Solid Compression - Explanation

Explanation

Compressed file formats often feature both compression (storing the data in a small space) and archiving (storing multiple files and metadata in a single file). One can combine these in two natural ways:

• compress the individual files, and then archive into a single file;
• archive into a single data block, and then compress.

The order matters (these operations do not commute), and this latter is solid compression.

In Unix, compression and archiving are traditionally separate operations, which allows one to understand this distinction:

• compressing individual files and then archiving would be a tar of gzip'ed files – this is very uncommon, while
• archiving via tar and then compressing yields a compressed archive: a .tar.gz – and this is solid compression.