df reports the space occupied in the partition, but there is no data

Question

Pasting the output of df -h for the /mtd_rwarea partition:

$ df -h 
Filesystem                Size      Used Available Use% Mounted on
/dev/mmcblk0p16         150.0M    138.5M     11.5M  92% /mtd_rwarea

But there is nothing in the directory. Directory /mtd_rwarea is empty. The output of ls -la from that partition:

ls -la
drwxr-xr-x    1 root     0                2 Jan  1 00:01 .
drwxrwxrwx   26 root     0              693 Aug 16  2012 ..

Could anyone please help in understanding what could be the reason here?

Thanks!

Answer 1

If the space is reclaimed upon reboot, most likely the difference is due to deleted files with open handlers.

Use lsof to see if there is any deleted files still open. Some applications like MySQL are notorious for misbehaving in this area, unlinking temporary files without closing the file handlers.

Answer 2

A few reasons can cause this, the ones I know of, sorted in the order that I thought about them:

1. Mounting\Umounting a filesystem when you are in the directory would give you stale information when doing things like ls.
2. In cases where a directory has two mountpoints attached to it, the last one mounted is the effective mount point.
3. File systems sometimes reserve a percentage of the space for root user usage and in some versions it appears as used space (can be found on extX using dumpe2fs).
4. An error had occurred that a file was unlinked and the data blocks were not properly freed (fixed using fsck).
5. When the file system was created it was created with a high inode/block ratio which caused the inode space to appear allocated.

Considering the filesystem is empty, I would just recreate it.

However, I would be glad if you could paste the dumpe2fs (without block lists) to find out which of the theories is correct, if any.