VMware OVF Tool automatically compresses disk files. In the streaming VMDK files that OVF Tool generates, the tool compresses each 64KB disk grain. It is possible to achieve even better compression using the --compress option. In addition, if a package contains multiple virtual machines, it is possible to compress an OVF package even more using a technique called delta disk compression. This compression algorithm is invoked using the --makeDeltaDisks option.

ovftool --makeDeltaDisks package.ovf output-dir/

Delta disk compression identifies disk segments that are equal and combines these equal parts in a parent disk. This process prevents storing the same segment twice.

As an example, consider a software solution that consists of an Apache Web server virtual machine and a MySQL database virtual machine, both installed on top of a single-disk Ubuntu server. The two virtual machines were created with the following process:

  1. Create a plain Ubuntu installation on one virtual machine.
  2. Clone the virtual machine.
  3. Install Apache on the first virtual machine.
  4. Install MySQL on the second virtual machine.

Using delta disk compression on the two virtual machine disks creates a parent disk containing all of the information they share, which is essentially the entire operation system and two child disks containing the MySQL and Apache parts.

A plain Ubuntu server can use 400–500MB of space, and two would use 800–1000MB of space. By contrast, using delta disk compression, an OVF package with these two servers uses only 400–500MB (plus the size of the MySQL and Apache installations), which saves 400–500MB by not duplicating the Ubuntu server.

vSphere 4 and later support the deployment of OVF packages that contain delta disk hierarchies. Any number of disks can be combined creating various disk trees and saving more space.

For delta disk compression, keep in mind the following:

  • Only disks with equal capacity can be combined. If you expect to use delta disk compression, you must keep disk capacities equal.
  • Delta disk compression necessitates that segments that might be put in a parent disk are at the same offset from the beginning of their respective files. In the Ubuntu example, if the setup varies between the two installations, it can completely offset each segment on one of the disks from the segments on the other disk. In this case, delta disk compression does not produce any significant disk space savings. This is why the example specified cloning the Ubuntu server before installing the MySQL and Apache parts, respectively.
  • Delta disk compression takes OVF packages and vSphere and VMX files as input, but not OVA packages.
  • The delta disk compression algorithm needs to read the contents of each disk up to two times. It might make sense to invoke OVF Tool on a local copy of the OVF package.
  • The delta disk compression algorithm always generates an OVF package in the given output directory. To convert this OVF package into an OVA package, reinvoke OVF Tool.