About the file compression attributes

When you compress a file with the Storage> compress command, compress attaches the following information to the inode:

This information is referred to as the file compression attributes. The purpose of the attributes are to collect the parameters used to create the compressed file. The information can then be read by a backup program.

The file compression attributes guarantee that a particular compressed file can only use one type and strength of compression. Recompressing a file using different attributes fails. To change the file compression attributes, you must explicitly uncompress first, and then recompress with the new options, even in the case where all extents are already uncompressed.

The file compression attributes do not indicate if all extents are compressed. Some extents might be incompressible, and other extents or even all extents might be uncompressed due to writes, but the file compression attributes remain. Only an explicit file uncompression can remove the attributes.