Xen: Attaching the disk failed


This article addresses the failiure message "Attaching the disk to vm failed" with internal error DISK_VBD_MUST_BE_READWRITE_FOR_HVM.


Xen backups fail with the following backup message:

----- XProtect Messages ----
attaching the disk to vm failed
----- End XProtect Messages ----

The internal Xen API error message is:

2017-12-19 15:06:05.361 -0600 : E DISK_VBD_MUST_BE_READWRITE_FOR_HVM
2017-12-19 15:06:05.361 -0600 : E OpaqueRef:9ca6e6c9-6330-e760-d3dd-da41df0d97e8
2017-12-19 15:06:05.361 -0600 : E failed to vbd_plug


This error occurs as a result of  the UB being deployed in XenServer in HVM (Hardware Virtualized Mode). This hands off direct access of many devices directly to the VM but prevents the guest OS from being aware of certain restrictions set by the Xen host, including read-only restrictions on disks. In order to attach a read-only disk during the backup the UB must be in PV (ParaVirtualized) or PV-HVM (PV disks/network, HVM CPU/RAM).


  • Power off the UB
  • Identify the UBs VM UUID. From the XenServer host console run:
[root@xenserver72-237-122 ~]# xe vm-list
uuid ( RO)           : f9296c97-46bb-537e-b1f8-04d197e6fa27
     name-label ( RW): UB-1000-130
    power-state ( RO): halted
  • Using the VM UUID set the VM Parameters to start PV mode:
[root@xenserver72-237-122 ~]# xe vm-param-set uuid=f9296c97-46bb-537e-b1f8-04d197e6fa27 HVM-boot-policy="" PV-bootloader="pygrub" PV-args="utf8 console=tty1"
  • Restart the UB

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