Live snaphot cloning (VBoxManage clonevm) fails when basefolder is SMB share

[Aleksandr Kotlyar]

When the --basefolder is specified to point to a Samba share, the clonevm process fails for a previously-created live snapshot.

Interesting note: the issue does NOT occur when the snapshot was taken from a VM that was powered off (not live). Live snapshot clones do succeed, however, when the basefolder is on the local ext4 filesystem.

Host is Ubuntu 16.04 amd64 running kernel 4.15.

$ VBoxManage snapshot "{7a81c085-7fd5-49c6-a698-c64eda275119}" take backup.tmp --live
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Snapshot taken. UUID: d53e1bfb-2111-4945-ac1a-d1b10206c5f3

$ VBoxManage clonevm "{7a81c085-7fd5-49c6-a698-c64eda275119}" --snapshot backup.tmp --options keepallmacs,keepdisknames --basefolder /vmdata/_backups/tmp --name "iot-manager.bak"
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...
Progress state: VBOX_E_IPRT_ERROR
VBoxManage: error: Clone VM failed
VBoxManage: error: Code VBOX_E_IPRT_ERROR (0x80BB0005) - Runtime subsystem error (extended info not available)
VBoxManage: error: Context: "RTEXITCODE handleCloneVM(HandlerArg*)" at line 609 of file VBoxManageMisc.cpp

/vmdata/_backups is a Samba mountpoint on a Synology box. Relevant info from mount:

//mynas/NAS/VM stuff/backups on /vmdata/_backups type cifs (rw,relatime,vers=3.0,sec=ntlmssp,cache=strict,username=nasuser,domain=,uid=1000,forceuid,gid=0,noforcegid,addr=mynas,file_mode=0755,dir_mode=0755,soft,nounix,serverino,mapposix,rsize=1048576,wsize=1048576,echo_interval=60,actimeo=1)

[guideloom]

This is still occurring in 6.0.10, using an NFS share. See here for additional info : ​https://forums.virtualbox.org/viewtopic.php?f=7&t=94104

Can this get escalated ASAP please?

Note: I've opened a new ticket for this issue when dealing with NFS. https://www.virtualbox.org/ticket/18811