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, look at 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 where most of the files are large. Use a smaller fragment size for file systems where 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 the users 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 for modern disks. If you need to use this parameter, the default value provided when the file system is created is probably best for most cases.

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 there is enough free space, it is relatively easy to allocate disk blocks effectively, 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 inodes parameter determines the number of files you can have in the file system: one inode for each file. The number of bytes per inode determines the total number of inodes that are created when the file system is made: the total size of the file system divided by the number of bytes per inode. 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 one Gbyte. If the file system is larger than one Gbyte, the following formula is used:

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 approximately 16 terabytes of usable space, minus approximately 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 one terabyte 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.

Commands for Creating a Customized File System

This section describes the two commands that you use to create a customized file system:

• newfs

• mkfs

The newfs Command Syntax, Options, and Arguments

The newfs command is a friendlier version of the mkfs command that is used to create file systems.

The syntax is as follows:

/usr/sbin/newfs [-Nv] [mkfs_options] raw_device

The following table describes the options and arguments for the newfs command.

Table 44-4 The newfs Command Options and Arguments

Example--newfs Command Options and Arguments

This example shows how to use the -N option to display file system information, including the backup superblocks.

# newfs -N /dev/rdsk/c0t0d0s0
/dev/rdsk/c0t0d0s0:  37260 sectors in 115 cylinders of 9 tracks, 36 sectors
        19.1MB in 8 cyl groups (16 c/g, 2.65MB/g, 1216 i/g)
superblock backups (for fsck -b #) at:
 32, 5264, 10496, 15728, 20960, 26192, 31424, 36656,
#