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When mirroring, the goal is to ensure that the destination reads the same as the source; this goal is met whether the destination is sparse or fully-allocated (except when explicitly punching holes, then merely reading zero is not enough to know if it is sparse, so we still want to punch the hole). Avoiding a redundant write to zero (whether in the background because the zero cluster was marked in the dirty bitmap, or in the foreground because the guest is writing zeroes) when the destination already reads as zero makes mirroring faster, and avoids allocating the destination merely because the source reports as allocated. The effect is especially pronounced when the source is a raw file. That's because when the source is a qcow2 file, the dirty bitmap only visits the portions of the source that are allocated, which tend to be non-zero. But when the source is a raw file, bdrv_co_is_allocated_above() reports the entire file as allocated so mirror_dirty_init sets the entire dirty bitmap, and it is only later during mirror_iteration that we change to consulting the more precise bdrv_co_block_status_above() to learn where the source reads as zero. Remember that since a mirror operation can write a cluster more than once (every time the guest changes the source, the destination is also changed to keep up), and the guest can change whether a given cluster reads as zero, is discarded, or has non-zero data over the course of the mirror operation, we can't take the shortcut of relying on s->target_is_zero (which is static for the life of the job) in mirror_co_zero() to see if the destination is already zero, because that information may be stale. Any solution we use must be dynamic in the face of the guest writing or discarding a cluster while the mirror has been ongoing. We could just teach mirror_co_zero() to do a block_status() probe of the destination, and skip the zeroes if the destination already reads as zero, but we know from past experience that extra block_status() calls are not always cheap (tmpfs, anyone?), especially when they are random access rather than linear. Use of block_status() of the source by the background task in a linear fashion is not our bottleneck (it's a background task, after all); but since mirroring can be done while the source is actively being changed, we don't want a slow block_status() of the destination to occur on the hot path of the guest trying to do random-access writes to the source. So this patch takes a slightly different approach: any time we have to track dirty clusters, we can also track which clusters are known to read as zero. For sync=TOP or when we are punching holes from "detect-zeroes":"unmap", the zero bitmap starts out empty, but prevents a second write zero to a cluster that was already zero by an earlier pass; for sync=FULL when we are not punching holes, the zero bitmap starts out full if the destination reads as zero during initialization. Either way, I/O to the destination can now avoid redundant write zero to a cluster that already reads as zero, all without having to do a block_status() per write on the destination. With this patch, if I create a raw sparse destination file, connect it with QMP 'blockdev-add' while leaving it at the default "discard": "ignore", then run QMP 'blockdev-mirror' with "sync": "full", the destination remains sparse rather than fully allocated. Meanwhile, a destination image that is already fully allocated remains so unless it was opened with "detect-zeroes": "unmap". And any time writing zeroes is skipped, the job counters are not incremented. Signed-off-by: Eric Blake <eblake@redhat.com> Message-ID: <20250509204341.3553601-26-eblake@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> |
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=========== 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. Documentation ============= Documentation can be found hosted online at `<https://www.qemu.org/documentation/>`_. The documentation for the current development version that is available at `<https://www.qemu.org/docs/master/>`_ is generated from the ``docs/`` folder in the source tree, and is built by `Sphinx <https://www.sphinx-doc.org/en/master/>`_. 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://wiki.qemu.org/Hosts/Linux>`_ * `<https://wiki.qemu.org/Hosts/Mac>`_ * `<https://wiki.qemu.org/Hosts/W32>`_ Submitting patches ================== The QEMU source code is maintained under the GIT version control system. .. code-block:: shell git clone https://gitlab.com/qemu-project/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 `style section <https://www.qemu.org/docs/master/devel/style.html>`_ of the Developers Guide. Additional information on submitting patches can be found online via the QEMU website: * `<https://wiki.qemu.org/Contribute/SubmitAPatch>`_ * `<https://wiki.qemu.org/Contribute/TrivialPatches>`_ The QEMU website is also maintained under source control. .. code-block:: shell git clone https://gitlab.com/qemu-project/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 GitLab issues to track bugs. Bugs found when running code built from QEMU git or upstream released sources should be reported via: * `<https://gitlab.com/qemu-project/qemu/-/issues>`_ 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 GitLab. For additional information on bug reporting consult: * `<https://wiki.qemu.org/Contribute/ReportABug>`_ ChangeLog ========= For version history and release notes, please visit `<https://wiki.qemu.org/ChangeLog/>`_ or look at the git history for more detailed information. 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://wiki.qemu.org/Contribute/StartHere>`_