Description of problem:

As per the bug 1420405, we are using qemu-img check to find the actual size of the volume. However, if the base image is having leaked cluster, then the qemu-img check will be having a return value of 3.

# qemu-img check /rhev/data-center/00000001-0001-0001-0001-000000000311/8257cf14-d88d-4e4e-998c-9f8976dac2a2/images/7455de38-1df1-4acd-b07c-9dc2138aafb3/be4a4d85-d7e6-4725-b7f5-90c9d935c336  ; echo $?
200 leaked clusters were found on the image.
This means waste of disk space, but no harm to data.
109461/180224 = 60.74% allocated, 15.29% fragmented, 0.00% compressed clusters
Image end offset: 7188578304
3

Anything other than 0 exit code will fail with below error in the vdsm side during cold merge.

===
2017-10-16 10:09:32,950+0530 DEBUG (tasks/0) [root] /usr/bin/taskset --cpu-list 0-3 /usr/bin/qemu-img check --output json -f qcow2 /rhev/data-center/mnt/blockSD/8257cf14-d88d-4e4e-998c-9f8976dac2a2/images/7455de38-1df1-4acd-b07c-9dc2138aafb3/be4a4d85-d7e6-4725-b7f5-90c9d935c336 (cwd None) (commands:69)
2017-10-16 10:09:33,576+0530 ERROR (tasks/0) [storage.TaskManager.Task] (Task='59404af6-b400-4e08-9691-9a64cdf00374') Unexpected error (task:872)
Traceback (most recent call last):
  File "/usr/share/vdsm/storage/task.py", line 879, in _run
    return fn(*args, **kargs)
  File "/usr/share/vdsm/storage/task.py", line 333, in run
    return self.cmd(*self.argslist, **self.argsdict)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/securable.py", line 79, in wrapper
    return method(self, *args, **kwargs)
  File "/usr/share/vdsm/storage/sp.py", line 1892, in finalizeMerge
    merge.finalize(subchainInfo)
  File "/usr/share/vdsm/storage/merge.py", line 271, in finalize
    optimal_size = subchain.base_vol.optimal_size()
  File "/usr/share/vdsm/storage/blockVolume.py", line 440, in optimal_size
    check = qemuimg.check(self.getVolumePath(), qemuimg.FORMAT.QCOW2)
  File "/usr/lib/python2.7/site-packages/vdsm/qemuimg.py", line 156, in check
    out = _run_cmd(cmd)
  File "/usr/lib/python2.7/site-packages/vdsm/qemuimg.py", line 416, in _run_cmd
    raise QImgError(cmd, rc, out, err)
QImgError: cmd=['/usr/bin/qemu-img', 'check', '--output', 'json', '-f', 'qcow2', '/rhev/data-center/mnt/blockSD/8257cf14-d88d-4e4e-998c-9f8976dac2a2/images/7455de38-1df1-4acd-b07c-9dc2138aafb3/be4a4d85-d7e6-4725-b7f5-90c9d935c336'], ecode=3, stdout={
QImgError: cmd=['/usr/bin/qemu-img', 'check', '--output', 'json', '-f', 'qcow2', '/rhev/data-center/mnt/blockSD/8257cf14-d88d-4e4e-998c-9f8976dac2a2/images/7455de38-1df1-4acd-b07c-9dc2138aafb3/be4a4d85-d7e6-4725-b7f5-90c9d935c336'], ecode=3, stdout={
    "image-end-offset": 7188578304,
    "total-clusters": 180224,
    "check-errors": 0,
    "leaks": 200,
    "leaks-fixed": 0,
    "allocated-clusters": 109461,
    "filename": "/rhev/data-center/mnt/blockSD/8257cf14-d88d-4e4e-998c-9f8976dac2a2/images/7455de38-1df1-4acd-b07c-9dc2138aafb3/be4a4d85-d7e6-4725-b7f5-90c9d935c336",
    "format": "qcow2",
    "fragmented-clusters": 16741
}
, stderr=Leaked cluster 109202 refcount=1 reference=0
===

And in engine log

===
2017-10-16 00:39:29,600-04 ERROR [org.ovirt.engine.core.bll.tasks.SPMAsyncTask] (DefaultQuartzScheduler7) [12cf8f7] BaseAsyncTask::logEndTaskFailure: Task '59404af6-b400-4e08-9691-9a64cdf00374' (Parent Command 'FinalizeMerge', Parameters Type 'org.ovirt.engine.core.common.asynctasks.AsyncTaskParameters') ended with failure:
-- Result: 'cleanSuccess'
-- Message: 'VDSGenericException: VDSErrorException: Failed to HSMGetAllTasksStatusesVDS, error = cmd=['/usr/bin/qemu-img', 'check', '--output', 'json', '-f', 'qcow2', '/rhev/data-center/mnt/blockSD/8257cf14-d88d-4e4e-998c-9f8976dac2a2/images/7455de38-1df1-4acd-b07c-9dc2138aafb3/be4a4d85-d7e6-4725-b7f5-90c9d935c336'], ecode=3, stdout={
2017-10-16 00:39:32,468-04 ERROR [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (DefaultQuartzScheduler2) [8b540950-5f08-432d-bcf9-5b2999284532] EVENT_ID: USER_REMOVE_SNAPSHOT_FINISHED_FAILURE(357), Correlation ID: 8b540950-5f08-432d-bcf9-5b2999284532, Job ID: d31f4487-2545-4ba7-a3e4-2cd03f0ea305, Call Stack: null, Custom ID: null, Custom Event ID: -1, Message: Failed to delete snapshot 'test_snap' for VM 'test_vm'.
===


The cold merge will make the image illegal and the user will not be able to start the VM because of illegal legality in the metadata. We need manual changes to recover the VM.

IIUC, the leaked cluster doesn't cause any harm to data and it's just wastage of  space.


Version-Release number of selected component (if applicable):

vdsm-4.19.31-1.el7ev.x86_64
ovirt-engine-4.1.6.2-0.1.el7.noarch


How reproducible:

100%

Steps to Reproduce:

1. Create a thin provisioned disk in RHV-M and assign to a VM.

2. Kill the qemu-kvm process while we write the data from the VM. While writing the data from the VM, the qcow2 will allocate new cluster and killing the process at this time will cause leaked cluster in the image.

3. Create a snapshot for the VM.

4. Do a cold merge.


Actual results:

Cold merge will fail if there is leaked cluster in the base image

Expected results:

Cold merge should work if the base image is having leaked cluster.

Additional info: