No space left on device on xfs filesystem with 7.7TB free

[atyu30 atyu30]

I'm running RedHat Linux Enterprise Server 5.2, on a 64-bit x86_64 Linux
machine with kernel version 2.6.18-128.el5 smp.  I appear to have version
2.9.4 of xfsprogs.

I have a 22TB xfs filesystem ,Yesterday, the hard disk is full, today
released a 7.7T disk space.But still can not write to new file.

software ambience:

[root at Production data5]# uname -a
Linux Production 2.6.18-128.el5 #1 SMP Wed Jan 21 10:41:14 EST 2009 x86_64
x86_64 x86_64 GNU/Linux

[root at Production data5]# rpm -qa | grep xfs
xfsprogs-2.9.4-1.el5.centos
xorg-x11-xfs-1.0.2-4
kmod-xfs-0.4-2


problem:

[root at Production data5]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              48G  6.7G   39G  15% /
/dev/sda2              81G  4.0G   73G   6% /opt
/dev/sdb1              22T   15T  7.7T  65% /data2
tmpfs                 2.0G     0  2.0G   0% /dev/shm

[root at Production data5]# df -i
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/sda1            12812288  136847 12675441    2% /
/dev/sda2            21867648    8440 21859208    1% /opt
/dev/sdb1            23429382016 4625792 23424756224    1% /data2
tmpfs                 504701       1  504700    1% /dev/shm

[root at Production data5]# touch test.log
touch: cannot touch `test.log': No space left on device
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