Journaled File System Size Limitations

Understanding Journaled File System Size Limitations

The maximum size for a journaled file system (JFS) is defined when the file system is created. When you create a JFS, there are five significant issues to consider:

Number of Inodes

The total number of i-nodes in a file system limits the total number of files and the total size of the file system. The JFS provides the nbpi (number of bytes per i-node) parameter that affects the number of i-nodes in a file system. JFS supports nbpi values of 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, and 131072. The values 32768, 65536, and 131072 only apply to AIX Version 4.2 or later.

One i-node is created for every NBPI bytes of allocation group space allocated to the filesystem. 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.

Allocation Group Size

AIX Version 4.2 or later supports various allocation group sizes. The JFS segregates file system space into groupings of i-nodes and disk blocks for user data. These groupings are called allocation groups. The allocation group size can be specified when the file system is created. The allocation group sizes are 8M, 16M, 32M, and 64M. Each allocation group size has an associated npbi range. The ranges are defined by the following table:

Allocation Group
Size in Megabytes      Allowable NBPI Values

 8                     512, 1024, 2048, 4096, 8192, 16384 

16                          1024, 2048, 4096, 8192, 16384, 32768 

32                                2048, 4096, 8192, 16384, 32768, 65536 

64                                      4096, 8192, 16384, 32768, 65536, 131072 

File System Fragment Addressability

The JFS supports four fragment sizes of 512, 1024, 2048, and 4096 byte units of contiguous disk space. The JFS maintains fragment addresses in i-nodes and indirect blocks as 28-bit numbers. Each fragment must be addressable by a number from 0 to (2²⁸).

Journaled File System Log Size Issues

Another size-related issue is the size of the JFS log. In most instances, multiple journaled file systems use a common log configured to be 4MB in size. For example, after initial installation, all file systems within the root volume group use logical volume hd8 as a common JFS log. The default logical volume partition size is 4MB, and the default log size is one partition, therefore, the root volume group normally contains a 4MB JFS log. When file systems exceed 2GB or when the total amount of file system space using a single log exceeds 2GB, the default log size may not be sufficient. In either case, the log sizes should be scaled upward as the file system size increases. The JFS log is limited to a maximum size of 256MB.

Maximum Journaled File System Size

The maximum JFS size is defined when the file system is created. For example, selecting an nbpi ratio of 512 will limit the file system to a size of 8GB (512 * 2²⁴ = 8GB). When creating a JFS file system, the factors listed above (nbpi, fragment size, and allocation group size) need to be weighed carefully. The filesystem size limitation is the minimum of NPBI * 2²⁴ or Fragment Size * 2²⁸