JFS2 file system layout

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A file system is a set of files, directories and other structures.

The file systems maintain information and identify where the data is located on the disk for a file or directory. In addition to files and directories a JFS2 file system contains a superblock, allocation maps and one or more allocation groups. Each file system occupies one logical volume.

JFS2 superblock

The superblock is 4096 bytes in size and starts at byte offset 32768 on the disk. The superblock maintains information about the entire file system and includes the following fields:

  • Size of the file system
  • Number of data blocks in the file system
  • A flag indicating the state of the file system
  • Allocation group sizes
  • File system block size

JFS2 allocation maps

The file system contains the following allocation maps:

  • The i-node allocation map records the location and allocation of all i-nodes in the file system.
  • The block allocation map records the allocation state of each file system block.

JFS2 disk i-nodes

A logical block contains a file or directory's data in units of file system blocks. Each logical block is allocated file system blocks for the storage of its data. Each file and directory has an i-node that contains access information such as file type, access permissions, owner's ID, and number of links to that file. These i-nodes also contain a B+ tree for finding the location on the disk where the data for a logical block is stored.

JFS2 allocation groups

Allocation groups divide the space on a file system into chunks. Allocation groups are used only for a problem-solving technique in which the most appropriate solution, found by attempting alternative methods, is selected at successive stages of a program for using in the next step of the program. Allocation groups allow JFS2 resource-allocation policies to use well-known methods for achieving optimum I/O performance. First, the allocation policies try to cluster disk blocks and disk i-nodes for related data to achieve good locality for the disk. Files are often read and written sequentially and the files within a directory are often accessed together. Second, the allocation policies try to distribute unrelated data throughout the file system in order to accommodate disk locality.

Allocation groups within a file system are identified by a zero-based allocation group index, the allocation group number.

Allocation group sizes must be selected that yield allocation groups that are large enough to provide for contiguous resource allocation over time. Allocation groups are limited to a maximum number of 128 groups. The minimum allocation group size is 8192 file-system blocks.

Partial allocation groups

A file system whose size is not a multiple of the allocation group size will contain a partial allocation group; the last allocation group of the file system is not fully covered by disk blocks. This partial allocation group will be treated as a complete allocation group, except that the nonexistent disk blocks will be marked as allocated in the block allocation map.