Description of problem: When running VM with NVDIMM device, the next warning is logged in qemu.log: 2020-10-29 16:03:11.880+0000: Domain id=8 is tainted: custom-hypervisor-feature Warning: requesting persistence across crashes for backend file /dev/pmem0 failed. Proceeding without persistence, data might become corrupted in case of host crash. Not sure what is the impact of this warning. Version-Release number of selected component (if applicable): ovirt-engine-4.4.3.8-0.1.el8ev vdsm-4.40.35-1.el8ev.x86_64 libvirt-daemon-6.6.0-6.module+el8.3.0+8125+aefcf088.x86_64 qemu-kvm-5.1.0-13.module+el8.3.0+8382+afc3bbea.x86_64 How reproducible: 100% Steps to Reproduce: 1. Add NVDIMM device to VM. 2. Run VM 3. Actual results: Warning is logged in qemu.log: Warning: requesting persistence across crashes for backend file /dev/pmem0 failed. Proceeding without persistence, data might become corrupted in case of host crash. Expected results: Additional info: vdsm.log (vm started at 2020-10-29 12:03:09,906-0400 INFO (jsonrpc/0) [api.virt] START create), engine.log and qemu.log attached.
Created attachment 1725110 [details] engine.log
Created attachment 1725111 [details] vdsm.log
Created attachment 1725112 [details] qemu log
According to QEMU documentation, https://github.com/qemu/qemu/blob/2c6605389c1f76973d92b69b85d40d94b8f1092c/docs/nvdimm.txt#L153, host-crash-proof persistence is available only when using a DAX backing device: Though QEMU supports multiple types of vNVDIMM backends on Linux, the only backend that can guarantee the guest write persistence is: A. DAX device (e.g., /dev/dax0.0, ) or B. DAX file(mounted with dax option) ... If these conditions are not satisfied i.e. if either 'pmem' or 'share' are not set, if the backend file does not support DAX or if MAP_SYNC is not supported by the host kernel, write persistence is not guaranteed after a system crash. For compatibility reasons, these conditions are ignored if not satisfied. That means that in this case, where fsdax mode was probably used on the host NVDIMM device, the persistence cannot be guaranteed (and it cannot be currently guaranteed at all, since DAX host device mode doesn't work due to Bug 1855336). Note that it concerns only a host crash, as cited above and in the warning, not a regular flow. We should document this persistence limitation. Arik, I'll update the feature page, how to handle the downstream documentation? Should I file a doc bug?
(In reply to Milan Zamazal from comment #4) > We should document this persistence limitation. Arik, I'll update the > feature page, how to handle the downstream documentation? Should I file a > doc bug? Yes please
Documentation bug: BZ 1894067
Just waiting for the bugs this one depends on.
Closing as per bz 1855336