Compressing files reduces the space used, while retaining the accessibility of the files and being transparent to applications. Compressed files look and behave almost exactly like uncompressed files: the compressed files have the same name, and can be read and written as with uncompressed files. Reads cause data to be uncompressed in memory, only; the on-disk copy of the file remains compressed. In contrast, after a write, the new data is uncompressed on disk.
Only user data is compressible. You cannot compress Veritas File System (VxFS) metadata.
After you compress a file, the inode number does not change, and file descriptors opened before the compression are still valid after the compression.
Compression is a property of a file. Thus, if you compress all files in a directory, for example, any files that you later copy into that directory do not automatically get compressed. You can compress the new files at any time by compressing the files in the directory again.
You compress files with the vxcompress command.
vxcompress(1) manual page.
To compress files, you must have VxFS file systems with disk layout Version 9 or later.