In virtualisation, so-called 'hot-swap backups' (other names such as 'snapshots' are also commonly used) are almost always implemented. 'Hot-swap' means that, prior to backing up, the machine is put into the static condition, so that backing up (without guest shutdown) can then be carried out.
In order that guests can be retrieved 'without problems' at some later point, the main memory (RAM) and the hard drive files are normally backed up in such a way that the guests are temporarily 'frozen in'. An image of the main memory (RAM) is then created and backed up together with the contents of the hard drive file(s). During restore, the backed-up content of the main memory (RAM) and the hard drive files are shifted back to the time of the original backup, i.e. the guests are back-dated to the point in the operation when the hot-swap data backup was implemented.
This ensures that the exact conditions that existed at the time of the data backup are recreated. However, such backups cannot be regarded as static, because during hot-swap operations, no checks are carried out as to whether the data actually exists in a static condition. In the worst case scenario, the backup is in a state where the operating system itself has stalled. Alternatively, the backed-up files in the database server are corrupted; in the best case scenario, these files can be retrieved, though retrieval is usually extremely time-consuming.
For database applications (which are the rule in the server environment), plug-ins are often installed in the guests, so that the databases are briefly paused prior to the creation of the memory image (for example); this, however, requires plug-ins to be available and also that the corresponding versions, the virtualisation software and the guest software are compatible. So, what can go wrong? Very simply, the virtualised guests can only be backed up if the virtualisation software provider makes the operating system plug-ins available. Or to put it another way, if there is no plug-in being used for the operating system, no accurate data backups can be carried out. Ultimately, this is sold as 'certified for XYZ', but it should be labelled 'limited to XYZ'.