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We are running a VMWare environment.

We created a new iSCSI datastore for some of our VMs due to the previous Fastclass Fibrechannel datastore having issues.

All of our VMs are running well on this new datastore but every Veeam backup job fails for VMs housed on it. All backups for VMs running on other datastores are fine. The VMs show up in Veeam and are able to be added to jobs and show their file sizes, but fail when the job is run.

Any ideas? I’ve pasted the typical error message below that we get, with some of our info redacted. Note that some of the files do actually back up, namely the smaller files like the .vmx, .vmxf, and .nvram, but the .vmdk file seems to be where the error occurs.

 

10/1/2025 3:18:59 PM :: Processing ******** Error: Failed to open VDDK disk [[***NAME OF ISCSI DATASTORE***] ***NAMEOFVM***.vmdk] ( is read-only mode - [true] )
Logon attempt with parameters [VC/ESX: [***VMWare HOST IP***];Port: 443;Login: [***USERNAME***];VMX Spec: [moref=9];Snapshot mor: [9-snapshot-3];Transports: [san];Read Only: [true]] failed because of the following errors:
Failed to open disk for read.
Failed to upload disk. Skipped arguments: [vddkConnSpec>];
Agent failed to process method {DataTransfer.SyncDisk}.

Hi,

 

it’s failing in Direct SAN mode which is likely that your proxy/proxies don’t have iSCSI access to the datastore. If you’re using a separate datastore for snapshots for the VMs on that datastore also remember to configure the permissions for this too.

 

If you enable failover mode to network I trust this works ok?

Or further to Michael’s answer set the Proxies to Automatic mode and see what it uses then you can tell what mode is working.

Thank you! Failing over to network mode seems to have done the trick. I’m running a test backup right now and it’s gotten to 39% on the VMDK which is way further than it’s gotten in the past.

I’ll post back if it fails again, but it looks like that has helped!

 

 

​@Chris.Childerhose we actually were already set to Automatic previously on the proxy.  Checking the “Failover to network mode if primary mode fails, or is unavailable” box on the transport mode settings allowed us to complete our test backup successfully.

I’ll follow-up here if our nightlies fail again, but right now it’s looking promising!

Hi ​@MOIT -

As Michael shared...did you enable iSCSI access on your Proxy server? What OS are you running?..Windows or Linux? There a several steps to enable iSCSI in Windows, and a few on Linux...namely enabling iscsi initiator...and assigning the IQN of your Proxy server Access to the Datastore housing your source VMs on your SAN. Once you do that, you should be good to go.

Share further info and we’ll try and get you sorted. Also, good to take a look at a few articles to fill in the gaps:

https://helpcenter.veeam.com/docs/backup/vsphere/direct_san_access.html?ver=120

https://veducate.co.uk/how-to-setup-veeam-direct-san-backup-over-iscsi-unleash-the-speed/

If using Linux Proxy:

And: https://www.veeam.com/kb1446

 

Our Veeam server is running Windows.

Thanks for the additional info - I’ll read over the links and try to get the iscsi initiator enabled. 

Ok, let us know how it goes. 

​@Chris.Childerhose we actually were already set to Automatic previously on the proxy.  Checking the “Failover to network mode if primary mode fails, or is unavailable” box on the transport mode settings allowed us to complete our test backup successfully.

I’ll follow-up here if our nightlies fail again, but right now it’s looking promising!

Sounds good.  Glad it was something simple.  Hoping for completion. 💪

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