du and df mismatch with quota enabled

Issue

One GlusterFS volume XXXX has discrepancy between "df" and "du". "df" (volume used) shows 9.2GB, but "du" (actual usage) is about 500MB.
We tried restart glusterd, restart volume XXXX and it didn't help.

# gluster volume quota XXXX list
                  Path                   Hard-limit  Soft-limit      Used  Available  Soft-limit exceeded? Hard-limit exceeded?
-------------------------------------------------------------------------------------------------------------------------------
/                                         10.0GB     80%(8.0GB)    9.2GB 818.6MB             Yes                   No

# du -sh /data/brick2/XXXX
515M    /data/brick2/XXXX