@olivierlambert Hi Olivier. I'll see what I can do. I've spent the weekend cleaning up my backups and catching up on mirror transfers. Once completed, I'll do a few custom backups at various nconnect values.

@acomav Replying to myself again. After working for a few days, the issue restarted. I'll raise a ticket.

@olivierlambert
I can confirm it was my side. I had to do a few things to get the VMware Virtual disks to free up empty space and once I did, the VM Import to XCP-NG to an NFS SR successfully copied the virtual disk in a thin mode.
For anyone reading this who will be preparing to jump ship off VMware.

I am using vSphere 6.7. I have not tested against vSphere 7 yet. Not bothering with vSphere 8 for obvious reasons. My VM was a CentOS 7 VM with LVM to manage the 3 virtual disks.

1. Make sure you Virtual Hardware is at least version 11. My test VM was a very old one still on version 8.
2. For the ESXi host the VM lives on (but you should probably go all hosts in the cluster), go into Advanced settings, and enable (change 0 to 1) VMFS3.EnableBlockDelete. I thought I had this enabled but only 2 of the 5 hosts in the cluster did. You may need to check this is not reset after updates.
3. Due to using CentOS 7 (perhaps) I could not used 'fstrim' with the discard mount option. It was not supported. I filled up the diskspace with zeros, synced, and then removed the zeroes.

# cd /mount point; dd if=/dev/zero of=./zeroes bs=1M count=1024; sync; rm zeroes

Change count=1024 (Which will create 1 GB of zeroes in a file) to however big a file you require to nearly fill up the partition / volume. eg count=10240 will make a 10 GB file.
Windows users can use 'sdelete'.

I could have waited for vSphere to automatically clean up the datastore in the background at this stage, but I was impatient and 'storage motioned' the virtual disks to NFS storage in Thin mode. I confirmed only the used space was copied across. I then migrated the disks back to my HP Nimble SAN and retained thin provisioning.

@olivierlambert Hi.
The disk sizes (and vmdk file size) are 150GB and 170GB. Both are in a Volume group and one Logical Volume using 100% of the Volume group mounted using XFS.

Disk space in use is 81%:

# pvs
  PV         VG         Fmt  Attr PSize    PFree
  /dev/sda2  centos     lvm2 a--   <15.51g    0 
  /dev/sdb   VolGroup01 lvm2 a--  <150.00g    0 
  /dev/sdc   VolGroup01 lvm2 a--  <170.00g    0 

# vgs
  VG         #PV #LV #SN Attr   VSize   VFree
  VolGroup01   2   1   0 wz--n- 319.99g    0 
  centos       1   2   0 wz--n- <15.51g    0

# lvs
  LV        VG         Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  IMAPSpool VolGroup01 -wi-ao---- 319.99g 

# df -h
/dev/mapper/VolGroup01-IMAPSpool  320G  257G   64G  81% /var/spool/imap

The vmdk files live on an HPE/Nimble CS3000 (Block iscsi). I am now thinking I will need to get into the VM and free up discarded/deleted blocks....which would make the vmdk sizes smaller. (as they are set to thin provisioned with vmfs)
I'll do that and retry and report back if I see the the full disk being written out to XCP-NG.

@acomav

Import completed. Great work @florent.