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 MB/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.