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Virtual Disk Development Kit library reports the error: Failed to open NBD extent, NBD_ERR_GENERIC (1022543)

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  • You cannot open <VM Name>.vmdk files
  • You see the error:

    Failed to open NBD extent, NBD_ERR_GENERIC

  • Virtual Disk Development Kit (VDDK) logs found in Server 2003: C:\Documents and Settings\<username>\Local Settings\Temp\vmware-<username>\, Server 2008: C:\Users\<username>\AppData\Local\Temp\2\vmware-<username>, Linux: /tmp/vmware-root/ report errors similar to:
[NFC ERROR] NfcFssrvrRecv: failed with code = 8
[NFC ERROR] NfcFssrvr_DiskOpen: failed to receive open reply
[NFC ERROR] NfcNetTcpWrite: bWritten: -1
[NFC ERROR] NfcNet_Send: requested 264, sent only -1 bytes
[NFC ERROR] NfcSendMessage: send failed
  • The vpxa.log file located at /var/log/vmware/vpx  on ESX/ESXi 4.x hosts reports errors similar to:

    F5AC8B90 info 'Libs'] NfcNetTcpRead: timed out waiting for data
    F5AC8B90 warning 'Libs'] [NFC ERROR] NfcFssrvrRecv: failed with code = 9

  • The vpxa.log file located at /var/log/ on ESXi 5.x hosts reports errors similar to:

    [NFC ERROR] NfcCheckAndReserveMem: Cannot allocate any more memory as NFC is already using 31457329 and allocating 2097152 will make it more than the maximum allocated: 33554432. Please close some sessions and try again


This issue occurs when the number of network file copy (NFC) connections to the ESX\ESXi host exceeds the buffer space. When the NFC client does not shut down properly, the ESX\ESXi host keeps the NFC sessions open for another 10 minutes. This might also increase the pool of connections to the ESX\ESXi host.


To resolve this issue:

  • Ensure that the unused NFC connections are closed properly.
  • Limit concurrent scheduled backups per ESX\ESXi host or vCenter Server.

    • VixDiskLib_Open() uses one connection for every virtual disk that it accesses on an ESX/ESXi host. 
    • VixDiskLib_Clone() also requires a connection.
    • It is not possible to share a connection across disks.
    • The ESX\ESXi limits depend on consumed buffer space, not the number of connections.

 For more information on the number of concurrent NFC connections, see Virtual Disk Transport Methods.

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  • 15 Ratings