Parallels Compressor Working Principles

Compressor processes a virtual machine in the following way:

The actions performed on your particular virtual machine depend upon the running mode:

in automatic mode Parallels Compressor compresses only the current system disk performing the pre-defined set of actions
in manual mode you are able to choose disks to compress and actions to perform

More about running modes and other Compressor properties can be found in Parallels Compressor Help.

Steps of the Compression Procedure

Parallels Compressor is designed to perform the most efficient compression of a virtual machine. The procedure consists of two steps:

a preparatory step performed in the guest operating system (deleting temporary and unnecessary files, defragmenting virtual disks and cleaning unused disk space);
a compacting step (reducing the size of the virtual disk files) performed in the primary operating system.