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Why I cannot compress some types of files (avi, mp3, pdf...)



can not compress files



It is a quite common experience that not all file types can be compressed with equal efficiency: some data structures, like text files, or text based containers (spreadsheets, documents, databases) usually compresses well, reaching high compression ratio, while other formats, usually multimedia files, does not compress properly and reaches very poor compression ratio not matter how powerful (and slow) compression method is used.
Most common reasons why a file can't be efficiently compressed are:

cannot reduce size of file with compression

The file contains lots of non significant or duplicate data, or noise
Some data in the file may be not relevant and should be edited off (with the own editing tools appropriate for the input file type) before archiving the data, in order to reduce disk occupation without losing actual information.

Some miscellaneous examples of non-meaningful data in files
  1. Disk images / virtual machine disks (QCOW2, VMDK, Microsoft VHD, VDI) often contain data which should be cleared, as recycle bin files, temporary files, RAM content (if machine was stopped rather than being shut down), and data of erased files still written on disk sectors if free space is not overwritten by zero - not shown in disk occupation when running the image, as marked as free space after deletion.
  2. Databases can contain deleted entries until purged with appropriate actions for specific database format; also, running proper data duplication routines is a good practice before archiving / backing up the database.
  3. Backup should be checked to reduce occurrencies of duplicate or temporary files, incremental backup strategy (save only files modified between the two backup checkpoints) can be employed to dramatically reduce the size of backup sets.
  4. Document files (MS Office DOCX, Adobe Acrobat PDF...) can contain data of obsolete revisions, or multimedia data not matching the resolution of the document which should be authored to a more appropriate definition / size either on source multimedia files, or during document editing.
  5. Graphic and multimedia can contain high level of enthropy entropyinformation theory compression that should be accurately taken in account, in example a poor quality photo sensor or a not adequate microphone may lead to low signal / noise ratio, which will lead boot to a poor result and poorly compressible data.

How to reduce size of data before compression

It is recommended to use specific editing tools for the format or the scope, before starting actual compression.
PeaZip free archiver can help in some of those tasks providing tools to zero free space, find and remove duplicate files, and to resize, convert and re-compress graphic files.

cannot compress multimedia files

The file is already compressed
Some file formats are designed with built in compression, so no further compression is possible (or practical).
File size is already efficiently reduced by design often employing dedicated compression types (lossy compression is common in compressed multimedia files), more efficient than general purpose compression algorithms designed to work on a plurality of data structures.

Examples of already compressed file formats

Graphic files as JPEG and PNG, will usually compress poorly compared to uncompressed bitmap graphic as RAW, BMP and uncompressed TIFF files (read more on how to optimize compression of images).
Audio files as MP3, will usually compress very poorly compared to uncopmpressed audio formats like WAV files, and multimedia video files like AVI, DIVX, MPEG, MKV...  are very poorly compressible.
Some document types as Adobe Acrorbat PDF, Open Office formats, and new Microsoft Office file formats (DOCX, XLSX, PPTX...), and some databases, contains already compressed data (usually employing lossless compression designed around Deflate algorithm), and generally does not compress well compared to older uncompressed documents formats, like DOC, XLS, PPT...
Please note a document or database is itself a container, so if it stores compressed bitmap graphic and other multimedia data, as a role of thumb it will compress poorly if compared to file of same format containing plain text data, this is the reason why even some uncompressed documents (DOC, XLS, PPT) will compress less than other documents of the same type.

Possible solutions to compress already compressed files

  1. Extract existing archives (ZIP, ACE, ...) and recompress the content in a more powerful compression format, such as RAR or 7Z: in this case the final archives will be smaller than the original ones (depending on the nature of the input data), while trying to compress the archives will not provide comparable results. PeaZip can automate extraction and re-compression of existing archives using stronger compression, using its file conversion tool.
  2. In some cases the built-in file format compression is a trade off between the need to reduce the final size of the file and the need to quickly access the content of the file, avoiding excessively powerful and complicate compression algorithms. In those cases (i.e. JPEG, PDF, DOCX, XLSX files...) applying a powerful compression (i.e.compress to RAR or 7Z) can reduce the size of those files.
  3. In worst cases it is simply not possible to further improve current compression level of the files, and other strategies should be considered, i.e. use utilities like PeaZip to deduplicate files (search for identical files to avoid archiving / backing up multiple copies of the same data), or split output archive in multiple volumes of desired size if it is needed to keep the output under mandatory maximum sizes (i.e. maximum e-mail attachment sizemaximum email size allowed, max upload files size, filesystem limits or built-in file size limitations, etc). Also in those cases it may be useful to opt for a lower compression settings (or no-compression archiving, option generally labelled as "store" compression level), which will save time and computing power with no practical inpact on final file size.

why some files types does not compress efficiently

The file is not compressible at all
Encrypted files are not compressible, containing pseudo random data - except, usually, for a few bytes in the file header. Plainly, there is not a shorter, more efficient way to represent the content of the file no matter how hard the compressor tries.

Examples of non compressible files types

Encryption can be employed to protect many types of databases, backup files, some documents formats as PDF, and it is a common option to protect archives. Archive formats supporting strong encryption standards are 7Z, PEA, RAR, ZIP, and ZIPX.

Possible solutions to reduce space occupation of non compressible files

It is not possible to compress a single encrypted file, and switching to more powerful compression algorithms / settings is of no use, but - as well as for previous cases - it is possible to de-duplicate content of archive / backup removing unnecessary duplicate copies of the same data, reducing final size by consequence.
If even this strategy is not viable, it is possible to span (split) the output in multiple volumes of desired size, smaller than limitations in trasmission (email attachment, sharing, max upload size) or in storing the data (filesystem of archive format size limitations).
An alternative approach is to decrypt the archive / backup image / database before re-compressing it with a more powerful algorithm or higher compression settings, and then re-applying encryption if security is a concern.


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FAQ > Archiving > Why some files types does not compress efficiently

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