Hello,
it happened two times, always with VMs composed by more than 1 disk.
Now I have a VM with 9 disks and a total of about 370Gb:
1 x 90Gb
5 x 50Gb
3 x 10Gb 

I export vm to an export domain and it started 2 hours and 10 minutes ago at 15:46.
At beginning the write rate on export domain was 120MB/s (in line with I/O capabilities of storage subsystem).
It seems 5 disks completed ok, while 4 don't complete, even if it seems to be some reading activity:

with command
iotop -d 3 -k -o -P 
I get this on hypervisor where qemu-img convert command is executing

Total DISK READ :    5712.75 K/s | Total DISK WRITE :     746.58 K/s
Actual DISK READ:    6537.11 K/s | Actual DISK WRITE:       6.89 K/s
  PID  PRIO  USER    DISK READ>  DISK WRITE  SWAPIN      IO    COMMAND                                            
21238 idle vdsm     1344.18 K/s  183.12 K/s  0.00 %  0.15 % qemu-img convert -p -t non~1d23-4829-8350-f81fa16ea8b0
21454 idle vdsm     1344.18 K/s  183.12 K/s  0.00 %  0.06 % qemu-img convert -p -t non~443d-4563-8879-3dbd711b6936
21455 idle vdsm     1344.18 K/s  189.68 K/s  0.00 %  0.08 % qemu-img convert -p -t non~b67d-4acd-9bb5-485bc3ffdf2e
21548 idle vdsm     1344.18 K/s  190.67 K/s  0.00 %  0.06 % qemu-img convert -p -t non~7fc

On export share
[root@xfer ~]# ll -td /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/*
drwxr-xr-x 2 36 36 4096 Jan 21 17:21 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/ac33a8fe-f10e-4cbb-b121-2956f9925ade
drwxr-xr-x 2 36 36 4096 Jan 21 17:06 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/20c07082-d5ed-4804-867d-1f3f7c202b0e
drwxr-xr-x 2 36 36 4096 Jan 21 17:02 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/f8b1c6bc-de13-4416-9fe6-d7c26fb082b1
drwxr-xr-x 2 36 36 4096 Jan 21 17:02 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/1b5313ef-0ee5-4f9a-b2a6-baa153ff0984
drwxr-xr-x 2 36 36 4096 Jan 21 17:02 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/3f21c566-bc00-4dab-acbb-db9a3a1d76fa
drwxr-xr-x 2 36 36 4096 Jan 21 15:47 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/ed3a721e-fd1b-4fea-ad41-7ca56e94890e
drwxr-xr-x 2 36 36 4096 Jan 21 15:46 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/213f9ce9-6c34-4911-820e-4d1a96ba1791
drwxr-xr-x 2 36 36 4096 Jan 21 15:46 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/28fef57c-4254-4469-9f3f-0d1f114d4f78
drwxr-xr-x 2 36 36 4096 Jan 21 15:46 /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/ea4fa5b1-a93a-4cb9-9552-e8ec67c5ff75

[root@xfer ~]# du -sh /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/*
11G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/1b5313ef-0ee5-4f9a-b2a6-baa153ff0984
51G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/20c07082-d5ed-4804-867d-1f3f7c202b0e
51G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/213f9ce9-6c34-4911-820e-4d1a96ba1791
51G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/28fef57c-4254-4469-9f3f-0d1f114d4f78
11G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/3f21c566-bc00-4dab-acbb-db9a3a1d76fa
85G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/ac33a8fe-f10e-4cbb-b121-2956f9925ade
51G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/ea4fa5b1-a93a-4cb9-9552-e8ec67c5ff75
51G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/ed3a721e-fd1b-4fea-ad41-7ca56e94890e
11G /export/ovirt/a6a289ea-f160-4d35-b3aa-e59d171b4633/images/f8b1c6bc-de13-4416-9fe6-d7c26fb082b1
[root@xfer ~]# 

It seems more than 30 minutes no more progress....

I see no errors in vdsm.log of hypervisor and in engine.log I keep this message every 10 seconds:

2019-01-21 18:08:19,060+01 INFO  [org.ovirt.engine.core.bll.ConcurrentChildCommandsExecutionCallback] (EE-ManagedThreadFactory-engineScheduled-Thread-10) [a1c0e39f-1924-4ae8-8b48-30bcce669278] Command 'ExportVm' (id: 'dc0a58bd-e8ca-42a9-8de0-dd3ccabe0512') waiting on child command id: '29d40db8-a0b9-4408-9e62-1abf3bccca65' type:'CopyImageGroup' to complete

All the disks except the "apparently" not completed one (the 90Gb disk) are preallocated.
Any other hint on what to check?

Thanks,
Gianluca