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System Management Concepts: Operating System and Devices

Enhanced Journaled File System

Understanding File System Block Size

The enhanced journaled file system (JFS2) supports multiple file system block sizes of 512, 1024, 2048, and 4096. 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 also result in additional operational overhead.

The block size for a JFS2 is specified during its creation. Different file systems can have diferent block sizes, but only one block size can be used within a single file system.

Variable Number of I-nodes for Enhanced Journaled File System

JFS2 allocates i-nodes as needed. Therefore, the number of i-nodes available is limited by the size of the file system itself.

Specifying File System Block Size

File system block size is specified during the file system's creation with the crfs and mkfs commands or by using the System Management Interface Tool (SMIT). The decision of file system block size should be based on the projected size of files contained by the file system.

Identifying File System Block Size

The file system block size value can be identified through the lsfs command or the System Management Interface Tool (SMIT). For application programs, the statfs subroutine can be used to identify the file system block size.

Compatibility and Migration

Note: The enhanced journaled file system (JFS2) is a new file system and is not compatible with any previous version of this operating system.

Device Driver Limitations

A device driver must provide disk block addressability that is the same or smaller than the file system block size.

Performance Costs

Although file systems that use block sizes smaller than 4096 bytes as their allocation unit might require substantially less disk space than those using the default allocation unit of 4096 bytes, the use of smaller block sizes can incur performance degradation.

Increased Allocation Activity

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.

Increased Block Allocation Map Size

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. 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.

Understanding Enhanced Journaled File System Size Limitations

The maximum size for an enhanced journaled file system is architecturally limited to 4 Petabytes. I-nodes are dynamically allocated by JFS2, so you don't need to consider how many i-nodes you may need when creating a JFS2 file system. You need to consider the size of the file system log.

Enhanced Journaled File System Log Size Issues

In most instances, multiple journaled file systems use a common log configured to be 4 MB in size. When file systems exceed 2 GB or when the total amount of filesystem space using a single log exceeds 2 GB, the default log size might not be sufficient. In either case, scale log sizes upward as the file system size increases. The JFS log is limited to a maximum size of 256 MB.

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