Thanks for your help. The result is:

df -k
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/wd0a      274487   260775      -13   100%    /
/dev/wd0e     2188755   251931  1827386    12%    /usr
kernfs              1        1        0   100%    /kern

You are right. It is the root partition that if full. This reminds
me that I've created home directory for the user at /home/username.
I should move user home directory under /usr, and that may solve
the problem. Am I right?

-Huaiyu

>From: maximum entropy <entropy@tappedin.com>
>To: huaiyuliu@hotmail.com
>Subject: Re: "Disk full" while there is still plenty space left, help 
>please.
>Date: Fri, 29 Jun 2001 11:12:38 -0400 (EDT)
>
> >From: "Huaiyu Liu" <huaiyuliu@hotmail.com>
> >Date: Fri, 29 Jun 2001 10:58:33 -0400
> >
> >I've installed NetBSD 1.4.1 on a Dell box. There is 3000M space
> >in the NetBSD partition. However, when I tried to download something
> >(as a common user) to the disk, I was told "/: write failed, file system 
>is
> >full" during the downloading. I checked the disk usage by "du -sk", and 
>the
> >result is : 512692, which is about 512M. There should be plenty of space
> >left.
> >[...]
> >  a:   567441       63     4.2BSD     1024  8192    16   # (Cyl.    0*- 
>562)
>
>Your root (a) partition seems to be full.  What does it say if you do
>"df -k /" ?
>
>--
>entropy -- it's not just a good idea, it's the second law.

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