JFS and JFS2 size limitations
You define the maximum size for a JFS when you create the file system. The decision of what size to define for a JFS is based on several significant issues.
The recommended maximum size for a JFS2 is 16 TBs. The minimum file system size for a JFS2 is 16 MBs.
Although file systems that use allocation units smaller than 4096 bytes require substantially less disk space than those using the default allocation unit of 4096 bytes, the use of smaller fragments might incur performance costs.
The allocation state of each fragment (JFS) or block (JFS2) within a file system is recorded in the file system allocation map. More virtual memory and file system disk space might be required to hold allocation maps for file systems with a fragment or block size smaller than 4096 bytes.
Because disk space is allocated in smaller units for a file system with a fragment (JFS) or block (JFS2) size other than 4096 bytes, allocation activity can occur more often when files or directories are repeatedly extended in size. For example, a write operation that extends the size of a zero-length file by 512 bytes results in the allocation of one 512-byte fragment or block to the file, depending on the file system type. If the file size is extended further by another write of 512 bytes, an additional fragment or block must be allocated to the file. Applying this example to a file system with 4096-byte fragments or blocks, disk space allocation occurs only once, as part of the first write operation. No additional allocation activity must be performed as part of the second write operation since the initial 4096-byte allocation is large enough to hold the data added by the second write operation. Allocation activity can be minimized if the files are extended by 4096 bytes at a time.
One size-related issue is the size of the file system log.
For JFS, in most instances, multiple 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.
There is a practical limit to the size of the combined file systems that a single JFS log can support. As a guideline, one trillion bytes of total file system capacity is the recommended limitation for a single JFS log. When this guideline is exceeded or is close to being exceeded, or when out-of-memory errors occur from the logredo command (which is called by the fsck command), add an additional JFS log and then share the load between the two JFS log files.
For JFS2, in most instances, multiple file systems also use a common 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, you can scale log sizes upward as the file system size increases or you can add an additional JFS2 log and then share the load between the two JFS2 log files.