Thanks Joerg.
I can confirm I disabled delayedACK as advised by Dell and rebooted the servers and then check via ssh and all the iscsi connections were showing 0.
Below is a log of one of our VM's being backed up. This is a full backup which too over 7 hours.
[2021-10-01 23:30:28.890]: vRanger Backup & Replication - v7.8.3.0
[2021-10-01 23:30:28.890]: Selected Options: Backup powered on machines only. | Check destination for free space. | Update notes with the latest backup results. | Enable Active Block Mapping™ (ABM). | Enable Change Block Tracking (CBT). | Selected SpaceSavingTech: Incremental.
[2021-10-01 23:30:28.906]: Task for virtual machine sccm-srv was queued.
[2021-10-02 00:54:52.477]: SourceVm:sccm-srv | Uuid:421ffd2e-4a92-ea97-0d31-567789843285 | VC:vcenter-srv.domain.local, Host:esxi2.domain.local [ESXi 6.0.0]
[2021-10-02 00:54:52.477]: Beginning backup task for Backup Plan 2 NAS C-sccm-srv
[2021-10-02 00:54:52.493]: Starting task validation.
[2021-10-02 00:54:52.493]: Connection to vcenter-srv.domain.local was properly validated.
[2021-10-02 00:54:52.493]: esxi2.domain.local is properly licensed.
[2021-10-02 00:54:52.493]: Validating virtual appliance VA2-vRangerVA completed.
[2021-10-02 00:54:52.509]: Test connection to repository C-Vranger-Backup-Volume starting...
[2021-10-02 00:54:53.305]: Test connection to repository C-Vranger-Backup-Volume successful!
[2021-10-02 00:54:53.305]: Ending task validation... success!
[2021-10-02 00:54:53.305]: Beginning initialization of backup information.
[2021-10-02 00:54:53.352]: Retrieving the tasks parent information.
[2021-10-02 00:54:53.384]: Retrieving save points for any full backups associated with this job.
[2021-10-02 00:54:53.540]: Full backup days policy is enforced.
[2021-10-02 00:54:53.540]: Finished initialization of backup information successfully.
[2021-10-02 00:54:53.540]: Initialization was sucessful. Backup type to run: Full
[2021-10-02 00:54:57.587]: Retrieving the VM BIOS configuration completed.
[2021-10-02 00:54:59.524]: Checking free space on C-Vranger-Backup-Volume completed.
[2021-10-02 00:55:05.727]: Creating a snapshot for vRanger completed.
[2021-10-02 00:55:16.133]: Loading virtual machine 'sccm-srv' information completed.
[2021-10-02 00:55:19.305]: Local machine is a VMware virtual machine.
[2021-10-02 00:55:19.383]: Backup task will attempt to use VA-based VDDK HotAdd.
[2021-10-02 00:55:37.914]: Using filter type(s) change block and active map for disk: vix:1:r:vcenter-srv.domain.local\:443:0:[VM-SAN-EMC-18] sccm-srv/sccm-srv.vmdk
[2021-10-02 08:29:43.350]: Backing up disk 'vix:1:r:vcenter-srv.domain.local\:443:0:[VM-SAN-EMC-18] sccm-srv/sccm-srv.vmdk:moref=vm-73142:snapshot-85256:4' completed.
[2021-10-02 08:31:51.926]: Removing snapshot for vRanger completed.
[2021-10-02 08:31:53.551]: Committing savepoint data to C-Vranger-Backup-Volume completed.
[2021-10-02 08:31:58.598]: Verifying the content of the repository completed.
[2021-10-02 08:31:58.614]: Checking and enforcing retention policy completed.
[2021-10-02 08:31:59.411]: Updating notes for sccm-srv completed.
[2021-10-02 08:31:59.411]: Updating VM notes completed.
[2021-10-02 08:31:59.504]: Setting VM event completed.
Thanks again for looking at this.
It is much appreciated.