Solid compression

In computing, solid compression refers to a method for data compression of multiple files, wherein all the compressed files are concatenated and treated as a single data block. Such an archive is called a solid archive. It is used natively in the 7z and RAR formats, as well as indirectly in tar-based formats such as .tar.gz and .tar.bz2. By contrast, the ZIP format is not solid because it stores separate compressed files (even though solid compression can be portably emulated).

The term is ostensible because the data is compressed as a single solid block, rather than as individual files.

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.

Rationale

Benefits

Solid compression allows for much better compression rates when all the files are similar, which is often the case if they are of the same file format. It is also very efficient when archiving a large number of rather small files.

Costs

On the other hand, getting a single file out of a solid archive requires processing all the files before it, so modifying solid archives can be slow and inconvenient.

Additionally, if the archive becomes even slightly damaged, some of the data (sometimes even all data) after the damaged part can be unusable (depending on the compression and archiving format), whereas in a non-solid archive format, usually only one file is unusable and the subsequent files can usually still be extracted.