OK, so feedback from Veeam support;
Firstly need to ensure that Veeam proxies have sufficient memory to hold metadata required for accelerated restores.
Secondly VMware has known limitations with NFC protocol and NBD.
For large environments and bulk restore operations, this does raise some questions around how to scale out VMware.
We have a known limitation for Accelerated Restore of Entire VM:
"If you restore a VM using the Network transport mode, the number of VM disks restored in parallel cannot exceed the number of allowed connections to an ESXi host"https://helpcenter.veeam.com/docs/backu ... tml?ver=95
NBD employs the network file copy (NFC) protocol. NFC Session Connection Limits shows limits on the number of connections for various host types. These are host limits, not per process limits. Additionally vCenter Server imposes a limit of 52 connections. VixDiskLib_Open() uses one connection for every virtual disk that it accesses on an ESXi host. Clone with VixDiskLib_Clone() also requires a connection. It is not possible to share a connection across physical disks.
So this message just indicates you exceeding 52 connections limit.
For further references:https://pubs.vmware.com/vsphere-60/inde ... t.5.5.html
(section "NFC Session Limits")