JFS and JFS2 disk space segmentation

Many UNIX file systems only allocate contiguous disk space in units equal in size to the logical blocks used for the logical division of files and directories. These allocation units are typically referred to as disk blocks and a single disk block is used exclusively to store the data contained within a single logical block of a file or directory.

Using a relatively large logical block size (4096 bytes for example) and maintaining disk block allocations that are equal in size to the logical block are advantageous for reducing the number of disk I/O operations that must be performed by a single file system operation. A file or directory data is stored on disk in a small number of large disk blocks rather than in a large number of small disk blocks. For example, a file with a size of 4096 bytes or less is allocated a single 4096-byte disk block if the logical block size is 4096 bytes. A read or write operation therefore only has to perform a single disk I/O operation to access the data on the disk. If the logical block size is smaller requiring more than one allocation for the same amount of data, then more than one disk I/O operation is be required to access the data. A large logical block and equal disk block size are also advantageous for reducing the amount of disk space allocation activity that must be performed as new data is added to files and directories, because large disk blocks hold more data.

Restricting the disk space allocation unit to the logical block size can, however, lead to wasted disk space in a file system containing numerous files and directories of a small size. Wasted disk space occurs when a logical block worth of disk space is allocated to a partial logical block of a file or directory. Because partial logical blocks always contain less than a logical block worth of data, a partial logical block only consumes a portion of the disk space allocated to it. The remaining portion remains unused because no other file or directory can write its contents to disk space that has already been allocated. The total amount of wasted disk space can be large for file systems containing a large number of small files and directories.

The journaled file system (JFS) divides disk space into allocation units called fragments. The enhanced journaled file system (JFS2) segments disk space into blocks. The objective is the same: to efficiently store data.

JFS fragments are smaller than the default disk allocation size of 4096 bytes. Fragments minimize wasted disk space by more efficiently storing the data in a file or directory partial logical blocks. The functional behavior of JFS fragment support is based on that provided by Berkeley Software Distribution (BSD) fragment support.

JFS2 supports multiple file system block sizes of 512, 1024, 2048, and 4096. Smaller block sizes minimize wasted disk space by more efficiently storing the data in a file or directory's partial logical blocks. Smaller block sizes also result in additional operational overhead. The block size for a JFS2 is specified during its creation. Different file systems can have different block sizes, but only one block size can be used within a single file system.