Backup compression format
By default, Virtualmin uses gzip compression on its backup files. However, this option can be used to select the space efficient (but CPU-intensive) bzip2 format instead. You can also use un-compressed tar format backups, which may be useful if you want a speedy backup, or if you are writing to tape. Or the zip format, which is easier to extract on other operating systems.

This option is only used when backing up virtual servers to a directory, with one file per domain. When backing up to a single file, the filename determines the compression format (like domains.tar.gz or domains.tar).