The enhanced journaled file system segments disk space into blocks. JFS2 supports multiple file system block sizes of 512, 1024, 2048, and 4096.
Different file systems can have different block sizes, but only one block size can be used within a single file system.
Smaller block sizes minimize wasted disk space by more efficiently storing the data in a file or directory's partial logical blocks. Smaller block sizes can also result in additional operational overhead. Also, device drivers must provide disk block addressability that is the same or smaller than the file system block size.
Because disk space is allocated in smaller units for a file system with a block 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 block to the file, assuming a block size of 512 bytes. If the file size is extended further by another write of 512 bytes, an additional block must be allocated to the file. Applying this example to a file system with 4096-byte blocks, disk space allocation occurs only once, as part of the first write operation. No additional allocation activity is performed as part of the second write operation since the initial 4096-byte block allocation is large enough to hold the data added by the second write operation.
File system block size is specified during the file system's creation with the System Management Interface Tool (SMIT), or the crfs and mkfs commands. The decision of which file system block size to choose should be based on the projected size of files contained by the file system.
The file system block size value can be identified through the System Management Interface Tool (SMIT), or the lsfs command. For application programs, the statfs subroutine can be used to identify the file system block size.
Blocks serve as the basic unit of disk space allocation, and the allocation state of each block within a file system is recorded in the file system block allocation map. More virtual memory and file system disk space might be required to hold block allocation maps for file systems with a block size smaller than 4096 bytes.