On most UNIX systems, you can use the df or bdf command to tell how full all currently mounted partitions are. Occasionally you will notice that a partition is more than 100% full. This happens because performance slows down when a partition gets above 90% full, and so to avoid this problem (at the cost of wasting 10% of every disk), when disks are 90% full UNIX pretends that they are 100% full, and will not allow anyone but root to write to them.

When an actively used partition fills up, all sorts of problems can occur. If it contains your home directory, you will not be able to create or edit files and could lose data. If it is the root partition (as is happening right now on a system I use), and /tmp doesn't have its own partition, programs such as elm which need to write to /tmp won't work. If /var fills up, this can also cause serious problems, as not only do many programs keep their log files there, the mail spool is also typically found in /var/spool/mail.

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