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General Programming Concepts: Writing and Debugging Programs

Working with JFS2 i-nodes

Files in the enhanced journaled file system (JFS2) are represented internally as index nodes (i-nodes). JFS2 i-nodes exist in a static form on the disk and they contain access information for the files as well as pointers to the real disk addresses of the file's data blocks. The i-nodes are allocated dynamically by JFS2.

When a file is opened, an in-core i-node is created by the operating system. The in-core i-node contains a copy of all the fields defined in the disk i-node, plus additional fields for tracking the in-core i-node. In-core i-nodes are defined in the /usr/include/j2/j2_inode.h file.

Disk i-node Structure for JFS2

Each disk i-node in JFS2 is a 512 byte structure. The index of a particular i-node allocation map of the file system produces the unique number (i-number) by which the operating system identifies the i-node. The i-node allocation map tracks the location of the i-nodes on the disk as well as their availability.

Disk i-nodes include the following information:

Field Contents
di_mode Type of file and access permission mode bits
di_size Size of file in bytes
di_uid Access permissions for the user ID
di_gid Access permissions for the group ID
di_nblocks Number of blocks allocated to the file
di_mtime Last time file was modified
di_atime Last time file was accessed
di_ctime Last time i-node was modified
di_nlink Number of hard links to the file
di_btroot Root of B+ tree describing the disk addresses of the data

It is impossible to change the data of a file without changing the i-node, but it is possible to change the i-node without changing the contents of the file. For example, when permission is changed, the information within the i-node (di_mode) is modified, but the data in the file remains the same.

The di_btroot describes the root of the B+ tree. It describes the data for the i-node. di_btroot has a field indicating how many of its entries in the i-node are being used and another field describing whether they are leaf nodes or internal nodes for the B+ tree. File space allocation geometries are discussed in the article JFS2 File Space Allocation.

Disk i-nodes do not contain file or path name information. Directory entries are used to link file names to i-nodes. Any i-node can be linked to many file names by creating additional directory entries with the link or symlink subroutine. To discover the i-node number assigned to a file, use the ls -i command.

The i-nodes that represent files that define devices contain slightly different information from i-nodes for regular files. Files associated with devices are called special files. There are no data block addresses in special device files, but the major and minor device numbers are included in the di_rdev field.

In normal situations, a disk i-node is released when the link count (di_nlink) to the i-node equals 0. Links represent the file names associated with the i-node. When the link count to the disk i-node is 0, all the data blocks associated with the i-node are released to the bit map of free data blocks for the file system. The i-node is then placed on the free i-node map.

In-core i-node Structure

When a file is opened, the information in the disk i-node is copied into an in-core i-node for easier access. The in-core i-node structure contains additional fields which manage access to the disk i-node's valuable data. The fields of the in-core i-node are defined in the j2_inode.h file. Some of the additional information tracked by the in-core i-node is:

When an in-core i-node is released (for instance with the close subroutine), the in-core i-node reference count is reduced by 1. If this reduction results in the reference count to the in-core i-node becoming 0, the i-node is released from the in-core i-node table, and the contents of the in-core i-node are written to the disk copy of the i-node (if the two versions differ).

Related Information

Chapter 5, File Systems and Directories

JFS File Space Allocation

JFS2 File Space Allocation

Using File Descriptors

Linking Files and Directories

Understanding Generic I-nodes (G-nodes)

The close subroutine

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