Rescuing a running virtual machine with dd when datastore metadata is inacces

Details

This is an alternative Linux command to clone vmdks without powering off a virtual machine. Some possible situations for use include the following:

• Metadata is corrupted or lost
• Part of the LUN has been over-written, for example, by an installation of ESX to incorrect LUN

This method may present some advantages over alternatives where the virtual machine is powered on:

• It is faster than Converter as data is transferred directly to SAN instead of over network
• It is possible to restart the guest operating system if the procedure fails

vmkfstools -i is not an option where:

• The virtual machine is powered on and the vmdk is locked
• You cannot create snapshot because you cannot risk an update to metadata

VMware Converter may not be an option where:

• The guest operating system is non-Windows, for example  Linux (pre-VMware Converter 4.0), Netware, or Solaris
• Insufficient diskspace on LUN for snapshots to continue to grow while the guest operating system is running

Solution

To rescue a virtual machine with dd when metadata is inaccessible:

1. Click Edit Settings > Add Hard Disk. Create a new hard drive of the same size on a different datastore.
2. Reboot within the guest operating system. Press F2 to enter the BIOS and change boot order to boot from CD.
3. From the virtual machine console toolbar, connect the live Linux boot CD ISO as a virtual CD-ROM. Press F10 to save the BIOS changes and exit.
4. Time how long it takes for this command to run:
   dd if=/dev/sda of=/dev/sdb bs=16M conv=sync,noerror
   The time to run is dependent on your data transfer rate.

Note: The virtual machine must not be powered off or reset during this process if the metadata may be corrupt.

Note: Test your backup method to ensure it functions as expected in your environment.