Re: [libvirt] Error when creating VM with persistent memory

After adding the memoryBacking tag in xml as below (in addition, to other xml changes to add nvdimm), virsh could allocate AD memory larger than the system RAM and VMs could start successfully. <memoryBacking> <access mode='shared'/> <discard/> </memoryBacking> This adds share=yes in command line. -object memory-backend-file,id=memnvdimm0,prealloc=yes,mem-path=/mnt/pmem0/file1,share=yes,size=414464344064 -device nvdimm,node=0,label-size=131072,memdev=memnvdimm0,id=nvdimm0,slot=0 For reference qemu command line where VM starts quickly: qemu-system-x86_64 \ -name qemu-gold29 \ -drive file=/var/lib/libvirt/images/gold29-ad.qcow2,format=qcow2,index=0,media=disk \ -m 2G,slots=4,maxmem=428G \ -smp 2 \ -machine pc,accel=kvm,nvdimm=on \ -enable-kvm \ -object memory-backend-file,id=pmem1,share=on,mem-path=/mnt/pmem0/file1,size=386G,align=4K \ -device nvdimm,memdev=pmem1,id=nv1 \ -daemonize Qemu command line generated from virsh: (please note VM now starts with this command line, shared=yes.) /usr/bin/qemu-system-x86_64 -machine accel=kvm -name guest=mix-test,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-18-mix-test/master-key.aes -machine pc-i440fx-3.0,accel=kvm,usb=off,vmport=off,dump-guest-core=off,nvdimm=on -cpu Skylake-Server-IBRS,hypervisor=on -m size=2097152k,slots=16,maxmem=419430400k -realtime mlock=off -smp 2,sockets=2,cores=1,threads=1 -object memory-backend-file,id=ram-node0,mem-path=/var/lib/libvirt/qem /ram/libvirt/qemu/18-mix-test/ram-node0,discard-data=yes,share=yes,size=2147483648 -numa node,nodeid=0,cpus=0-1,memdev=ram-node0 -object memory-backend-file,id=memnvdimm0,prealloc=yes,mem-path=/mnt/pmem0/file1,share=yes,size=414464344064 -device nvdimm,node=0,label-size=131072,memdev=memnvdimm0,id=nvdimm0,slot=0 -uuid 318c0529-0330-460b-8d0a-3b253e9decdd -no-user-config -nodefaults -chardev socket,id=charmonitor,fd=32,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew -global kvm-pit.lost_tick_policy=delay -no-hpet -no-shutdown -global PIIX4_PM.disable_s3=1 -global PIIX4_PM.disable_s4=1 -boot strict=on -device ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x5.0x7 -device ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x5 -device ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x5.0x1 -device ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x5.0x2 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x6 -drive file=/var/lib/libvirt/images/mix-test.qcow2,format=qcow2,if=none,id=drive-virtio-disk0 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -drive if=none,id=drive-ide0-0-0,readonly=on -device ide-cd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -netdev tap,fd=34,id=hostnet0 -device e1000,netdev=hostnet0,id=net0,mac=52:54:00:06:db:55,bus=pci.0,addr=0xa -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -chardev socket,id=charchannel0,fd=35,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0 -chardev spicevmc,id=charchannel1,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=com.redhat.spice.0 -device usb-tablet,id=input0,bus=usb.0,port=1 -spice port=5901,addr=127.0.0.1,disable-ticketing,image-compression=off,seamless-migration=on -device qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,vram64_size_mb=0,vgamem_mb=16,max_outputs=1,bus=pci.0,addr=0x2 -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -chardev spicevmc,id=charredir0,name=usbredir -device usb-redir,chardev=charredir0,id=redir0,bus=usb.0,port=2 -chardev spicevmc,id=charredir1,name=usbredir -device usb-redir,chardev=charredir1,id=redir1,bus=usb.0,port=3 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x8 -object rng-random,id=objrng0,filename=/dev/urandom -device virtio-rng-pci,rng=objrng0,id=rng0,bus=pci.0,addr=0x9 -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg timestamp=on VM start takes longer that qemu. Any thoughts?? Why is prealloc=yes default on for nvdimm? Any other important deltas?