| From: Howard Gibson via talk <talk@gtalug.org> | I figured that separate email files would be using up a lot of | sectors, but my bandwidth is limited by my Blu-ray discs. My actual | backup is a gzipped tar file. Sectors should not be a problem, should | they? Uncompressed TAR format: <https://www.gnu.org/software/tar/manual/html_node/Standard.html> tar uses blocks. The default block-size is 512 bytes (that dates back to 7th edition UNIX in the late 1970s) (for many applications you want to use a larger block size but not for maximal compression). Each file uses one header block plus the number of blocks to hold the contents. So maildir is probably significantly worse than mbox if no compression is used. I expect any compression technique to be fairly good at compressing a run of 0 bytes. Such bytes are expected for filling out each header block and the last block of each file. I cannot quantify "fairly good". mbox has none of that per-message overhead, compressed or not. Without testing, I cannot say how much worse compressed tar files of maildir would be, compared with compressed tar files of mbox.