JFS data compression
JFS supports fragmented and compressed file systems, which save disk space by allowing a logical block to be stored on the disk in units or "fragments" smaller than the full block size of 4096 bytes.
Data compression is not supported for JFS2.
In a fragmented file system, only the last logical block of files no larger than 32KB are stored in this manner, so that fragment support is only beneficial for file systems containing numerous small files. Data compression, however, allows all logical blocks of any-sized file to be stored as one or more contiguous fragments. On average, data compression saves disk space by about a factor of two.
The use of fragments and data compression does, however, increase the potential for fragmentation of the disk free space. Fragments allocated to a logical block must be contiguous on the disk. A file system experiencing free space fragmentation might have difficulty locating enough contiguous fragments for a logical block allocation, even though the total number of free fragments may exceed the logical block requirements. The JFS alleviates free space fragmentation by providing the defragfs program which "defragments" a file system by increasing the amount of contiguous free space. This utility can be used for fragmented and compressed file systems. The disk space savings gained from fragments and data compression can be substantial, while the problem of free space fragmentation remains manageable.
Data compression in the current JFS is compatible with previous versions of this operating system. The API comprised of all the system calls remains the same in both versions of the JFS.