This section contains information about creating large files and file allocation. Large files are only applicable to AIX 4.2 or later.
Note: 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 a new option (bf=true) to specify file systems enabled for large files. The JFS SMIT menus can create these file systems also.
Note: Large file geometry is specific to JFS.
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, a 132 MB file in a file system enabled for large files has 1024 4 KB disk blocks and 1024 128KB disk blocks. In a regular file system, the 132 MB file requires 33 single indirect blocks (each filled with 1024 4 KB disk addresses). However, the large file geometry requires only two single indirect blocks for the 132 MB file.
Note: Free space fragmentation is specific to JFS.
Large disk blocks require 32 contiguous 4KB blocks. If you write to large files beyond the 4MB, file offset will fail with ENOSPC if the file system does not contain 32 unused contiguous 4KB blocks.
Note: The file system may have thousands of free blocks, but if 32 of them are not contiguous, the allocation will fail.
The defragfs command reorganizes disk blocks to provide larger contiguous free block areas.
Note: Zeroing kproc is specific to JFS.
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.