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What is an archive file
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What is a compressed / archive file?



what is file compression

File archival definition

File archival
means to combine multiple files together for easier management of the data (i.e. backup, mail attachments, sharing by FTP, torrent, cloud, or any kind of network service, etc) as for the host filesystem all the data will be treated as a single file rather than as multiple ones, eliminating the overhead of handling multiple objects - for each single file, locating the physical data on disk, locating possible fragments, checking file level security permissions, and so on.

The idea of archiving files pre-dates .zip format by many years, as in AR Unix format (later superseded by TAR, released in 1979 and standardized in 1988), and LBR format in CP/M / DOS world in early '80s.

File compression definition

File compression
means to reduce size of data on disk encoding it to a smaller output, employing various strategies to efficiently map (most cases of) a larger input to a smaller output, i.e. using statistical analisys to reduce redundancy in inputa data.
Data compression, too, predates development of ZIP standard, as once the input files were merged into a single output archive, the operation was often concatenated to lossless data compression to reduce the size of the archive using various utilities available at the time as SQ (DOS, CP/M), CRUNCH (CP/M), and compress (Unix).
TAR format, for example, is still an uncompressed archive standard, and uses external compressors, nowadays usually GZ (fast deflate based compression, same as in ZIP format), BZ2 (more powerful compression), XZ (modern, very powerful LZMA based compression - the default compression algorithm used in 7Z format).

Learn more about similarities and differences in Lossless data compression and Lossy data compression. For general purpose compressed archive file, however, compression means Lossless Compression, a 1:1 mapping of input to a smaller output.

SEA's ARC format (1985) combined the archival and (lossless) compression in a single pass, providing probably the first example of general purpose of archive manager, which allowed both to spare storage for backup, and save upload and download bandwidth (and time) for sharing - at the time, mainly BBS.
A few years later, after a controversy with SEA about alleged derived work in PKARC, Phil Katz superseded previous works releasing PKZIP, which knew great success due multiple factors, as superior speed and efficiency, and being the specs released under public domain, and having relatively few competitors in years of fast PC market expansion.

What is ZIP a file

ZIP is a lossless data compression and archival format created in 1989 by Phil Katz, implemented for the first time in PKWARE's PKZIP.
The ZIP file format specifications were released under public domain and the format had long and lasting success, to the point often "zip" is colloquially used for any generic compressed archive, and many package formats are based on deflate compression and/or same or very similar specs: Java JAR / WAR / EAR, Android APK, Apple iOS IPA files (iPhone and iPad devices), Microsoft CAB and Office compound files.
WinZip 12.1 (2009) introduced the new ZIPX file format specifications for identifying a new archive standard which supports newer and more powerful compression algorithms.

What are RAR, ACE, 7Z formats

During '90s and beyond, multiple alternative archival standard emerged, as ARJ, RAR (1993), ACE, and 7Z (1999), introducing unique features to distinguish them from the growing number of competitors, in example:
  • usually, stronger compression ratio than ZIP at the cost of slower operation - but that disadvantage would have been paid off by slower transfer time (especially on slow and public networks) of smaller output file
  • multi volume archival, splitting the output to met constrains as mail attachment size limit
  • encryption, to enforce end user's privacy if the file is stolen, or passed through unsecure servers (unencrypted public network, or any third party controlled channel as a mail server, or remote storage service)
  • error detection and correction, to prevent extraction in the event data gets corrupted (i.e. faulty connection, damaged disk) and attempt recovery from known good data.
Archival file format tends to be more geared towards powerful, computing intensive features to enhance manageability of data (high compression, strong encryption), rather than enhancing ability to work on live data (rapid read and write access) like filesystems, even if some archive management utilities offers various mechanisms to edit or update data inside archives.
More choice in standards brought users more features and healthy competition between standards (see comparison of archive formats) and implementations, but also brought the need for flexible multi-purpose archival applications, like PeaZip, to deal with different formats users may encounter, and to make full use of the feature's potential of the different supported file formats.

Definition of compressed / archive file is broadening, with package and disk image format standards implementing native compression (in some cases even encryption) features - Microsoft CAB and WIM, Apple DMG.

More online resources: ARAR wiki (Unix), LBRwikipedia lbr file (CP/M), SEAsea pkware controversy company, WinZip's ZIPX standardwinzip's zipx standard.



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