JFS fragments
In JFS, the disk space allocation unit is called a fragment, and it can be smaller than the logical block size of 4096 bytes.
With the use of fragments smaller than 4096 bytes, the data contained within a partial logical block can be stored more efficiently by using only as many fragments as are required to hold the data. For example, a partial logical block that only has 500 bytes could be allocated a fragment of 512 bytes (assuming a fragment size of 512 bytes), thus greatly reducing the amount of wasted disk space. If the storage requirements of a partial logical block increase, one or more additional fragments are allocated.
The fragment size for a file system is specified during its creation. The allowable fragment sizes for journaled file systems (JFS) are 512, 1024, 2048, and 4096 bytes. Different file systems can have different fragment sizes, but only one fragment size can be used within a single file system. Different fragment sizes can also coexist on a single system (machine) so that users can select a fragment size most appropriate for each file system.
JFS fragment support provides a view of the file system as a contiguous series of fragments rather than as a contiguous series of disk blocks. To maintain the efficiency of disk operations, however, disk space is often allocated in units of 4096 bytes so that the disk blocks or allocation units remain equal in size to the logical blocks. A disk-block allocation in this case can be viewed as an allocation of 4096 bytes of contiguous fragments.
Both operational overhead (additional disk seeks, data transfers, and allocation activity) and better utilization of disk space increase as the fragment size for a file system decreases. To maintain the optimum balance between increased overhead and increased usable disk space, the following factors apply to JFS fragment support:
- Where possible, disk space allocations of 4096 bytes of fragments are maintained for a file or the logical blocks of a directory.
- Only partial logical blocks for files or directories less than 32KB in size can be allocated less than 4096 bytes of fragments.
As the files and directories within a file system grow beyond 32 KB in size, the benefit of maintaining disk space allocations of less than 4096 bytes for partial logical blocks diminishes. The disk space savings as a percentage of total file system space grows small while the extra performance cost of maintaining small disk space allocations remains constant. Since disk space allocations of less than 4096 bytes provide the most effective disk space utilization when used with small files and directories, the logical blocks of files and directories equal to or greater than 32 KB are always allocated 4096 bytes of fragments. Any partial logical block associated with such a large file or directory is also allocated 4096 bytes of fragments.