FreeBSD 6.1-PRERELEASE : rm swapfile after swapoff can't release the disk space

[Honest Qiao]

Environment: 
FreeBSD www141.igogo8.com 6.1-PRERELEASE FreeBSD 6.1-PRERELEASE #0: Fri May 
12 12:12:17 CST 2006 
HonestQiao at www141.igogo8.com:/usr/src/sys/i386/compile/iGOGO8-PAE i386
Description: 
Several months ago , I read the follow article, and add a 12G swapfile.
The article url is 
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/adding-swap-space.html


The swapfile is /usr/swap0 , and I have a swap mount which size is 2G.
top show:
Swap: 14G Total, 14G Free

Today , I use swapoff /usr/swap0 to disable swapfile , and then rm 
/usr/swap0.
/usr/swap0 was deleted, but its disk space can't release.
Now, top show:
Swap: 2048M Total, 2048M Free

www141# df -hi
Filesystem Size Used Avail Capacity iused ifree %iused Mounted on
/dev/amrd0s1a 248M 41M 187M 18% 1609 31413 5% /
devfs 1.0K 1.0K 0B 100% 0 0 100% /dev
/dev/amrd0s1f 19G 965M 17G 5% 1737 2636085 0% /db
/dev/amrd0s1g 19G 12G 5.6G 69% 769803 1868019 29% /home
/dev/amrd0s1d 248M 43M 185M 19% 585 32437 2% /tmp
/dev/amrd0s1h 25G 19G 3.4G 85% 354372 3013562 11% /usr
/dev/amrd0s1e 248M 65M 163M 29% 6430 26592 19% /var

www141# du -csh /usr
9.3G /usr
9.3G total

www141# mount
/dev/amrd0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/amrd0s1f on /db (ufs, local, soft-updates)
/dev/amrd0s1g on /home (ufs, local, soft-updates)
/dev/amrd0s1d on /tmp (ufs, local, soft-updates)
/dev/amrd0s1h on /usr (ufs, local, soft-updates)
/dev/amrd0s1e on /var (ufs, local, soft-updates)

from , df -hi show /usr use 19G
but, du -csh /usr show use use 9.3G

Why there has a big difference size between df and du?

Also , you can visit it on 
http://www.freebsd.org/cgi/query-pr.cgi?pr=103867

Thanks!

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