IT Services



Backing Up Virtual Machines/VMware With TSM


We are trialling a new TSM for Virtual Environments service which can directly backup VMs via a vCenter server. If you would like to trial this service please see the page on TSM for Virtual Environments for more details.

If you wish to back up virtual machines, then this can be done either as part of the regular backup of the host machine, backing up the virtual machine files; or by registering the virtual machine itself as a TSM node and backing up from within it. For several reasons, it is better for both client and server if you use the latter method, excluding your virtual machine files from backup on the host machine and only backing it up when logged into it. The reasons are:

  1. Because each virtual machine is one or more very large file(s), the smallest change will result in that large file being fully backed up all over again every time a backup occurs. Even running a virtual machine without actively making any changes is probably enough to make these large files change, and TSM treats each changed file as qualified for backup.
  2. Running a backup of a virtual machine externally on two consecutive days, where any change has occurred to the machine in between, would therefore result in two copies being sent to the HFS. Since the HFS only keeps a maximum of two copies of any file, this would mean that our backup for the virtual machine would only go back as far as two days. Any files that a user needed to be restored but which were deleted over two days ago would no longer exist on the HFS. However, if a virtual machine is backed up from within, the HFS would have the usual data for a TSM node - up to two versions of each file, going back as far as 90 days. The user would then be able to restore individual files from this file store.
  3. If a virtual machine is backed up externally while it is in use then it is very possible that the resulting backup will be unusable. Backing up an open file risks a corrupt backup because the result may not accurately reflect what is in the file at time of backup - it might contain some, but not all, of the changes that are being made.
  4. The repeated and frequent sending of such large files results in an unnecessarily large amount of network traffic.

We therefore recommend that you register your virtual machines individually and exclude them from backup on the host machine. Files of the type *.vmdk and *.vmem are excluded by default in HFS releases from March 2007 onwards (TSM 5.4). If you have an earlier release of TSM then you will need to exclude them yourself: for more information on how to do this, see our page on how to exclude files and folders from backup.

However, if you have virtual machines that rarely change (such as a development machine), and if you therefore feel that the above points do not apply to you because there would be no frequent sending of large files, then it may be appropriate for you to back them up as normal files. In this case you would need to be sure to remove the aforementioned exclusions in order for the virtual machine files to be backed up.