How does filling a disk to capacity affect performance?

[Doug Gwyn]

In article <92 at iravcl.ira.uka.de> fsinf at iravcl.ira.uka.de writes:
>I've heard (but not verified) you can crash *every* unix-system using the
>CP-command when there is not enough space on disk. CP will not check
>whether the disk is full and overwrite blocks which are not free. The original
>data will be lost; also the machine is likely to go down.

This is totally off-the-wall.  "cp" is just a user-mode utility;
it is the KERNEL that maintains file system integrity.  It is true
that "cp" does not check whether there is enough space to hold the
copy, but it cannot know that in advance (there are other processes
asynchronously modifying the file system).  I know of only two UNIX
utilities that even attempt to check for sufficient space before
writing a file (one is "ed").

The only real problems I know of with user-mode programs running a
filesystem out of space are:
	If the filesystem contains /tmp, several critical system
	utilities may malfunction.

	Other applications that don't check for successful system
	call operation may not realize that their output is being
	truncated.  This is an application design error.