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transport mode: [hotadd] async I/O: [unsupported], WHY?

  • May 19, 2026
  • 6 comments
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P

Hello!

I look through logs, and see this text

transport mode: [hotadd], max sectors to read/write: [8192], async I/O: [unsupported]

The backup operation ends successfully, but why could possibly async I/O be unsupported for hotadd? I've changed VBR VM (it's a Default Proxy) SCSI Controller type to VMware Paravirtual and it didn't help and sincerely I'm out of options to guess. Can you suggest me a direction to dig?

Best answer by eblack

Broadcom VDDK documentation documents asynchronous I/O as an NBD/NBDSSL feature. 

    6 comments

    Chris.Childerhose
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    Based on what I am reading and can find is that Async I/O is not supported with HotAdd, but I need to do some more digging.

    To get a good answer for this, I would suggest opening a support ticket with the logs to show this to Support.  They will be able to give you a definitive answer at that point.

     
     
     
    Chris Childerhose (Enterprise Architect) - VMCA | VMCE | VMCE-SP | Veeam Vanguard 8* | Veeam Legend 5* | VUG Canada Leader | vExpert 7* | Toronto VMUG Leader | VCAP-DCV/VCP-DCV/VCP-VVF | Object First Ace | Cisco Champion | Twitter: @cchilderhose | Blog Site – https://just-virtualization.tech
      coolsport00
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      • coolsport00
      • Veeam Legend
      • May 19, 2026

      What specific issue are you having? You said your backups are running fine? With any “trouble” you experience within VBR or its components, it’s always best to reach out to Veeam Support. 

      But, if you share what specific issue you’re having, maybe someone who may have had a similar experience can comment on how to resolve it.

      Best.

      Shane Williford - Veeam VMCA/VMCE | Veeam Legend | VUG Leader | VCP-DCV | Twitter: @coolsport00
        AndrePulia
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        • AndrePulia
        • Veeam Vanguard
        • May 19, 2026

        ​@pgadmin I would recommend you to use surebackup just to be sure if the vbk is OK, did you realize any other different behavior as usual, like, the job took longer to complete?
        But for sure as mentioned above, opening a ticket is a good idea 

        André Pulia - VMCA | VMCE | VMCT |Veeam Vanguard | VUG Leader Brazil | ObjectFirst ACE | HPECI | VCI | Nutanix Certified Instructor | Twitter: @andrepulia | Blog Site – https://pulia.com.br | https://www.credly.com/users/andre-pulia
          eblack
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          • eblack
          • Influencer
          • Answer
          • May 19, 2026

          Broadcom VDDK documentation documents asynchronous I/O as an NBD/NBDSSL feature. 

            P
            • pgadmin
            • Author
            • Comes here often
            • May 20, 2026

            Broadcom VDDK documentation documents asynchronous I/O as an NBD/NBDSSL feature. 

            So it’s like Chris.Childerhose said, no async I/O for hotadd at all?

             

            Thank you and thanks everyone for the help!

              eblack
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              • eblack
              • Influencer
              • May 20, 2026

              Broadcom VDDK documentation documents asynchronous I/O as an NBD/NBDSSL feature. 

              So it’s like Chris.Childerhose said, no async I/O for hotadd at all?

               

              Thank you and thanks everyone for the help!

              I’m inclined to say it’s not feasible /w hotadd. I took a look at some of the hotadd proxies I have in the environment, I see similarities in the logging behavior.

               

              VixDiskLib_WriteAsync for hotadd is synchronous in the older behavior, with async support called out only for NBDSSL when the ESXi host supports async NBD.

               

              The VDDK 7.0.3/8.0 U3 release notes reinforce that the configurable async I/O buffer feature is described for NBD(SSL) transport, not HotAdd. 

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