JFS and large files
You can create large files with the JFS file system type.
All JFS2 file systems support large files.
File systems enabled for large files can be created with the crfs and mkfs commands.
Both commands have an option (
bf=true) to specify
file systems enabled for large files. You can also use SMIT to create
these file systems.
In file systems enabled for large files, file data stored before the 4 MB file offset is allocated in 4096-byte blocks. File data stored beyond the 4 MB file offset is allocated with large disk blocks of 128 KB in size. The large disk blocks are actually 32 contiguous 4096-byte blocks.
For example, in a regular file system, the 132-MB file requires 33K 4-KB disk blocks (33 single indirect blocks each filled with 1024 4 KB disk addresses). A 132-MB file in a file system enabled for large files has 1024 4-KB disk blocks and 1024 128-KB disk blocks. The large file geometry requires only two single indirect blocks for the 132 MB file. Both large and regular file types require one double indirect block.
Large disk blocks require 32 contiguous 4 KB blocks. If you write to large files beyond the 4 MB, file offset will fail with ENOSPC if the file system does not contain 32 unused contiguous 4 KB blocks.
The defragfs command reorganizes disk blocks to provide larger contiguous free block areas.
The JFS is required to initialize all new disk allocations. The JFS starts the kernel kproc procedure used to zero initial file allocations when mounting the first large file enabled file system on your system. The kproc procedure remains once the file system enabled for large files has successfully unmounted.