the 10% factor

[Rich Winkel UMC Math Department]

I understand that allowing bsd filesystems to exceed 90% of their
capacity results in a significant reduction in performance.  This makes
a certain amount of sense to me, at least when files are being created, deleted
or appended to.  However, I've been told this is also true of static
filesystems.  For instance, if I have a 20MB partition set aside for
an nfs client's swap activity, I can't utilize the entire 20MB for the swap
file, I'm supposed to leave 10% unused to avoid a performance impact, even
though the swap file's size is unchanging.  Is
this true?  Could someone explain this to me?

Thanks,
Rich