[Bug 1860759] Re: [REGRESSION] option `-snapshot` ignored with blockdev

[Max Reitz]

Hi,

The technical difference is that -blockdev requires you (the user or
management software) to create all block graph nodes explicitly.  -drive
snapshot=on implicitly creates a qcow2 node above the actual disk image
(and that node points to a temporary image in /tmp).  So because it’s
implicit and not explicit, it can’t work with -blockdev.

With -blockdev, the user has to create this temporary overlay, open it
in qemu (with blockdev-add or -blockdev), and then delete it.

Max

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Title:
  [REGRESSION] option `-snapshot` ignored with blockdev

Status in QEMU:
  New

Bug description:
  After upgrade of qemu 3.1.0 → 4.2.0 I found that running with libvirt doesn't 
honor `-snapshot` option anymore. I.e. disk images get modified.
  Using `-hda` option honors `-snapshot`

  So I made a test case without libvirt. Testcase using 4.2.0:

  > qemu -hda tmp-16G.img -cdrom regular-rescue-latest-x86_64.iso -m 2G

  This works fine and tmp-16G.img stays unmodified.

  But:
  > /usr/bin/qemu-system-x86_64 -name guest=test-linux,debug-threads=on -S 
-machine pc-i440fx-3.1,accel=kvm,usb=off,vmport=off,dump-guest-core=off -cpu 
Broadwell-noTSX,vme=on,ss=on,f16c=on,rdrand=on,hypervisor=on,arat=on,tsc-adjust=on,xsaveopt=on,pdpe1gb=on,abm=on
 -m 2048 -overcommit mem-lock=off -smp 3,sockets=3,cores=1,threads=1 -uuid 
d32a9191-f51d-4fae-a419-b73d85b49198 -no-user-config -nodefaults -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 
-blockdev 
\{\"driver\":\"file\",\"filename\":\"/tmp/regular-rescue-latest-x86_64.iso\",\"node-name\":\"libvirt-2-storage\",\"auto-read-only\":true,\"discard\":\"unmap\"}
 -blockdev 
\{\"node-name\":\"libvirt-2-format\",\"read-only\":true,\"driver\":\"raw\",\"file\":\"libvirt-2-storage\"}
 -device ide-cd,bus=ide.0,unit=0,drive=libvirt-2-format,id=ide0-0-0,bootindex=1 
-blockdev 
\{\"driver\":\"file\",\"filename\":\"/tmp/tmp-2G.img\",\"node-name\":\"libvirt-1-storage\",\"auto-read-only\":true,\"discard\":\"unmap\"}
 -blockdev 
\{\"node-name\":\"libvirt-1-format\",\"read-only\":false,\"driver\":\"qcow2\",\"file\":\"libvirt-1-storage\",\"backing\":null}
 -device 
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=libvirt-1-format,id=virtio-disk0
 -netdev user,id=hostnet0 -device 
e1000,netdev=hostnet0,id=net0,mac=52:54:00:ab:d8:29,bus=pci.0,addr=0x3 -chardev 
pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -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 -device 
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6 -snapshot -sandbox 
on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg 
timestamp=on

  This modifies tmp-16G.img.

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