IBM JFS (Journaling File System) Introduction

IBM JFS is a 128 bit file system available in AIX Operating System. Currently available versions are JFS v1 & v2.

JFS means "keep track of file system changes", before committing them to the main file system. So, in the event of "sudden power failure or system crash", such file systems are quicker to bring back online & less likely to become corrupted. 

JFS & i-nodes

JFS allows you to specify the number of disk i-nodes created within a file system in case more or fewer than the default number of disk i-nodes is desired.

NBPI = The number of disk i-nodes at file system creation is specified in a value called as the number of bytes per i-node or NBPI

For example, an NBPI value of 1024 causes a disk i-node to be created for every 1024 bytes of file system disk space. 

Another way to look at this is that a small NBPI value (512 for instance) results in a large number of i-nodes, while a large NBPI value (such as 16,384) results in a small number of i-nodes.

For JFS file systems, one i-node is created for every NBPI bytes of allocation group space allocated to the file system. The total number of i-nodes in a file system limits the total number of files and the total size of the file system. An allocation group can be partially allocated, though the full number of i-nodes per allocation group is still allocated. NBPI is inversely proportional to the total number of i-nodes in a file system.

The JFS restricts all file systems to 16M (2²⁴) i-nodes

The set of allowable NBPI values vary according to the allocation group size (agsize). 

The default is 8 MB. The allowable NBPI values are 512, 1024, 2048, 4096, 8192, and 16,384 with an agsize of 8 MB. A larger agsize can be used. 

The allowable values for agsize are 8, 16, 32, and 64. The range of allowable NBPI values scales up as agsize increases. If the agsize is doubled to 16 MB, the range of NBPI values also double: 1024, 2048, 4096, 8193, 16384, and 32768.

JFS2 & i-nodes

JFS2 allocates i-nodes as needed.

If there is room in the file system for additional i-nodes, they are automatically allocated. Therefore, the number of i-nodes available is limited by the size of the file system itself.

Nishith N.Vyas