How to compress data for archival

Using tar command

The tar command is used to archive files, that is to put many files into one single archive file, with no compression. In some cases this is the goal, one file simplifies file management, it is fast, and it is suitable for streaming the archive to an archival storage device, such as tape storage.

However, nowadays, mostly it is desired to add some kind of compression of the archived data. tar can do this by using external compression programs, such as gzip or bzip2.

gzip, GNU Zip, provides reasonable compression and relatively fast.

bzip2 compresses better, but significantly slower. In many cases, gzip is good enough.

One directory

For this example we use a directory that contains 6172 files containing 165 MB of data.

To create an archive, the cvf options are used, which means Create Verbose a File. These options instruct tar to create a new file for the archive and also print file names of the files being added to the archive. Archival without compression takes 8.5 sec and produces a file of 150MB in size.

# 8.5 sec
$ tar cvf archive.tar my_data
....
$ ls -lh archive.tar
-rw-r--r-- 1 drozmano drozmano 150M Jul  4 08:42 gromacs.tar

Individual files use some integral number of storage blocks, this is why the archive is smaller than the total size of the original files. Data padding to fill the last block is only done once in the case of the archive.

Archival with compression using the gzip is requested by using the z option, and it is actually faster (5.9 sec) due to less data that needs to be written to the compressed archive. The archive size is 33MB, that is 20% of the original size.

# 5.9 sec
$ tar czvf archive.tar.gz my_data
....
$ ls -lh archive.tar.gz
-rw-r--r--  1 drozmano drozmano  33M Jul  4 08:46 archive.tar.gz

Archival with the bzip2 compression is requested by the j option, and takes about twice as long, 16.8 sec. The archive size is 26MB, that is 16% of the original size.

# 5.9 sec
$ tar cjvf archive.tar.bz2 my_data
....
$ ls -lh archive.tar.bz2
-rw-r--r--  1 drozmano drozmano  26M Jul  4 08:58 archive.tar.bz2

Archival with the newer XZ compression is requested by the J option, and takes the longest, 4-times as long, 30.7 sec: The archive size is the smallest, 22MB, which is 13% of the original size.

# 5.9 sec
$ tar cJvf archive.tar.xz my_data
....
$ ls -lh archive.tar.xz
-rw-r--r--  1 drozmano drozmano  22M Jul  4 09:02 archive.tar.xz

One file

If one wants to compress just one file. For example, there is an uncompressed archive, archive.tar and we want to compress it. Here are the possible commands:

$ls -lh
-rw-r--r--   1 drozmano drozmano 150M Jul  4 08:57 archive.tar

$ tar czvf  archive.tar.gz archive.tar
....

$ tar cjvf  archive.tar.bz2 archive.tar
....

$ tar cJvf  archive.tar.xz archive.tar
.....

$ ls -lh
-rw-r--r--  1 drozmano drozmano 150M Jul  4 08:57 archive.tar
-rw-r--r--  1 drozmano drozmano  26M Jul  4 09:16 archive.tar.bz2
-rw-r--r--  1 drozmano drozmano  33M Jul  4 09:15 archive.tar.gz
-rw-r--r--  1 drozmano drozmano  22M Jul  4 09:17 archive.tar.xz