[Bug 1914282] Re: block copy job sometimes hangs on the last block for minutes

[Thomas Huth]

Ticket has been moved to the gitlab issue tracker:
https://gitlab.com/qemu-project/qemu/-/issues/287
... thus I'm closing this on Launchpad now.

** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #287
   https://gitlab.com/qemu-project/qemu/-/issues/287

** Changed in: qemu
       Status: Incomplete => Invalid

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https://bugs.launchpad.net/bugs/1914282

Title:
  block copy job sometimes hangs on the last block for minutes

Status in QEMU:
  Invalid

Bug description:
  In openstack nova we use the block copy API to copy disks for volume
  swap requests. In our CI gate we observed that sometimes the block
  copy job progress will reach the last or next to last block and hang
  there for  minutes at a time, causing CI jobs to fail as jobs timeout.

  On the client (nova-compute) side, using the python bindings we see
  the following in the openstack nova logs:

  ---------------

  Jan 21 05:31:02.207785 ubuntu-focal-vexxhost-ca-ymq-1-0022641000 nova-
  compute[93823]: DEBUG nova.virt.libvirt.guest [None req-d6170fbb-e023
  -4cdb-93dc-a2e9ae9b0a56 tempest-TestVolumeSwap-1117975117 tempest-
  TestVolumeSwap-1117975117] COPY block job progress, current cursor: 0
  final cursor: 1073741824 {{(pid=93823) is_job_complete
  /opt/stack/nova/nova/virt/libvirt/guest.py:873}}

  Jan 21 05:31:55.688227 ubuntu-focal-vexxhost-ca-ymq-1-0022641000 nova-
  compute[93823]: DEBUG nova.virt.libvirt.guest [None req-d6170fbb-e023
  -4cdb-93dc-a2e9ae9b0a56 tempest-TestVolumeSwap-1117975117 tempest-
  TestVolumeSwap-1117975117] COPY block job progress, current cursor:
  1049624576 final cursor: 1073741824 {{(pid=93823) is_job_complete
  /opt/stack/nova/nova/virt/libvirt/guest.py:873}}

  [...]

  Jan 21 05:31:55.706698 ubuntu-focal-vexxhost-ca-ymq-1-0022641000 nova-
  compute[93823]: DEBUG nova.virt.libvirt.guest [None req-d6170fbb-e023
  -4cdb-93dc-a2e9ae9b0a56 tempest-TestVolumeSwap-1117975117 tempest-
  TestVolumeSwap-1117975117] COPY block job progress, current cursor:
  1049624576 final cursor: 1073741824 {{(pid=93823) is_job_complete
  /opt/stack/nova/nova/virt/libvirt/guest.py:873}}

  Jan 21 05:31:56.175248 ubuntu-focal-vexxhost-ca-ymq-1-0022641000 nova-
  compute[93823]: DEBUG nova.virt.libvirt.guest [None req-d6170fbb-e023
  -4cdb-93dc-a2e9ae9b0a56 tempest-TestVolumeSwap-1117975117 tempest-
  TestVolumeSwap-1117975117] COPY block job progress, current cursor:
  1073741823 final cursor: 1073741824 {{(pid=93823) is_job_complete
  /opt/stack/nova/nova/virt/libvirt/guest.py:873}}

  [...]

  ~2.5 minutes later, it's still going at current cursor: 1073741823
  final cursor: 1073741824

  Jan 21 05:34:30.952371 ubuntu-focal-vexxhost-ca-ymq-1-0022641000 nova-
  compute[93823]: DEBUG nova.virt.libvirt.guest [None req-d6170fbb-e023
  -4cdb-93dc-a2e9ae9b0a56 tempest-TestVolumeSwap-1117975117 tempest-
  TestVolumeSwap-1117975117] COPY block job progress, current cursor:
  1073741823 final cursor: 1073741824 {{(pid=93823) is_job_complete
  /opt/stack/nova/nova/virt/libvirt/guest.py:873}}

  then current cursor == final cursor at 05:34:31.460595

  Jan 21 05:34:31.460595 ubuntu-focal-vexxhost-ca-ymq-1-0022641000 nova-
  compute[93823]: DEBUG nova.virt.libvirt.guest [None req-d6170fbb-e023
  -4cdb-93dc-a2e9ae9b0a56 tempest-TestVolumeSwap-1117975117 tempest-
  TestVolumeSwap-1117975117] COPY block job progress, current cursor:
  1073741824 final cursor: 1073741824 {{(pid=93823) is_job_complete
  /opt/stack/nova/nova/virt/libvirt/guest.py:873}}

  ---------------

  In this excerpt the cursor reaches the next to last block at Jan 21
  05:31:56.175248 and hangs there repeating status at the next to last
  block until Jan 21 05:34:30.952371 (~2.5 minutes) and then the job
  shows current cursor == final cursor at Jan 21 05:34:31.460595.

  In the corresponding qemu log, we see the block copy job report being
  on the last block for minutes:

  ---------------

  021-01-21 05:31:02.206+0000: 60630: debug : qemuMonitorJSONIOProcessLine:220 
: Line [{"return": [{"auto-finalize": true, "io-status": "ok", "device": 
"copy-vdb-libvirt-5-format", "auto-dismiss": false, "busy": true, "len": 
1073741824, "offset": 0, "status": "running", "paused": false, "speed": 0, 
"ready": false, "type": "mirror"}], "id": "libvirt-501"}]
  2021-01-21 05:31:02.206+0000: 60630: info : qemuMonitorJSONIOProcessLine:239 
: QEMU_MONITOR_RECV_REPLY: mon=0x7fd07813ec80 reply={"return": 
[{"auto-finalize": true, "io-status": "ok", "device": 
"copy-vdb-libvirt-5-format", "auto-dismiss": false, "busy": true, "len": 
1073741824, "offset": 0, "status": "running", "paused": false, "speed": 0, 
"ready": false, "type": "mirror"}], "id": "libvirt-501"}

  [...]

  len == offset at 05:31:56.174

  2021-01-21 05:31:56.174+0000: 60630: debug : qemuMonitorJSONIOProcessLine:220 
: Line [{"return": [{"auto-finalize": true, "io-status": "ok", "device": 
"copy-vdb-libvirt-5-format", "auto-dismiss": false, "busy": true, "len": 
1073741824, "offset": 1073741824, "status": "running", "paused": false, 
"speed": 0, "ready": false, "type": "mirror"}], "id": "libvirt-581"}]
  2021-01-21 05:31:56.174+0000: 60630: info : qemuMonitorJSONIOProcessLine:239 
: QEMU_MONITOR_RECV_REPLY: mon=0x7fd07813ec80 reply={"return": 
[{"auto-finalize": true, "io-status": "ok", "device": 
"copy-vdb-libvirt-5-format", "auto-dismiss": false, "busy": true, "len": 
1073741824, "offset": 1073741824, "status": "running", "paused": false, 
"speed": 0, "ready": false, "type": "mirror"}], "id": "libvirt-581"}

  [...]

  ~2.5 minutes later, still len == offset but it's still going

  2021-01-21 05:34:31.459+0000: 60630: debug : qemuMonitorJSONIOProcessLine:220 
: Line [{"return": [{"auto-finalize": true, "io-status": "ok", "device": 
"copy-vdb-libvirt-5-format", "auto-dismiss": false, "busy": false, "len": 
1073741824, "offset": 1073741824, "status": "ready", "paused": false, "speed": 
0, "ready": true, "type": "mirror"}], "id": "libvirt-855"}]
  2021-01-21 05:34:31.459+0000: 60630: info : qemuMonitorJSONIOProcessLine:239 
: QEMU_MONITOR_RECV_REPLY: mon=0x7fd07813ec80 reply={"return": 
[{"auto-finalize": true, "io-status": "ok", "device": 
"copy-vdb-libvirt-5-format", "auto-dismiss": false, "busy": false, "len": 
1073741824, "offset": 1073741824, "status": "ready", "paused": false, "speed": 
0, "ready": true, "type": "mirror"}], "id": "libvirt-855"}

  and then the job finishes soon after

  2021-01-21 05:34:31.467+0000: 60630: debug :
  qemuProcessHandleJobStatusChange:1002 : job 'copy-vdb-
  libvirt-5-format'(domain: 0x7fd070075720,instance-00000013) state
  changed to 'waiting'(6)

  2021-01-21 05:34:31.467+0000: 60630: debug :
  qemuProcessHandleJobStatusChange:1002 : job 'copy-vdb-
  libvirt-5-format'(domain: 0x7fd070075720,instance-00000013) state
  changed to 'pending'(7)

  2021-01-21 05:34:31.467+0000: 60630: debug :
  qemuProcessHandleJobStatusChange:1002 : job 'copy-vdb-
  libvirt-5-format'(domain: 0x7fd070075720,instance-00000013) state
  changed to 'concluded'(9)

  2021-01-21 05:34:31.468+0000: 60630: debug :
  qemuProcessHandleJobStatusChange:1002 : job 'copy-vdb-
  libvirt-5-format'(domain: 0x7fd070075720,instance-00000013) state
  changed to 'null'(11)

  2021-01-21 05:34:31.468+0000: 60634: debug :
  qemuBlockJobProcessEventConcludedCopyPivot:1221 : copy job 'copy-vdb-
  libvirt-5-format' on VM 'instance-00000013' pivoted

  2021-01-21 05:34:32.292+0000: 60634: debug : qemuDomainObjEndJob:9746
  : Stopping job: modify (async=none vm=0x7fd070075720
  name=instance-00000013)

  ---------------

  Is this normal for a block copy job to hang on the last block like
  this for minutes at a time? Why doesn't the job close out once offset
  == len?

  Thanks for any help you can offer.

  Additional log messages have been pasted here:

  https://pastebin.com/FQRu76Sn

  and for completeness, these logs were taken from the following
  openstack nova gate job:

  https://zuul.opendev.org/t/openstack/build/a078a17aa9924517b329cafc3f54fed4

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