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:
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 number of bytes per i-node (NBPI) 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.
One i-node is created for every NBPI bytes of allocation group space allocated to 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 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 NBPI 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
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).
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 4 MB 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 4 MB, and the default log size is one partition, therefore, the root volume group normally contains a 4 MB JFS log. When file systems exceed 2 GB or when the total amount of file system space using a single log exceeds 2 GB, the default log size might not be sufficient. In either case, the log sizes are scaled upward as the file system size increases. When the size of the log logical volume is changed, the logform command must be run to reinitialize the log before the new space can be used. The JFS log is limited to a maximum size of 256 MB.
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 8 GB (512 * 224 = 8 GB). When creating a JFS file system, the factors listed above (NBPI, fragment size, and allocation group size) need to be weighed carefully. The file system size limitation is the minimum of NBPI * 224 or Fragment Size * 228