Compression

When a file is written into a file system for which compression is specified, the compression algorithm compresses the data 4096 bytes (a page) at a time, and the compressed data is then written in the minimum necessary number of contiguous fragments. Obviously, if the fragment size of the file system is 4KB, there is no disk-space payback for the effort of compressing the data. (Compression and fragments smaller than 4KB are new in AIX Version 4.1.)

Although compression should result in conserving space overall, there are at least two reasons for leaving some space in the file system unused:

• Since the degree to which each 4096-byte block of data will compress is not known in advance, the file system initially reserves a full block of space. The unneeded fragments are released after compression, but the conservative initial allocation policy may lead to premature "out of space" indications.
• Some free space is necessary to allow the defragfs command to operate.