system. .. code-block:: shell git clone https://git.qemu.org/git/qemu.git When submitting patches, one common approach is to use 'git format-patch' and/or 'git send-email' to format & send the mail to the qemu-devel@nongnu.org mailing list. All patches submitted must contain a 'Signed-off-by' line from the author. Patches should follow the guidelines set out in the CODING_STYLE.rst file. Additional information on submitting patches can be found online via the QEMU website * `<https://qemu.org/Contribute/SubmitAPatch>`_ * `<https://qemu.org/Contribute/TrivialPatches>`_ The QEMU website is also maintained under source control. .. code-block:: shell git clone https://git.qemu.org/git/qemu-web.git * `<https://www.qemu.org/2017/02/04/the-new-qemu-website-is-up/>`_ A 'git-publish' utility was created to make above process less cumbersome, and is highly recommended for making regular contributions, or even just for sending consecutive patch series revisions. It also requires a working 'git send-email' setup, and by default doesn't automate everything, so you may want to go through the above steps manually for once. For installation instructions, please go to * `<https://github.com/stefanha/git-publish>`_ The workflow with 'git-publish' is: .. code-block:: shell $ git checkout master -b my-feature $ # work on new commits, add your 'Signed-off-by' lines to each $ git publish Your patch series will be sent and tagged as my-feature-v1 if you need to refer back to it in the future. Sending v2: .. code-block:: shell $ git checkout my-feature # same topic branch $ # making changes to the commits (using 'git rebase', for example) $ git publish Your patch series will be sent with 'v2' tag in the subject and the git tip will be tagged as my-feature-v2. Bug reporting ============= The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs found when running code built from QEMU git or upstream released sources should be reported via: * `<https://bugs.launchpad.net/qemu/>`_ If using QEMU via an operating system vendor pre-built binary package, it is preferable to report bugs to the vendor's own bug tracker first. If the bug is also known to affect latest upstream code, it can also be reported via launchpad. For additional information on bug reporting consult: * `<https://qemu.org/Contribute/ReportABug>`_ Contact ======= The QEMU community can be contacted in a number of ways, with the two main methods being email and IRC * `<mailto:qemu-devel@nongnu.org>`_ * `<https://lists.nongnu.org/mailman/listinfo/qemu-devel>`_ * #qemu on irc.oftc.net Information on additional methods of contacting the community can be found online via the QEMU website: * `<https://qemu.org/Contribute/StartHere>`_