DF command shows incorrect usage in VXFS

I have observed “df -h”command will show that veritas filesystems is used so and so space, but actually if you go to that mount point and calculate disk space using “du -sh” there will be nothing. In few cases, when df command will not show the correct usage of the filesystems. Also,if the filesystem created is with big size like 500GB, you might see that some 19-22GB of space is shown as used while filesystem is completely empty.

From veritas training books,
   1. VxFS will use some of this space for structural data such as the Intent Log, Free Extent Bitmaps, Superblocks, etc.
    2.  Due to the dynamic inode allocation functionality on a VXFS, free space is not the same as available space.

This space will be released by Veritas if user area needs that space.Correct way of identifying the actual free space available would be to use “df -b” command,
bash-3.00# df -h /mnt
Filesystem size used avail capacity Mounted on
/dev/vx/dsk/examdg/vol3 20M 3.1M 16M 17% /mnt
bash-3.00# cd /mnt
bash-3.00# ls -lrt
total 0
drwxr-xr-x 2 root root 96 Jul 3 13:51 lost+found

To find the right value,use the below command.

bash-3.00# df -b /mnt
Filesystem avail
/dev/vx/dsk/examdg/vol3 17315

As you can see it reports correct way.This is not a bug but rather is the way VxFS keeps the structural data of filesystem.

Thanks  to www.symantec.com