4.2BSD Filesystem troubles....

[Guy Harris]

> I am working with a vanilla 4.2BSD VAX system, and I have been noticing that
> disk space is vanishing from my filesystems. I have a 45Mb filesystem that
> fsck says has 13Mb free, but df states is full.!!..??? (and can't be
> written to)

4.2BSD reserves a certain percentage of the free space on a file system;
only processes running as superuser can get at that space.  The normal
percentage is 10%, although it's tunable.  The reason for this is that the
block allocation code gets slower when the free space drops below about 10%;
see the paper "A Fast File System for UNIX", in the section "Layout
Policies".  That paper comes with the 4.2BSD documentation.

"fsck" reports the total free space; "df" reports only the free space
available to non-superusers.  (That's why it sometimes reports that a file
system is more than 100% full.)

	Guy Harris