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.
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).