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 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 file system space. NBPI is inversely proportional to the total number of i-nodes in a file system.
The JFS restricts all file systems to 16MB (2 ¬24 ) i-nodes. This restriction yields the following relationship:
Maximum file system size in bytes = <npbi> * 2¬24
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
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 (2 ¬28 ). The following relationship is produced:
Maximum file system size in bytes = <Fragment size> * 2¬28
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.
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 * 2 ¬28 = 8GB) . When creating a JFS file system, the factors listed above (nbpi, fragment size, and allocation group size) need to be weighed carefully.