Ulli Hankeln (aka continuum) recently started an interesting thread in the VMware communities asking what cloning method people prefer with VMware Converter.
Some time ago we started a fairly large server virtualization project (several hundred servers) and ask ourselves exactly this question. After some discussions we identified some evaluation criteria and judged both of two methods following these criteria:
1. Risk of data loss
This is clear point for cold clone, because it will make the machine absolutely unavailable and unchangeable during the conversion process. With a hot clone it is possible that data on the machine is changed during the conversion process that will then not be replicated to the virtual clone.
2. Operations complexity
This in contrast is a clear point for hot cloning. This process is fully automated through the Converter GUI. For cold cloning you need to boot the server from a CD. If you are lucky you can do that remotely be using HP iLO or similar remote control adapters. But you will always stumble over some servers that you will need to visit physically to mount the boot CD.
3. Operations success rate and possible errors
What can go wrong with a cold clone? You might not be able to even start it, because you cannot boot the server through iLO (or similar means) and do not have physical access to the system (at the time you need it) to mount the CD physically. If this is not a problem the second possible error is that the boot CD does not include the necessary drivers for the conversion: You need a working mass storage driver and a working network driver, but the CD only includes a limited set of drivers that might not be suitable for you if the server you are going to virtualize has quite modern hardware. It is possible though to rebuild the boot CD and add additional drivers to it, but this is an extra step and needs extra time and effort.
What can go wrong with a hot clone? You will always be able to start the hot cloning, but in some situations it will fail half way through or at the end. Possible errors are e.g. corrupt file systems or bugs in converter. In such a case troubleshooting is difficult, although you will find some hints in the VMware KB. Fortunately, this does not happen too often. In our experience in clearly less than 5% of all cases.
So, what's working better? It very much depends on your setup, the types of hardware you have, the OSs you are virtualizing and the applications you are running in them.
For us hot cloning wins this criteria, but we were not able to decide that until we already started our virtualization project and made good progress with it.
4. VMware support
In recent releases of Converter (including 5.0 being in beta right now) VMware has dropped support for cold cloning. I guess the reason for this is that they wanted to get rid of maintaining the boot CD and instead concentrate on improving the hot conversion process. And they really did that e.g. by adding incremental data synchronization after the first data copy run.
So, this is a win for hot cloning. Anyway, it is still possible to use cold cloning with the current releases of Converter, because it has support for importing the images of third party cloning tools (like Symantec Ghost and Acronis TrueImage). The reconfiguration process of Converter will make these images bootable with the virtual clone by injecting the necessary mass storage drivers into it.
Conclusions: In our project we have a "hot cloning first"-policy. To mitigate the risk of data loss we stop all unneeded services on the source machine (like application and database services) before starting the process. On machines that are used interactively (like Windows Terminal Servers) we disable logons.
Only if the hot clone fails we try a cold clone as the next step.
As stated earlier, your mileage may vary. What do you think? Like always comments are welcome.