The minimum free space is the percentage of the total disk space 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% and 10%, inclusively. Free space is important because file access becomes less and less efficient as a file system gets full. As long as there is an adequate amount of free space, 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 free space reserve. You can change the value of the minimum free space for an existing file system by using the tunefs command.