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With RHEL 6.2, virDomainBlockPull(dom, dev, bandwidth, 0) has a race with non-zero bandwidth: there is a window between the block_stream and block_job_set_speed monitor commands where an unlimited amount of data was let through, defeating the point of a throttle. This race was first identified in commit a9d3495e, and libvirt was able to reduce the size of the window for that race. In the meantime, the qemu developers decided to fix things properly; per this message: https://lists.gnu.org/archive/html/qemu-devel/2012-04/msg03793.html the fix will be in qemu 1.1, and changes block-job-set-speed to use a different parameter name, as well as adding a new optional parameter to block-stream, which eliminates the race altogether. Since our documentation already mentioned that we can refuse a non-zero bandwidth for some hypervisors, I think the best solution is to do just that for RHEL 6.2 qemu, so that the race is obvious to the user (anyone using stock RHEL 6.2 binaries won't have this patch, and anyone building their own libvirt with this patch for RHEL can also rebuild qemu to get the modern semantics, so it is no real loss in behavior). Meanwhile the code must be fixed to honor actual qemu 1.1 naming. Rename the parameter to 'modern', since the naming difference now covers more than just 'async' block-job-cancel. And while at it, fix an unchecked integer overflow. * src/qemu/qemu_monitor.h (enum BLOCK_JOB_CMD): Drop unused value, rename enum to match conventions. * src/qemu/qemu_monitor.c (qemuMonitorBlockJob): Reflect enum rename. * src/qemu_qemu_monitor_json.h (qemuMonitorJSONBlockJob): Likewise. * src/qemu/qemu_monitor_json.c (qemuMonitorJSONBlockJob): Likewise, and support difference between RHEL 6.2 and qemu 1.1 block pull. * src/qemu/qemu_driver.c (qemuDomainBlockJobImpl): Reject bandwidth during pull with too-old qemu. * src/libvirt.c (virDomainBlockPull, virDomainBlockRebase): Document this.
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>