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System Administration Guide: Devices and File Systems
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Customizing UFS File System Parameters

Before you alter the default file system parameters that are assigned by the newfs command, you need to understand them. This section describes these parameters:

For a description of the command options that customize these parameters, see newfs(1M) and mkfs_ufs(1M).

Logical Block Size

The logical block size is the size of the blocks that the UNIX® kernel uses to read or write files. The logical block size is usually different from the physical block size. The physical block size is usually 512 bytes, which is the size of the smallest block that the disk controller can read or write.

Logical block size is set to the page size of the system by default. The default logical block size is 8192 bytes (8 Kbytes) for UFS file systems. The UFS file system supports block sizes of 4096 or 8192 bytes (4 or 8 Kbytes). The recommended logical block size is 8 Kbytes.

SPARC only - You can specify only the 8192-byte block size on the sun-4uTM platform.

To choose the best logical block size for your system, consider both the performance you want and the available space. For most UFS systems, an 8-Kbyte file system provides the best performance, offering a good balance between disk performance and the use of space in primary memory and on disk.

As a general rule, to increase efficiency, use a larger logical block size for file systems when most of the files are very large. Use a smaller logical block size for file systems when most of the files are very small. You can use the quot -c filesystem command on a file system to display a complete report on the distribution of files by block size.

However, the page size set when the file system is created is probably the best size in most cases.

Fragment Size

As files are created or expanded, they are allocated disk space in either full logical blocks or portions of logical blocks called fragments. When disk space is needed for a file, full blocks are allocated first, and then one or more fragments of a block are allocated for the remainder. For small files, allocation begins with fragments.

The ability to allocate fragments of blocks to files, rather than just whole blocks, saves space by reducing fragmentation of disk space that results from unused holes in blocks.

You define the fragment size when you create a UFS file system. The default fragment size is 1 Kbyte. Each block can be divided into 1, 2, 4, or 8 fragments, which results in fragment sizes from 8192 bytes to 512 bytes (for 4-Kbyte file systems only). The lower bound is actually tied to the disk sector size, typically 512 bytes.

For multiterabyte file systems, the fragment size must be equal to the file system block size.

Note - The upper bound for the fragment is the logical block size, in which case the fragment is not a fragment at all. This configuration might be optimal for file systems with very large files when you are more concerned with speed than with space.

When choosing a fragment size, consider the trade-off between time and space: A small fragment size saves space, but requires more time to allocate. As a general rule, to increase storage efficiency, use a larger fragment size for file systems when most of the files are large. Use a smaller fragment size for file systems when most of the files are small.

Minimum Free Space

The minimum free space is the percentage of the total disk space that is held in reserve when you create the file system. The default reserve is ((64 Mbytes/partition size) * 100), rounded down to the nearest integer and limited between 1 percent and 10 percent, inclusively.

Free space is important because file access becomes less and less efficient as a file system gets full. As long as an adequate amount of free space exists, UFS file systems operate efficiently. When a file system becomes full, using up the available user space, only root can access the reserved free space.

Commands such as df report the percentage of space that is available to users, excluding the percentage allocated as the minimum free space. When the command reports that more than 100 percent of the disk space in the file system is in use, some of the reserve has been used by root.

If you impose quotas on users, the amount of space available to them does not include the reserved free space. You can change the value of the minimum free space for an existing file system by using the tunefs command.

Rotational Delay

This parameter is obsolete. The value is always set to 0, regardless of the value you specify.

Optimization Type

The optimization type parameter is set to either space or time.

  • Space – When you select space optimization, disk blocks are allocated to minimize fragmentation and disk use is optimized.

  • Time – When you select time optimization, disk blocks are allocated as quickly as possible, with less emphasis on their placement. When sufficient free space exists, allocating disk blocks is relatively easy, without resulting in too much fragmentation. The default is time.

    You can change the value of the optimization type parameter for an existing file system by using the tunefs command.

For more information, see tunefs(1M).

Number of Inodes (Files)

The number of bytes per inode specifies the density of inodes in the file system. The number is divided into the total size of the file system to determine the number of inodes to create. Once the inodes are allocated, you cannot change the number without re-creating the file system.

The default number of bytes per inode is 2048 bytes (2 Kbytes) if the file system is less than 1 Gbyte. If the file system is larger than 1 Gbyte, the following formula is used:

File System Size

Number of Bytes Per Inode

Less than or equal to 1 Gbyte


Less than 2 Gbytes


Less than 3 Gbytes


3 Gbytes up to 1 Tbyte


Greater than 1 Tbyte or created with -T option


If you have a file system with many symbolic links, they can lower the average file size. If your file system is going to have many small files, you can give this parameter a lower value. Note, however, that having too many inodes is much better than running out of inodes. If you have too few inodes, you could reach the maximum number of files on a disk slice that is practically empty.

Maximum UFS File and File System Size

The maximum size of a UFS file system is about 16 Tbytes of usable space, minus about one percent overhead. A sparse file can have a logical size of one terabyte. However, the actual amount of data that can be stored in a file is approximately one percent less than 1 Tbyte because of the file system overhead.

Maximum Number of UFS Subdirectories

The maximum number of subdirectories per directory in a UFS file system is 32,767. This limit is predefined and cannot be changed.

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