auto-compression-mode is a variable defined in `jka-cmpr-hook.el'.
Its value is

Non-nil if Auto-Compression mode is enabled.
See the command `auto-compression-mode' for a description of this minor mode.
Setting this variable directly does not take effect;
either customize it (see the info node `Easy Customization')
or call the function `auto-compression-mode'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.