Solid compression

This article is about Solid compression in computing. For compressibility of solids, see Compression (physics).

In computing, solid compression refers to a method for data compression of multiple files, wherein all the uncompressed 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 emulated for small archives by combining the files into an uncompressed zip archive and then compressing the zip archive inside a second compressed zip file).

The term is used ostensibly 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 the 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 originally required processing all the files before it, so modifying solid archives could be slow and inconvenient. Later versions of 7-zip use a variable solid block size, so that only a limited amount of data must be processed in order to extract one file. Parameters control the maximum solid block window size, the number of files in a block, and whether blocks are separated by file extension.

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.