What is a metadata

Before we start describe metadata file systems, we need to know what metadata is. Metadata[10] is simply data about the data. To be sure you got the right picture of metadata, I will describe what could be the metadata for some different type of files

A report

It could for instance be this report, or another document. The metadata could be document type (e.g. PDF, MS Word document - doc, OpenOffice document - ODF), Authors name, Organization where it have been written, Publishing date, number of pages, Name of report, Abstract.

A music file

It could for instance be an mp3-, WMA-, OGG-, flag- file, or yet another format. The metadata could be title of the song, compressions format, artist, track number.

An email

Suppose you have your email stores in a Maildir¹ then each email is stored in an individual file. The metadata for each file could be sender, receiver, subject, date of arrival.

A picture

The metadata could be compression format, geographical location, date, who have taken the picture, in connection to which event the picture have been taken, name of the people on the picture.

A library² file

The metadata for a library file could be a list of the function it provides, version number, license, dependency, link to the project web page.

An executable

The metadata could be links to the documentation, license, author, installation date, the maintainer of the application.

A file with unknown content

Suppose you have file with unknown content, then the metadata could be, who created the file, when it was created, last change, size, maybe even the previous path in a traditional file system.

Those was just a few examples, a lot of file types could be added to the list, and a lot of metadata could easy be added to each file type. And as you can see, every file and every kind of data have metadata, otherwise it can not exist in a file system (not even in a regular file system), every file will have the metadata listed in the example of ``A file with unknown content''.